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1
Bhattacharyya, Shaman, e Somnath Bhattacharyya. "Demonstration of the Holonomically Controlled Non-Abelian Geometric Phase in a Three-Qubit System of a Nitrogen Vacancy Center". Entropy 24, n. 11 (2 novembre 2022): 1593. http://dx.doi.org/10.3390/e24111593.
Abstract (sommario):
The holonomic approach to controlling (nitrogen-vacancy) NV-center qubits provides an elegant way of theoretically devising universal quantum gates that operate on qubits via calculable microwave pulses. There is, however, a lack of simulated results from the theory of holonomic control of quantum registers with more than two qubits describing the transition between the dark states. Considering this, we have been experimenting with the IBM Quantum Experience technology to determine the capabilities of simulating holonomic control of NV-centers for three qubits describing an eight-level system that produces a non-Abelian geometric phase. The tunability of the geometric phase via the detuning frequency is demonstrated through the high fidelity (~85%) of three-qubit off-resonant holonomic gates over the on-resonant ones. The transition between the dark states shows the alignment of the gate’s dark state with the qubit’s initial state hence decoherence of the multi-qubit system is well-controlled through a π/3 rotation.
2
Bashkirov, Eugene K. "Entanglement between two charge qubits taking account the Kerr media". Physics of Wave Processes and Radio Systems 27, n. 1 (29 marzo 2024): 26–34. http://dx.doi.org/10.18469/1810-3189.2024.27.1.26-34.
Abstract (sommario):
Background. The need to implement controlled coupling between qubits, which are the logical elements of quantum devices such as quantum computers and quantum networks, requires, along with the use of traditional methods, the development of new, more effective ways to organize the interaction of qubits with the microwave fields of resonators used to generate and control the entanglement of qubits. As one of these methods, a method based on the influence of frequency-regulated radio frequency signals on a superconducting Josephson qubit connected by a large Josephson junction to a free qubit has been proposed. Aim. The influence of the Kerr medium of the resonator, in which one of the two qubits is placed, on their entanglement induced by the coherent or thermal frequency-regulated radio frequency field of the resonator is considered. Methods. To analyze the dynamics of the system under consideration, the solution of the quantum Liouville equation for the full density matrix is studied. An exact solution o this equation is found in the case of initial separable and entangled states of qubits. The exact solution of the evolution equation is used to calculate the criterion of qubit-qubit entanglement – cconcurrence. Numerical modeling of the concurrence was carried out for various states of qubits, coherent and thermal fields of the resonator, as well as various values of the intensity of the resonator field and the Kerr nonlinearity parameter. Results. It is shown that for separable initial states of qubits, the inclusion of Kerr nonlinearity reduces the maximum degree of entanglement of qubits. For an entangled initial state of qubits, the possibility of creating long-lived entangled states in the presence of Kerr nonlinearity is shown. Conclusion. The type of initial states of qubits and the range of values of the intensities of the resonator fields and the Kerr nonlinearity parameters have been established, for which the most effective control and operation of the evolution of qubits, as well as the degree of their entanglement, in the physical system under consideration, is possible.
3
Dykman, M. I., L. F. Santos, M. Shapiro e F. M. Izrailev. "On-site localization of excitations". Quantum Information and Computation 5, n. 4&5 (luglio 2005): 335–49. http://dx.doi.org/10.26421/qic5.45-5.
Abstract (sommario):
We demonstrate that, in a quantum computer with perpetually coupled qubits, all excitations can be confined to their sites (qubits) even without refocusing. The on-site localization is obtained by constructing a sequence of qubit energies that efficiently suppresses resonant hopping. The time during which a many-excitation state remains strongly localized in an infinite chain can exceed the reciprocal hopping frequency by $\agt 10^5$ already for a moderate bandwidth of qubit energies. The proposed energy sequence is also convenient for performing quantum operations on the qubits.
4
Tholén, Mats O., Riccardo Borgani, Giuseppe Ruggero Di Carlo, Andreas Bengtsson, Christian Križan, Marina Kudra, Giovanna Tancredi et al. "Measurement and control of a superconducting quantum processor with a fully integrated radio-frequency system on a chip". Review of Scientific Instruments 93, n. 10 (1 ottobre 2022): 104711. http://dx.doi.org/10.1063/5.0101398.
Abstract (sommario):
We describe a digital microwave platform called Presto, designed for measurement and control of multiple quantum bits (qubits) and based on the third-generation radio-frequency system on a chip. Presto uses direct digital synthesis to create signals up to 9 GHz on 16 synchronous output ports, while synchronously analyzing responses on 16 input ports. Presto has 16 DC-bias outputs, four inputs and four outputs for digital triggers or markers, and two continuous-wave outputs for synthesizing frequencies up to 15 GHz. Scaling to a large number of qubits is enabled through deterministic synchronization of multiple Presto units. A Python application programming interface configures a firmware for synthesis and analysis of pulses, coordinated by an event sequencer. The analysis integrates template matching (matched filtering) and low-latency (184–254 ns) feedback to enable a wide range of multi-qubit experiments. We demonstrate Presto’s capabilities with experiments on a sample consisting of two superconducting qubits connected via a flux-tunable coupler. We show single-shot readout and active reset of a single qubit; randomized benchmarking of single-qubit gates showing 99.972% fidelity, limited by the coherence time of the qubit; and calibration of a two-qubit iSWAP gate.
5
MASTELLONE, A., A. D'ARRIGO, E. PALADINO e G. FALCI. "PROTECTED COMPUTATIONAL SUBSPACES OF COUPLED SUPERCONDUCTING QUBITS". International Journal of Quantum Information 06, supp01 (luglio 2008): 645–50. http://dx.doi.org/10.1142/s0219749908003906.
Abstract (sommario):
We discuss the effect of low-frequency noise on interacting superconducting qubits in a fixed coupling scheme. By properly choosing operating conditions, within the adiabatic framework the systems develops two decoupled subspaces. The subspace where a SWAP operation takes place turns out to be resilient to low frequency fluctuations. The possibility to encode a single qubit in a protected two-physical-qubit system subspace is briefly discussed.
6
Kubo, Kentaro, e Hayato Goto. "Fast parametric two-qubit gate for highly detuned fixed-frequency superconducting qubits using a double-transmon coupler". Applied Physics Letters 122, n. 6 (6 febbraio 2023): 064001. http://dx.doi.org/10.1063/5.0138699.
Abstract (sommario):
High-performance two-qubit gates have been reported with superconducting qubits coupled via a single-transmon coupler (STC). Most of them are implemented for qubits with a small detuning since reducing residual ZZ coupling for highly detuned qubits by an STC is challenging. In terms of the frequency crowding and crosstalk, however, highly detuned qubits are desirable. Here, we numerically demonstrate a high-performance parametric gate for highly detuned fixed-frequency qubits using a recently proposed tunable coupler called a double-transmon coupler (DTC). Applying an ac flux pulse, we can perform a maximally entangling universal gate ([Formula: see text]) with an average fidelity over 99.99% and a short gate time of about 24 ns. This speed is comparable to resonance-based gates for slightly detuned tunable qubits. Moreover, using a dc flux pulse alternatively, we can achieve another kind of entangling gate called a CZ gate with an average fidelity over 99.99% and a gate time of about 18 ns. Given the flexibility and feasible settings, we can expect that the DTC will contribute towards realizing a high-performance quantum computer in the near future.
7
Greenaway, Sean, Adam Smith, Florian Mintert e Daniel Malz. "Analogue Quantum Simulation with Fixed-Frequency Transmon Qubits". Quantum 8 (22 febbraio 2024): 1263. http://dx.doi.org/10.22331/q-2024-02-22-1263.
Abstract (sommario):
We experimentally assess the suitability of transmon qubits with fixed frequencies and fixed interactions for the realization of analogue quantum simulations of spin systems. We test a set of necessary criteria for this goal on a commercial quantum processor using full quantum process tomography and more efficient Hamiltonian tomography. Significant single qubit errors at low amplitudes are identified as a limiting factor preventing the realization of analogue simulations on currently available devices. We additionally find spurious dynamics in the absence of drive pulses, which we identify with coherent coupling between the qubit and a low dimensional environment. With moderate improvements, analogue simulation of a rich family of time-dependent many-body spin Hamiltonians may be possible.
8
Takeda, Kenta, Jun Kamioka, Tomohiro Otsuka, Jun Yoneda, Takashi Nakajima, Matthieu R. Delbecq, Shinichi Amaha et al. "A fault-tolerant addressable spin qubit in a natural silicon quantum dot". Science Advances 2, n. 8 (agosto 2016): e1600694. http://dx.doi.org/10.1126/sciadv.1600694.
Abstract (sommario):
Fault-tolerant quantum computing requires high-fidelity qubits. This has been achieved in various solid-state systems, including isotopically purified silicon, but is yet to be accomplished in industry-standard natural (unpurified) silicon, mainly as a result of the dephasing caused by residual nuclear spins. This high fidelity can be achieved by speeding up the qubit operation and/or prolonging the dephasing time, that is, increasing the Rabi oscillation quality factor Q (the Rabi oscillation decay time divided by the π rotation time). In isotopically purified silicon quantum dots, only the second approach has been used, leaving the qubit operation slow. We apply the first approach to demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet that is optimally designed for fast spin control. This optimized design allows access to Rabi frequencies up to 35 MHz, which is two orders of magnitude greater than that achieved in previous studies. We find the optimum Q = 140 in such high-frequency range at a Rabi frequency of 10 MHz. This leads to a qubit fidelity of 99.6% measured via randomized benchmarking, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum dot–based qubits. This result can inspire contributions to quantum computing from industrial communities.
9
Fabre, Nicolas. "Teleportation-Based Error Correction Protocol of Time–Frequency Qubit States". Applied Sciences 13, n. 16 (21 agosto 2023): 9462. http://dx.doi.org/10.3390/app13169462.
Abstract (sommario):
We present a linear optical protocol for teleporting and correcting both temporal and frequency errors in two time–frequency qubit states. The first state is the frequency (or time-of-arrival) cat qubit, which is a single photon in a superposition of two frequencies (or time-of-arrival), while the second is the time–frequency Gottesman–Kitaev–Preskill (GKP) state, which is a single photon with a frequency comb structure. The proposed optical scheme could be valuable for reducing the error rate in quantum communication protocols involving one of these qubits.
10
Гринберг, Я. С., e А. А. Штыгашев. "Импульсное возбуждение в двухкубитных системах". Физика твердого тела 60, n. 11 (2018): 2069. http://dx.doi.org/10.21883/ftt.2018.11.46641.02nn.
Abstract (sommario):
AbstractThe temporal dynamics of absorption of a single-photon pulse by two qubits interacting with a microwave field of a one-dimensional waveguide have been studied. The theory, which allows one to use arbitrary shapes of the input single-photon wave packet as an initial condition, as well as investigate the excitation dynamics of each qubit, has been developed. The numerical calculation is performed for the packet of a Gaussian shape, at different parameters of frequency detuning and duration of the input pulse. The excitation dynamics of both identical and nonidentical qubits have been studied. It has been specifically shown that it is possible to form symmetric and antisymmetric entangled states for identical qubits.
11
Choi, Jeong Ryeol. "Dynamics of Dispersive Measurements of Flux-Qubit States: Energy-Level Splitting Connected to Quantum Wave Mechanics". Nanomaterials 13, n. 17 (23 agosto 2023): 2395. http://dx.doi.org/10.3390/nano13172395.
Abstract (sommario):
Superconducting flux qubits have many advantages as a storage of quantum information, such as broad range tunability of frequency, small-size fabricability, and high controllability. In the flux qubit–oscillator, qubits are connected to SQUID resonators for the purpose of performing dispersive non-destructive readouts of qubit signals with high fidelity. In this work, we propose a theoretical model for analyzing quantum characteristics of a flux qubit–oscillator on the basis of quantum solutions obtained using a unitary transformation approach. The energy levels of the combined system (qubit + resonator) are analyzed in detail. Equally spaced each energy level of the resonator splits into two parts depending on qubit states. Besides, coupling of the qubit to the resonator brings about an additional modification in the split energy levels. So long as the coupling strength is not zero, the energy-level splitting of the resonator does not disappear even when the tunnel splitting in the qubit is zero. We conclude that quantum nondemolition dispersive measurements of the qubit states are possible by inducing bifurcation of the resonator states through the coupling.
12
Knaut, C. M., A. Suleymanzade, Y. C. Wei, D. R. Assumpcao, P. J. Stas, Y. Q. Huan, B. Machielse et al. "Entanglement of nanophotonic quantum memory nodes in a telecom network". Nature 629, n. 8012 (15 maggio 2024): 573–78. http://dx.doi.org/10.1038/s41586-024-07252-z.
Abstract (sommario):
AbstractA key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected by fibre optical infrastructure1–3. Here we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centres in nanophotonic diamond cavities integrated with a telecommunication fibre network. Remote entanglement is generated by the cavity-enhanced interactions between the electron spin qubits of the SiVs and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits to telecommunication frequencies (1,350 nm), we demonstrate the entanglement of two nuclear spin memories through 40 km spools of low-loss fibre and a 35-km long fibre loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.
13
Nuerbolati, Wuerkaixi, Zhikun Han, Ji Chu, Yuxuan Zhou, Xinsheng Tan, Yang Yu, Song Liu e Fei Yan. "Canceling microwave crosstalk with fixed-frequency qubits". Applied Physics Letters 120, n. 17 (25 aprile 2022): 174001. http://dx.doi.org/10.1063/5.0088094.
Abstract (sommario):
Scalable quantum information processing requires that modular gate operations can be executed in parallel. The presence of crosstalk decreases the individual addressability, causing erroneous results during simultaneous operations. For superconducting qubits which operate in the microwave regime, electromagnetic isolation is often limited due to design constraints, leading to signal crosstalk that can deteriorate the quality of simultaneous gate operations. Here, we propose and demonstrate a method based on the alternative-current Stark effect for calibrating the microwave signal crosstalk. The method is suitable for processors based on fixed-frequency qubits, which are known for high coherence and simple control. The optimal compensation parameters can be reliably identified from a well-defined interference pattern. We implement the method on an array of seven superconducting qubits and show its effectiveness in removing the majority of crosstalk errors.
14
Nuerbolati, Wuerkaixi, Zhikun Han, Ji Chu, Yuxuan Zhou, Xinsheng Tan, Yang Yu, Song Liu e Fei Yan. "Canceling microwave crosstalk with fixed-frequency qubits". Applied Physics Letters 120, n. 17 (25 aprile 2022): 174001. http://dx.doi.org/10.1063/5.0088094.
Abstract (sommario):
Scalable quantum information processing requires that modular gate operations can be executed in parallel. The presence of crosstalk decreases the individual addressability, causing erroneous results during simultaneous operations. For superconducting qubits which operate in the microwave regime, electromagnetic isolation is often limited due to design constraints, leading to signal crosstalk that can deteriorate the quality of simultaneous gate operations. Here, we propose and demonstrate a method based on the alternative-current Stark effect for calibrating the microwave signal crosstalk. The method is suitable for processors based on fixed-frequency qubits, which are known for high coherence and simple control. The optimal compensation parameters can be reliably identified from a well-defined interference pattern. We implement the method on an array of seven superconducting qubits and show its effectiveness in removing the majority of crosstalk errors.
15
Hu, Rui-Zi, Rong-Long Ma, Ming Ni, Xin Zhang, Yuan Zhou, Ke Wang, Gang Luo et al. "An Operation Guide of Si-MOS Quantum Dots for Spin Qubits". Nanomaterials 11, n. 10 (24 settembre 2021): 2486. http://dx.doi.org/10.3390/nano11102486.
Abstract (sommario):
In the last 20 years, silicon quantum dots have received considerable attention from academic and industrial communities for research on readout, manipulation, storage, near-neighbor and long-range coupling of spin qubits. In this paper, we introduce how to realize a single spin qubit from Si-MOS quantum dots. First, we introduce the structure of a typical Si-MOS quantum dot and the experimental setup. Then, we show the basic properties of the quantum dot, including charge stability diagram, orbital state, valley state, lever arm, electron temperature, tunneling rate and spin lifetime. After that, we introduce the two most commonly used methods for spin-to-charge conversion, i.e., Elzerman readout and Pauli spin blockade readout. Finally, we discuss the details of how to find the resonance frequency of spin qubits and show the result of coherent manipulation, i.e., Rabi oscillation. The above processes constitute an operation guide for helping the followers enter the field of spin qubits in Si-MOS quantum dots.
16
Huang, Zhongkai, Alejandro D. Somoza, Cheng Peng, Jin Huang, Maolin Bo, Chuang Yao, JiBiao Li e Guankui Long. "Polaron dynamics of Bloch–Zener oscillations in an extended Holstein model". New Journal of Physics 23, n. 12 (1 dicembre 2021): 123020. http://dx.doi.org/10.1088/1367-2630/ac3ac7.
Abstract (sommario):
Abstract Recent developments in qubit engineering make circuit quantum electrodynamics devices promising candidates for the study of Bloch oscillations (BOs) and Landau–Zener (LZ) transitions. In this work, a hybrid circuit chain with alternating site energies under external electric fields is employed to study Bloch–Zener oscillations (BZOs), i.e. coherent superpositions of BOs and LZ transitions. We couple each of the tunable qubits in the chain to dispersionless optical phonons and build an extended Holstein polaron model with the purpose of investigating vibronic effects in the BZOs. We employ an extension of the Davydov ansatz in combination with the Dirac–Frenkel time-dependent variational principle to simulate the dynamics of the qubit chain under the influence of high-frequency quantum harmonic oscillators. Band gaps emerge due to energy differences in site energies at alternating qubit sites, and are shown to play key roles in tuning band structures and time periodic reconstructions of the wave patterns. In the absence of qubit–phonon interactions, the qubits undergo either standard BZOs or breathing modes, depending on whether the initial wave packet is formed by a broad or narrow Gaussian wave packet, respectively. The BZOs can get localized in space if the band gaps are sufficiently large. In the presence of qubit–phonon coupling, the periodic behavior of BZOs can be washed out and undergo dynamic localization. The influence of an ohmic bath on the dynamics of BZOs is investigated by means of a Markovian master equation approach. Finally, we calculate the von Neumann entropy as a measure of the entanglement between qubits and phonons.
17
McKemmish, Laura K., David J. Kedziora, Graham R. White, Noel S. Hush e Jeffrey R. Reimers. "Frequency-based Quantum Computers from a Chemist's Perspective". Australian Journal of Chemistry 65, n. 5 (2012): 512. http://dx.doi.org/10.1071/ch12053.
Abstract (sommario):
Quantum computer elements are often designed and tested using molecular or nanoscopic components that form registers of qubits in which memory is stored and information processed. Often such registers are probed and manipulated using frequency-based techniques such as nuclear-magnetic resonance spectroscopy. A major challenge is to design molecules to act as these registers. We provide a basis for rational molecular design through consideration of the generic spectroscopic properties required for quantum computing, bypassing the need for intricate knowledge of the way these molecules are used spectroscopically. Designs in which two-qubit gate times scale similarly to those for one-qubit gates are presented. The specified spectroscopic requirements are largely independent of the type of spectroscopy used (e.g. magnetic resonance or vibrational) and are often independent of technical details of the application (e.g. broadband or high-resolution spectroscopy). This should allow the design of much larger quantum registers than have currently been demonstrated.
18
Simonović, Svetomir. "On Photonic Implementation of Quantum Computers". Advanced Technologies & Materials 48, n. 2 (2 dicembre 2023): 61–68. http://dx.doi.org/10.24867/atm-2023-2-004.
Abstract (sommario):
The first section of the work investigates light modes as a means of implementing optical qubits and qudits. The modes considered are polarization mode, path mode, transverse spatial mode, frequency mode, temporal bin-mode and temporal mode. Subsequently, mathematical model of linear optical elements like beam splitters (BS) and phase shifters are deduced and their capability of representing any single qubit optical gate is exposed. Finally Knill, Laflamme and Milburn (KLM) method of using linear optical elements to promote nonlinear operations based on nonlinear (nondeterministic) sign-flip gate (NS) is explained, and designs of two qubits conditional sign flip gate (c-z1/16) and CNOT gate, both based on KLM method, are demonstrated. So, universal set of quantum gates based on linear optics is possible.
19
Yang, Xin-Xin, Xiao-Yan Yang, Liang-Liang Guo, Lei Du, Peng Duan, Zhi-Long Jia, Hai-Ou Li e Guo-Ping Guo. "Locating Two-Level Systems in a Superconducting Xmon Qubit". Applied Sciences 13, n. 11 (30 maggio 2023): 6672. http://dx.doi.org/10.3390/app13116672.
Abstract (sommario):
One significant source of decoherence in superconducting circuits is known as two-level systems (TLSs), found in amorphous oxide layers. These circuits can, however, also be utilized as spectral and temporal TLS probes. Comprehensive investigations on the physics of TLSs are now possible thanks to recent advancements in superconducting qubits. Here, we simultaneously measure the tunable Xmon qubit decoherence time as well as the resonance frequency for more than 3 days to investigate stochastic fluctuations. Time-domain Allan deviation and frequency-domain power spectral density analysis indicate that two TLSs in near resonance with the qubit are responsible for the fluctuations. From the extracted oscillation in T1 decay, we locate the two TLSs near the junctions.
20
Konno, Shunya, Warit Asavanant, Fumiya Hanamura, Hironari Nagayoshi, Kosuke Fukui, Atsushi Sakaguchi, Ryuhoh Ide et al. "Logical states for fault-tolerant quantum computation with propagating light". Science 383, n. 6680 (19 gennaio 2024): 289–93. http://dx.doi.org/10.1126/science.adk7560.
Abstract (sommario):
To harness the potential of a quantum computer, quantum information must be protected against error by encoding it into a logical state that is suitable for quantum error correction. The Gottesman-Kitaev-Preskill (GKP) qubit is a promising candidate because the required multiqubit operations are readily available at optical frequency. To date, however, GKP qubits have been demonstrated only at mechanical and microwave frequencies. We realized a GKP state in propagating light at telecommunication wavelength and verified it through homodyne measurements without loss corrections. The generation is based on interference of cat states, followed by homodyne measurements. Our final states exhibit nonclassicality and non-Gaussianity, including the trident shape of faint instances of GKP states. Improvements toward brighter, multipeaked GKP qubits will be the basis for quantum computation with light.
21
Sekatski, Pavel, Michalis Skotiniotis, Janek Kołodyński e Wolfgang Dür. "Quantum metrology with full and fast quantum control". Quantum 1 (6 settembre 2017): 27. http://dx.doi.org/10.22331/q-2017-09-06-27.
Abstract (sommario):
We establish general limits on how precise a parameter, e.g. frequency or the strength of a magnetic field, can be estimated with the aid of full and fast quantum control. We consider uncorrelated noisy evolutions of N qubits and show that fast control allows to fully restore the Heisenberg scaling (~1/N^2) for all rank-one Pauli noise except dephasing. For all other types of noise the asymptotic quantum enhancement is unavoidably limited to a constant-factor improvement over the standard quantum limit (~1/N) even when allowing for the full power of fast control. The latter holds both in the single-shot and infinitely-many repetitions scenarios. However, even in this case allowing for fast quantum control helps to increase the improvement factor. Furthermore, for frequency estimation with finite resource we show how a parallel scheme utilizing any fixed number of entangled qubits but no fast quantum control can be outperformed by a simple, easily implementable, sequential scheme which only requires entanglement between one sensing and one auxiliary qubit.
22
Hornibrook, J. M., J. I. Colless, A. C. Mahoney, X. G. Croot, S. Blanvillain, H. Lu, A. C. Gossard e D. J. Reilly. "Frequency multiplexing for readout of spin qubits". Applied Physics Letters 104, n. 10 (10 marzo 2014): 103108. http://dx.doi.org/10.1063/1.4868107.
23
Kattemölle, Joris, e Jasper van Wezel. "Conditions for superdecoherence". Quantum 4 (14 maggio 2020): 265. http://dx.doi.org/10.22331/q-2020-05-14-265.
Abstract (sommario):
Decoherence is the main obstacle to quantum computation. The decoherence rate per qubit is typically assumed to be constant. It is known, however, that quantum registers coupling to a single reservoir can show a decoherence rate per qubit that increases linearly with the number of qubits. This effect has been referred to as superdecoherence, and has been suggested to pose a threat to the scalability of quantum computation. Here, we show that superdecoherence is absent when the spectrum of the single reservoir is continuous, rather than discrete. The reason of this absence, is that, as the number of qubits is increased, a quantum register inevitably becomes susceptible to an ever narrower bandwidth of frequencies in the reservoir. Furthermore, we show that for superdecoherence to occur in a reservoir with a discrete spectrum, one of the frequencies in the reservoir has to coincide exactly with the frequency the quantum register is most susceptible to. We thus fully resolve the conditions that determine the presence or absence of superdecoherence. We conclude that superdecoherence is easily avoidable in practical realizations of quantum computers.
24
Tang, Jia-Liang, Gabriel Alvarado Barrios, Enrique Solano e Francisco Albarrán-Arriagada. "Tunable Non-Markovianity for Bosonic Quantum Memristors". Entropy 25, n. 5 (6 maggio 2023): 756. http://dx.doi.org/10.3390/e25050756.
Abstract (sommario):
We studied the tunable control of the non-Markovianity of a bosonic mode due to its coupling to a set of auxiliary qubits, both embedded in a thermal reservoir. Specifically, we considered a single cavity mode coupled to auxiliary qubits described by the Tavis–Cummings model. As a figure of merit, we define the dynamical non-Markovianity as the tendency of a system to return to its initial state, instead of evolving monotonically to its steady state. We studied how this dynamical non-Markovianity can be manipulated in terms of the qubit frequency. We found that the control of the auxiliary systems affects the cavity dynamics as an effective time-dependent decay rate. Finally, we show how this tunable time-dependent decay rate can be tuned to engineer bosonic quantum memristors, involving memory effects that are fundamental for developing neuromorphic quantum technologies.
25
Dheer, Vihaan. "The optimization of flux trajectories for the adiabatic controlled-Z gate on split-tunable transmons". AIP Advances 12, n. 9 (1 settembre 2022): 095306. http://dx.doi.org/10.1063/5.0087364.
Abstract (sommario):
In a system of two tunable-frequency qubits, it is well-known that adiabatic tuning into strong coupling-interaction regions between the qubit subspace and the rest of the Hilbert space can be used to generate an effective controlled-Z rotation. We address the problem of determining a preferable adiabatic trajectory along which the qubit frequency is tuned and apply this to the flux-tunable transmon model. The especially minimal anharmonic nature of these quantum processors makes them good candidates for qubit control using non-computational states as long as higher-level leakage is properly addressed. While the statement of this method has occurred multiple times in the literature, there have been few discussions on which trajectories may be used. We present a generalized method for optimizing parameterized families of possible flux trajectories and provide examples of use on five test families of one and two parameters.
26
Kim, Hyunseong, Christian Jünger, Alexis Morvan, Edward S. Barnard, William P. Livingston, M. Virginia P. Altoé, Yosep Kim et al. "Effects of laser-annealing on fixed-frequency superconducting qubits". Applied Physics Letters 121, n. 14 (3 ottobre 2022): 142601. http://dx.doi.org/10.1063/5.0102092.
Abstract (sommario):
As superconducting quantum processors increase in complexity, techniques to overcome constraints on frequency crowding are needed. The recently developed method of laser-annealing provides an effective post-fabrication method to adjust the frequency of superconducting qubits. Here, we present an automated laser-annealing apparatus based on conventional microscopy components and demonstrate preservation of highly coherent transmons. In addition, we perform noise spectroscopy to investigate the change in defect features, in particular, two-level system defects, after laser-annealing. Finally, we present a local heating model as well as demonstrate aging stability for laser-annealing on the wafer scale. Our work constitutes an important step toward both understanding the underlying physical mechanism and scaling up laser-annealing of superconducting qubits.
27
Parrado-Rodríguez, Pedro, Ciarán Ryan-Anderson, Alejandro Bermudez e Markus Müller. "Crosstalk Suppression for Fault-tolerant Quantum Error Correction with Trapped Ions". Quantum 5 (29 giugno 2021): 487. http://dx.doi.org/10.22331/q-2021-06-29-487.
Abstract (sommario):
Physical qubits in experimental quantum information processors are inevitably exposed to different sources of noise and imperfections, which lead to errors that typically accumulate hindering our ability to perform long computations reliably. Progress towards scalable and robust quantum computation relies on exploiting quantum error correction (QEC) to actively battle these undesired effects. In this work, we present a comprehensive study of crosstalk errors in a quantum-computing architecture based on a single string of ions confined by a radio-frequency trap, and manipulated by individually-addressed laser beams. This type of errors affects spectator qubits that, ideally, should remain unaltered during the application of single- and two-qubit quantum gates addressed at a different set of active qubits. We microscopically model crosstalk errors from first principles and present a detailed study showing the importance of using a coherent vs incoherent error modelling and, moreover, discuss strategies to actively suppress this crosstalk at the gate level. Finally, we study the impact of residual crosstalk errors on the performance of fault-tolerant QEC numerically, identifying the experimental target values that need to be achieved in near-term trapped-ion experiments to reach the break-even point for beneficial QEC with low-distance topological codes.
28
Mori, Takahiro. "(Invited, Digital Presentation) Silicon Compatible Quantum Computers: Challenges in Devices, Integration, and Circuits". ECS Meeting Abstracts MA2022-01, n. 29 (7 luglio 2022): 1297. http://dx.doi.org/10.1149/ma2022-01291297mtgabs.
Abstract (sommario):
Quantum computers have been attractive because they could realize large-scale and highly complicated calculations that conventional computers cannot solve within a finite time. The large-scale integration of qubits, which are the building block of quantum computers, is required to realize their practical application. Indeed, fault-tolerant quantum computers require the integration of one million qubits. Therefore, silicon qubits is a high-profile candidate because they have advanced process and miniaturization technologies developed with VLSI. In addition, silicon qubits are advantageous in operation temperature. Superconductor qubits operate at the cryogenic temperature at around a few tens mK; in contrast, the operation principle of silicon qubits can operate at a much higher temperature over 1 K. The high-temperature operation can realize quantum computers with small and high-power refrigerators; therefore, we can expect desktop quantum computers instead of ongoing supercomputer-size ones. We must promote integration technology development for silicon qubits; however, the silicon qubit research was mainly in the physics field. Then, nowadays, the integration technology development is accelerated in the world. The challenges are in all conventional research fields: devices, integration, and circuits. We must re-develop the silicon technologies for quantum. For example, on the device design, now we do not have a good tool to design the qubits like TCAD; therefore, we must re-develop the TCAD technologies for quantum [1]. Actually, this is the starting point of our recent research activities; we are going to develop a quantum device simulator, clarify the requirements on the fabrication process of silicon qubits, and propose new technologies to reduce the variability to realize large-scale integration [2]. As for the integration, the quantum calculation circuits require several integrated items: qubits, qubit couplers, micro-magnets, and readout systems. The situation is quite different from the conventional VLSI case for which only the transistors should be integrated. Therefore, we must go re-developing new technologies to integrate all these items. Regarding the circuits, we must use CMOS circuits to generate input signals for qubits and readout the results of quantum calculation, which should be operated at cryogenic temperature. This is so-called “cryo-CMOS.” We must explore a new side of the transistor technologies, which is not investigated so far, because the physics of the MOSFET operation is quite different from the conventional room-temperature operation, hampering the circuit design due to the lack of the device operation model. In this situation, despite the long history of MOSFETs, new phenomena of transistor operation are discovered. For example, the low-frequency current noise increases at a low temperature. The origin of the noise is on the interface traps, instead of the fixed charges in the gate oxides as is the case for room temperature operation [3]. Therefore, we must re-developing CMOS circuit technologies from the bottom of the technologies, device physics. In this presentation, I’m going to overview the status of silicon technology developments for quantum from the viewpoints of devices, integration, and circuits. Also, we introduce some of our recent results to contribute to the developments. Acknowledgment: Our work is supported by MEXT Quantum Leap Flagship Program (Q-LEAP) JPMXS0118069228. [1] H. Asai et al., IEEE Electron Devices Technology and Manufacturing Conference 2021. [2] S. Iizuka et al., Tech. Dig. Symp. VLSI Technology 2021. [3] H. Oka et al., Tech. Dig. Symp. VLSI Technology 2020.
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BERMAN, G. P., G. W. BROWN, M. E. HAWLEY, D. I. KAMENEV e V. I. TSIFRINOVICH. "IMPLEMENTATION OF QUANTUM LOGIC OPERATIONS AND CREATION OF ENTANGLEMENT BETWEEN TWO NUCLEAR SPIN QUBITS WITH CONSTANT INTERACTION". International Journal of Quantum Information 04, n. 06 (dicembre 2006): 975–1001. http://dx.doi.org/10.1142/s0219749906002353.
Abstract (sommario):
We describe how to implement quantum logic operations in a silicon-based quantum computer with phosphorus atoms serving as qubits. The information is stored in the states of nuclear spins and the conditional logic operations are implemented through the electron spins using nuclear–electron hyperfine and electron–electron exchange interactions. The electrons in our computer should stay coherent only during implementation of one Controlled-NOT gate. The exchange interaction is constant, and selective excitations are provided by a magnetic field gradient. The quantum logic operations are implemented by rectangular radio-frequency pulses. This architecture is scalable and does not require manufacturing nanoscale electronic gates. As shown in this paper, parameters of a quantum protocol can be derived analytically even for a computer with a large number of qubits using our perturbation approach. We present the protocol for initialization of the nuclear spins and the protocol for creation of entanglement. All analytical results are tested numerically using a two-qubit system.
30
Ye, Yangsen, Sirui Cao, Yulin Wu, Xiawei Chen, Qingling Zhu, Shaowei Li, Fusheng Chen et al. "Realization of High-Fidelity Controlled-Phase Gates in Extensible Superconducting Qubits Design with a Tunable Coupler". Chinese Physics Letters 38, n. 10 (1 novembre 2021): 100301. http://dx.doi.org/10.1088/0256-307x/38/10/100301.
Abstract (sommario):
High-fidelity two-qubit gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design an extensible 5-qubit system in which center transmon qubit can couple to every four near-neighboring qubits via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating central qubit and one near-neighboring qubit. Speckle purity benchmarking and cross entropy benchmarking are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69±0.04% and the average fidelity of the CZ gate is 99.65±0.04%, which means that the control error is about 0.04%. Our work is helpful for resolving many challenges in implementation of large-scale quantum systems.
31
Park, Kun Hee, Yung Szen Yap, Yuanzheng Paul Tan, Christoph Hufnagel, Long Hoang Nguyen, Karn Hwa Lau, Patrick Bore et al. "ICARUS-Q: Integrated control and readout unit for scalable quantum processors". Review of Scientific Instruments 93, n. 10 (1 ottobre 2022): 104704. http://dx.doi.org/10.1063/5.0081232.
Abstract (sommario):
We present a control and measurement setup for superconducting qubits based on the Xilinx 16-channel radio-frequency system-on-chip (RFSoC) device. The proposed setup consists of four parts: multiple RFSoC boards, a setup to synchronize every digital to analog converter (DAC) and analog to digital converter (ADC) channel across multiple boards, a low-noise direct current supply for tuning the qubit frequency, and cloud access for remotely performing experiments. We also designed the setup to be free of physical mixers. The RFSoC boards directly generate microwave pulses using sixteen DAC channels up to the third Nyquist zone, which are directly sampled by its eight ADC channels between the fifth and the ninth zones.
32
Makhlin, Yu, e A. Shnirman. "Dephasing of qubits by transverse low-frequency noise". Journal of Experimental and Theoretical Physics Letters 78, n. 8 (ottobre 2003): 497–501. http://dx.doi.org/10.1134/1.1637702.
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Bergli, J., Y. M. Galperin e B. L. Altshuler. "Decoherence in qubits due to low-frequency noise". New Journal of Physics 11, n. 2 (25 febbraio 2009): 025002. http://dx.doi.org/10.1088/1367-2630/11/2/025002.
34
Jiang, Junliang, Zishuo Li, Tingting Guo, Wenqu Xu, Xingyu Wei, Kaixuan Zhang, Tianshi Zhou et al. "Building compact superconducting microwave resonators with Hilbert space-filling curves". Applied Physics Letters 121, n. 25 (19 dicembre 2022): 254001. http://dx.doi.org/10.1063/5.0128964.
Abstract (sommario):
Superconducting quantum computing is currently one of the most promising platforms for universal quantum information processing. The readout resonator is an essential integral part of a superconducting qubit, while its size is much larger compared to the Josephson junction. We propose and realize a new readout resonator using space-filling curves, specifically Hilbert space-filling curves. We introduce the frequency analysis method and demonstrate a qubit sample, in which the Hilbert-space-filling-curves resonator (HSFCR) is used to read out the qubit states. We also propose to fabricate the HSFCRs and Josephson junctions simultaneously in the same processes of E-beam lithography and E-beam evaporation. Our design reduces the resonator area sufficiently and, thus, will help to improve the integration of superconducting qubits, as well as to design other superconducting quantum devices.
35
Wang, Z. T., Peng Zhao, Z. H. Yang, Ye Tian, H. F. Yu e S. P. Zhao. "Escaping detrimental interactions with microwave-dressed transmon qubits". Chinese Physics Letters, 27 giugno 2023. http://dx.doi.org/10.1088/0256-307x/40/7/070304.
Abstract (sommario):
Abstract Superconducting transmon qubits with fixed frequencies are widely used in many applications due to their advantages of better coherence and less control lines as compared to the frequency tunable qubits. However, any uncontrolled interactions with the qubits such as the two-level systems could lead to adverse impacts, degrading the qubit coherence and inducing crosstalk. To mitigate the detrimental effect from uncontrolled interactions between qubits and defect modes in fixedfrequency transmon qubits, we propose and demonstrate an active approach using an off-resonance microwave drive to dress the qubit and induce the ac-Stark shift on the qubit frequency. We show experimentally that the qubit frequency can be tuned well away from the defect mode so that the impact on qubit coherence is greatly reduced while maintaining the universal controls of the qubit initialization, readout, and single-qubit gate operations. Our approach provides an effective way for tuning the qubit frequency and suppressing the detrimental effect from the defect modes that happen to be located close to the qubit frequency.
36
Zhang, Eric J., Srikanth Srinivasan, Neereja Sundaresan, Daniela F. Bogorin, Yves Martin, Jared B. Hertzberg, John Timmerwilke et al. "High-performance superconducting quantum processors via laser annealing of transmon qubits". Science Advances 8, n. 19 (13 maggio 2022). http://dx.doi.org/10.1126/sciadv.abi6690.
Abstract (sommario):
Scaling the number of qubits while maintaining high-fidelity quantum gates remains a key challenge for quantum computing. Presently, superconducting quantum processors with >50 qubits are actively available. For these systems, fixed-frequency transmons are attractive because of their long coherence and noise immunity. However, scaling fixed-frequency architectures proves challenging because of precise relative frequency requirements. Here, we use laser annealing to selectively tune transmon qubits into desired frequency patterns. Statistics over hundreds of annealed qubits demonstrate an empirical tuning precision of 18.5 MHz, with no measurable impact on qubit coherence. We quantify gate error statistics on a tuned 65-qubit processor, with median two-qubit gate fidelity of 98.7%. Baseline tuning statistics yield a frequency-equivalent resistance precision of 4.7 MHz, sufficient for high-yield scaling beyond 10 3 qubit levels. Moving forward, we anticipate selective laser annealing to play a central role in scaling fixed-frequency architectures.
37
Vepsäläinen, Antti, Roni Winik, Amir H. Karamlou, Jochen Braumüller, Agustin Di Paolo, Youngkyu Sung, Bharath Kannan et al. "Improving qubit coherence using closed-loop feedback". Nature Communications 13, n. 1 (11 aprile 2022). http://dx.doi.org/10.1038/s41467-022-29287-4.
Abstract (sommario):
AbstractSuperconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10−4 to (5.9 ± 0.7) × 10−4. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.
38
Yun, Jonginn, Jaemin Park, Hyeongyu Jang, Jehyun Kim, Wonjin Jang, Younguk Song, Min-Kyun Cho et al. "Probing two-qubit capacitive interactions beyond bilinear regime using dual Hamiltonian parameter estimations". npj Quantum Information 9, n. 1 (29 marzo 2023). http://dx.doi.org/10.1038/s41534-023-00699-4.
Abstract (sommario):
AbstractWe report the simultaneous operation and two-qubit-coupling measurement of a pair of two-electron spin qubits, actively decoupled from quasi-static nuclear noise in a GaAs quadruple quantum dot array. Coherent Rabi oscillations of both qubits (decay time ≈2 μs; frequency few MHz) are achieved by continuously tuning their drive frequency using rapidly converging real-time Hamiltonian estimators. We observe strong two-qubit capacitive interaction (>190 MHz), combined with detuning pulses, inducing a state-conditional frequency shift. The two-qubit capacitive interaction is beyond the bilinear regime, consistent with recent theoretical predictions. We observe a high ratio (>16) between coherence and conditional phase-flip time, which supports the possibility of generating high-fidelity and fast quantum entanglement between encoded spin qubits using a simple capacitive interaction.
39
Asaad, Serwan, Christian Dickel, Nathan K. Langford, Stefano Poletto, Alessandro Bruno, Michiel Adriaan Rol, Duije Deurloo e Leonardo DiCarlo. "Independent, extensible control of same-frequency superconducting qubits by selective broadcasting". npj Quantum Information 2, n. 1 (23 agosto 2016). http://dx.doi.org/10.1038/npjqi.2016.29.
Abstract (sommario):
Abstract A critical ingredient for realising large-scale quantum information processors will be the ability to make economical use of qubit control hardware. We demonstrate an extensible strategy for reusing control hardware on same-frequency transmon qubits in a circuit QED chip with surface-code-compatible connectivity. A vector switch matrix enables selective broadcasting of input pulses to multiple transmons with individual tailoring of pulse quadratures for each, as required to minimise the effects of leakage on weakly anharmonic qubits. Using randomised benchmarking, we compare multiple broadcasting strategies that each pass the surface-code error threshold for single-qubit gates. In particular, we introduce a selective broadcasting control strategy using five pulse primitives, which allows independent, simultaneous Clifford gates on arbitrary numbers of qubits.
40
Niknam, Mohamad, Md Fahim F. Chowdhury, Md Mahadi Rajib, Walid Al Misba, Robert N. Schwartz, Kang L. Wang, Jayasimha Atulasimha e Louis-S. Bouchard. "Quantum control of spin qubits using nanomagnets". Communications Physics 5, n. 1 (12 novembre 2022). http://dx.doi.org/10.1038/s42005-022-01041-8.
Abstract (sommario):
AbstractSingle-qubit gates are essential components of a universal quantum computer. Without selective addressing of individual qubits, scalable implementation of quantum algorithms is extremely challenging. When the qubits are discrete points or regions on a lattice, selectively addressing magnetic spin qubits at the nanoscale remains a challenge due to the difficulty of localizing and confining a classical divergence-free field to a small volume of space. Herein we propose a technique for addressing spin qubits using voltage-control of nanoscale magnetism, exemplified by the use of voltage control of magnetic anisotropy. We show that by tuning the frequency of the nanomagnet’s electric field drive to the Larmor frequency of the spins confined to a nanoscale volume, and by modulating the phase of the drive, single-qubit quantum gates with fidelities approaching those for fault-tolerant quantum computing can be implemented. Such single-qubit gate operations require only tens of femto-Joules per gate operation and have lossless, purely magnetic field control. Their physical realization is also straightforward using foundry manufacturing techniques.
41
Mirza, Ali Raza, e Adam Zaman Chaudhry. "Improving the estimation of environment parameters via a two-qubit scheme". Scientific Reports 14, n. 1 (21 marzo 2024). http://dx.doi.org/10.1038/s41598-024-57150-7.
Abstract (sommario):
AbstractWe demonstrate how using two qubits can drastically improve the estimation of environment parameters compared to using only a single qubit. The two qubits are coupled to a common harmonic oscillator environment, and the properties of the environment are imprinted upon the dynamics of the two qubits. The reduced density matrix of only one of these qubits contains a decoherence factor and an additional factor taking into account the indirect interaction induced between the qubits due to the interaction with their common environment. This additional factor can drastically improve the estimation of the environment parameters, as quantified by the quantum Fisher information. In particular, we investigate the estimation of the cutoff frequency, the coupling strength, and the temperature using our two-qubit scheme compared to simply using a single qubit. We find that the precision of the estimates can be improved by orders of magnitude.
42
Ohfuchi, Mari, e Shintaro Sato. "Remote cross-resonance gate between superconducting fixed-frequency qubits". Quantum Science and Technology, 16 aprile 2024. http://dx.doi.org/10.1088/2058-9565/ad3f47.
Abstract (sommario):
Abstract High-fidelity quantum state transfer and remote entanglement between superconducting fixed-frequency qubits have not yet been realized. In this study, we propose an alternative remote cross-resonance gate. Considering multiple modes of a superconducting coaxial cable connecting qubits, we must find conditions under which the cross-resonance gate operates with a certain accuracy even in the presence of qubit frequency shifts due to manufacturing errors. For 0.25- and 0.5-m cables, remote cross-resonance gates with a concurrence of > 99.9% in entanglement generation are obtained even with ±10-MHz frequency shifts. For a 1-m cable with a narrow mode spacing, a concurrence of 99.5% is achieved by reducing the coupling between the qubits and cable. The optimized echoed raised-cosine pulse duration is 150–400 ns, which is similar to the operation time of cross-resonance gates between neighboring qubits on a chip. The dissipation through the cable modes does not considerably affect the obtained results. Such high-precision quantum interconnects pave the way not only for scaling up quantum computer systems but also for nonlocal connections on a chip. 
43
Landa, Haggai, e Grégoire Misguich. "Nonlocal correlations in noisy multiqubit systems simulated using matrix product operators". SciPost Physics Core 6, n. 2 (1 maggio 2023). http://dx.doi.org/10.21468/scipostphyscore.6.2.037.
Abstract (sommario):
We introduce an open-source solver for the Lindblad master equation, based on matrix product states and matrix product operators. Using this solver we study the dynamics of tens of interacting qubits with different connectivities, focusing on a problem where an edge qubit is being continuously driven on resonance, which is a fundamental operation in quantum devices. Because of the driving, induced excitations propagate through the qubits until the system reaches a steady state due to the incoherent terms. We find that with alternating-frequency qubits whose interactions with their off-resonant neighbors appear weak, the tunneling excitations lead to large correlations between distant qubits in the system. Some two-qubit correlation functions are found to increase as a function of distance in the system (in contrast to the typical decay with distance), peaking on the two edge qubits farthest apart from each other.
44
Landig, A. J., J. V. Koski, P. Scarlino, C. Müller, J. C. Abadillo-Uriel, B. Kratochwil, C. Reichl et al. "Virtual-photon-mediated spin-qubit–transmon coupling". Nature Communications 10, n. 1 (6 novembre 2019). http://dx.doi.org/10.1038/s41467-019-13000-z.
Abstract (sommario):
Abstract Spin qubits and superconducting qubits are among the promising candidates for realizing a solid state quantum computer. For the implementation of a hybrid architecture which can profit from the advantages of either approach, a coherent link is necessary that integrates and controllably couples both qubit types on the same chip over a distance that is several orders of magnitude longer than the physical size of the spin qubit. We realize such a link with a frequency-tunable high impedance SQUID array resonator. The spin qubit is a resonant exchange qubit hosted in a GaAs triple quantum dot. It can be operated at zero magnetic field, allowing it to coexist with superconducting qubits on the same chip. We spectroscopically observe coherent interaction between the resonant exchange qubit and a transmon qubit in both resonant and dispersive regimes, where the interaction is mediated either by real or virtual resonator photons.
45
Brehm, Jan David, Alexander N. Poddubny, Alexander Stehli, Tim Wolz, Hannes Rotzinger e Alexey V. Ustinov. "Waveguide bandgap engineering with an array of superconducting qubits". npj Quantum Materials 6, n. 1 (4 febbraio 2021). http://dx.doi.org/10.1038/s41535-021-00310-z.
Abstract (sommario):
AbstractWaveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits toward a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.
46
Carroll, M., S. Rosenblatt, P. Jurcevic, I. Lauer e A. Kandala. "Dynamics of superconducting qubit relaxation times". npj Quantum Information 8, n. 1 (17 novembre 2022). http://dx.doi.org/10.1038/s41534-022-00643-y.
Abstract (sommario):
AbstractSuperconducting qubits are a leading candidate for quantum computing but display temporal fluctuations in their energy relaxation times T1. This introduces instabilities in multi-qubit device performance. Furthermore, autocorrelation in these time fluctuations introduces challenges for obtaining representative measures of T1 for process optimization and device screening. These T1 fluctuations are often attributed to time varying coupling of the qubit to defects, putative two level systems (TLSs). In this work, we develop a technique to probe the spectral and temporal dynamics of T1 in single junction transmons by repeated T1 measurements in the frequency vicinity of the bare qubit transition, via the AC-Stark effect. Across 10 qubits, we observe strong correlations between the mean T1 averaged over approximately nine months and a snapshot of an equally weighted T1 average over the Stark shifted frequency range. These observations are suggestive of an ergodic-like spectral diffusion of TLSs dominating T1, and offer a promising path to more rapid T1 characterization for device screening and process optimization.
47
Lingenfelter, Andrew, e Aashish A. Clerk. "Surpassing spectator qubits with photonic modes and continuous measurement for Heisenberg-limited noise mitigation". npj Quantum Information 9, n. 1 (11 agosto 2023). http://dx.doi.org/10.1038/s41534-023-00748-y.
Abstract (sommario):
AbstractNoise is an ever-present challenge to the creation and preservation of fragile quantum states. Recent work suggests that spatial noise correlations can be harnessed as a resource for noise mitigation via the use of spectator qubits to measure environmental noise. In this work we generalize this concept from spectator qubits to a spectator mode: a photonic mode which continuously measures spatially correlated classical dephasing noise and applies a continuous correction drive to frequency-tunable data qubits. Our analysis shows that by using many photon states, spectator modes can surpass many of the quantum measurement constraints that limit spectator qubit approaches. We also find that long-time data qubit dephasing can be arbitrarily suppressed, even for white noise dephasing. Further, using a squeezing (parametric) drive, the error in the spectator mode approach can exhibit Heisenberg-limited scaling in the number of photons used. We also show that spectator mode noise mitigation can be implemented completely autonomously using engineered dissipation. In this case no explicit measurement or processing of a classical measurement record is needed. Our work establishes spectator modes as a potentially powerful alternative to spectator qubits for noise mitigation.
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Klemt, Bernhard, Victor Elhomsy, Martin Nurizzo, Pierre Hamonic, Biel Martinez, Bruna Cardoso Paz, Cameron Spence et al. "Electrical manipulation of a single electron spin in CMOS using a micromagnet and spin-valley coupling". npj Quantum Information 9, n. 1 (23 ottobre 2023). http://dx.doi.org/10.1038/s41534-023-00776-8.
Abstract (sommario):
AbstractFor semiconductor spin qubits, complementary-metal-oxide-semiconductor (CMOS) technology is a promising candidate for reliable and scalable fabrication. Making the direct leap from academic fabrication to qubits fully fabricated by industrial CMOS standards is difficult without intermediate solutions. With a flexible back-end-of-line (BEOL), functionalities such as micromagnets or superconducting circuits can be added in a post-CMOS process to study the physics of these devices or achieve proofs-of-concept. Once the process is established, it can be incorporated in the foundry-compatible process flow. Here, we study a single electron spin qubit in a CMOS device with a micromagnet integrated in the flexible BEOL. We exploit the synthetic spin orbit coupling (SOC) to control the qubit via electric fields and we investigate the spin-valley physics in the presence of SOC where we show an enhancement of the Rabi frequency at the spin-valley hotspot. Finally, we probe the high frequency noise in the system using dynamical decoupling pulse sequences and demonstrate that charge noise dominates the qubit decoherence in this range.
49
Wang Ning, Wang Bao-Chuan e Guo Guo-Ping. "New progress in silicon-based semiconductor quantum computation". Acta Physica Sinica, 2022, 0. http://dx.doi.org/10.7498/aps.71.20221900.
Abstract (sommario):
Spin qubits in silicon-based semiconductor quantum dots have become one of the prominent candidates for realizing fault-tolerant quantum computing due to their long coherence time, good controllability, and compatibility with modern advanced integrated circuit manufacturing processes. In recent years, thanks to the remarkable progress made in silicon-based materials, structure of quantum dot and its fabrication process, and qubit manipulation technology, high-fidelity state preparation and readout, single- and two-qubit gates have been demonstrated for silicon spin qubits. The control fidelities for single- and two-qubit gates all exceed 99%—fault tolerance threshold required by the surface code known for its exceptionally high tolerance to errors. In this paper, we briefly introduce the basic concepts of silicon-based semiconductor quantum dots, discuss the state-of-art technologies used to improve the fidelities of single- and two-qubit gates, and finally highlight the research directions that need to be focused on.<br />The paper is organized as follows. Firstly, we introduce three major types of quantum dots (QD) devices fabricated on different silicon-based substrate, including Si/SiGe heterojunction and Si/SiO<sub>2</sub>. The spin degree of electron or nuclear hosted in QD can be encoded to spin qubits. Electron spin qubit can be thermal initialized to ground state utilizing electron reservoirs and read out by spin-charge conversion mechanism energy-selective readout (Elzerman readout) with reservoirs or Pauli spin blockade (PSB) needless for a reservoir. Additionally, high fidelity single-shot readout has been demonstrated using radio-frequency gate reflectometry combined with PSB, which has unique advantages in large-scale qubit array. To coherent control the spin qubits, electron dipole renounce (ESR) or electron dipole spin resonance (EDSR) for electron and nuclear magnetic resonance (NMR) for nuclear are introduced. With help of isotope purification greatly improving the dephasing time of qubit and fast single-qubit manipulation based on EDSR, fidelity above 99.9 percent can be reached. For the two-qubit gates based on exchange interaction between electron spins, the strength of interaction <em>J</em> combined with Zeeman energy difference Δ<em>E</em><sub><em>Z</em></sub> determines the energy levels of system, which lead to the different two-qubit gates, such as controlled-Z (CZ), controlled-Rotation (CROT) and the square root of the SWAP gate ($\sqrt{\text { SWAP }}$) gates. In order to improve the fidelity of two-qubit gates, a series of key technologies are used in the experiments, not only isotope purification but also symmetry operation, careful Hamiltonian engineering and gate set tomography. Fidelity of two-qubit gates exceeding 99 percent has been demonstrated for electron spin qubits in Si/SiGe quantum dots and nuclear spin qubits in donors. These progresses have pushed the silicon-based spin qubits platform to constitute a major stepping stone towards fault-tolerant quantum computation. Finally, we discuss the next step for spin qubits, that is, how to effectively expand the number of qubits and there are still many problems to be explored and solved.
50
Shan, Zheng, Xuelian Gou, Huihui Sun, Shuya Wang, Jiandong Shang e Lin Han. "O-terminated interface for thickness-insensitive transport properties of aluminum oxide Josephson junctions". Scientific Reports 12, n. 1 (12 luglio 2022). http://dx.doi.org/10.1038/s41598-022-16126-1.
Abstract (sommario):
AbstractAlumina Josephson junction has demonstrated a tremendous potential to realize superconducting qubits. Further progress towards scalable superconducting qubits urgently needs to be guided by novel analysis mechanisms or methods to reduce the thickness sensitivity of the junction critical current to the tunnel barrier. Here, it is first revealed that the termination mode of AlOx interface plays a crucial role in the uniformity of critical current, and we demonstrate that the O-terminated interface has the lowest resistance sensitivity to thickness. More impressively, we developed atomically structured three-dimensional models and calculated their transport properties using a combination of quantum ballistic transport theory with first-principles DFT and NEGF to examine the effects of the Al2O3 termination mode and thickness variations. This work clarifies that O-terminated interface can effectively improve the resistance uniformity of Josephson junction, offering useful guidance for increasing the yield of fixed-frequency multi-qubit quantum chips which require tight control on qubit frequency.