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1
CAO, WEN-ZHEN, LI-JIE TIAN, HUI-JUAN JIANG, and CHONG LI. "SINGLE QUBIT MANIPULATION IN HETERONUCLEAR DIATOMIC MOLECULAR SYSTEM." International Journal of Quantum Information 06, no. 06 (2008): 1223–30. http://dx.doi.org/10.1142/s0219749908004390.
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We propose a scenario to realize quantum computers utilizing heteronuclear diatomic rovibrational states as qubits. We focused on rovibrational qubits created by simple transform limited infrared laser pulse instead of using chirped pulse. Numerical calculations show that single qubit gate operation in the electronic ground state of LiH molecule can be obtained. We also discuss the effect of temperature on the initially rotational states, and a suitable experiment condition is indicated.
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Gidney, Craig, Michael Newman, and Matt McEwen. "Benchmarking the Planar Honeycomb Code." Quantum 6 (September 21, 2022): 813. http://dx.doi.org/10.22331/q-2022-09-21-813.
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We improve the planar honeycomb code by describing boundaries that need no additional physical connectivity, and by optimizing the shape of the qubit patch. We then benchmark the code using Monte Carlo sampling to estimate logical error rates and derive metrics including thresholds, lambdas, and teraquop qubit counts. We determine that the planar honeycomb code can create a logical qubit with one-in-a-trillion logical error rates using 7000 physical qubits at a 0.1% gate-level error rate (or 900 physical qubits given native two-qubit parity measurements). Our results cement the honeycomb code as a promising candidate for two-dimensional qubit architectures with sparse connectivity.
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Xue, Xiao, Maximilian Russ, Nodar Samkharadze, et al. "Quantum logic with spin qubits crossing the surface code threshold." Nature 601, no. 7893 (2022): 343–47. http://dx.doi.org/10.1038/s41586-021-04273-w.
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AbstractHigh-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault tolerance—the ability to correct errors faster than they occur1. The central requirement for fault tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the approximately 1% error threshold of the well-known surface code2,3. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling, as they can leverage advanced semiconductor technology4. Here we report a spin-based quantum processor in silicon with single-qubit and two-qubit gate fidelities, all of which are above 99.5%, extracted from gate-set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighbouring qubit. Using this high-fidelity gate set, we execute the demanding task of calculating molecular ground-state energies using a variational quantum eigensolver algorithm5. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault tolerance and to possible applications in the era of noisy intermediate-scale quantum devices.
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Yirka, Justin, and Yiğit Subaşı. "Qubit-efficient entanglement spectroscopy using qubit resets." Quantum 5 (September 2, 2021): 535. http://dx.doi.org/10.22331/q-2021-09-02-535.
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One strategy to fit larger problems on NISQ devices is to exploit a tradeoff between circuit width and circuit depth. Unfortunately, this tradeoff still limits the size of tractable problems since the increased depth is often not realizable before noise dominates. Here, we develop qubit-efficient quantum algorithms for entanglement spectroscopy which avoid this tradeoff. In particular, we develop algorithms for computing the trace of the n-th power of the density operator of a quantum system, Tr(ρn), (related to the Rényi entropy of order n) that use fewer qubits than any previous efficient algorithm while achieving similar performance in the presence of noise, thus enabling spectroscopy of larger quantum systems on NISQ devices. Our algorithms, which require a number of qubits independent of n, are variants of previous algorithms with width proportional to n, an asymptotic difference. The crucial ingredient in these new algorithms is the ability to measure and reinitialize subsets of qubits in the course of the computation, allowing us to reuse qubits and increase the circuit depth without suffering the usual noisy consequences. We also introduce the notion of effective circuit depth as a generalization of standard circuit depth suitable for circuits with qubit resets. This tool helps explain the noise-resilience of our qubit-efficient algorithms and should aid in designing future algorithms. We perform numerical simulations to compare our algorithms to the original variants and show they perform similarly when subjected to noise. Additionally, we experimentally implement one of our qubit-efficient algorithms on the Honeywell System Model H0, estimating Tr(ρn) for larger n than possible with previous algorithms.
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Yamamoto, Satoru, Shigeaki Nakazawa, Kenji Sugisaki, et al. "Adiabatic quantum computing with spin qubits hosted by molecules." Physical Chemistry Chemical Physics 17, no. 4 (2015): 2742–49. http://dx.doi.org/10.1039/c4cp04744c.
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Moreno-Pineda, Eufemio, Clément Godfrin, Franck Balestro, Wolfgang Wernsdorfer, and Mario Ruben. "Molecular spin qudits for quantum algorithms." Chemical Society Reviews 47, no. 2 (2018): 501–13. http://dx.doi.org/10.1039/c5cs00933b.
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Molecules are promising building blocks for Quantum information processing. Herein we describe how a molecular multilevel nuclear spin qubit (or qudit, where d = 4), known as TbPc<sub>2</sub>, showing all necessary requirements to perform as a molecular hardware platform with a first generation of molecular devices enabling even quantum algorithm operations.
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Tahan, Charles. "Opinion: Democratizing Spin Qubits." Quantum 5 (November 18, 2021): 584. http://dx.doi.org/10.22331/q-2021-11-18-584.
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I've been building Powerpoint-based quantum computers with electron spins in silicon for 20 years. Unfortunately, real-life-based quantum dot quantum computers are harder to implement. Materials, fabrication, and control challenges still impede progress. The way to accelerate discovery is to make and measure more qubits. Here I discuss separating the qubit realization and testing circuitry from the materials science and on-chip fabrication that will ultimately be necessary. This approach should allow us, in the shorter term, to characterize wafers non-invasively for their qubit-relevant properties, to make small qubit systems on various different materials with little extra cost, and even to test spin-qubit to superconducting cavity entanglement protocols where the best possible cavity quality is preserved. Such a testbed can advance the materials science of semiconductor quantum information devices and enable small quantum computers. This article may also be useful as a light and light-hearted introduction to quantum dot spin qubits.
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Johnson, Alexander I., Fhokrul Islam, C. M. Canali, and Mark R. Pederson. "A multiferroic molecular magnetic qubit." Journal of Chemical Physics 151, no. 17 (2019): 174105. http://dx.doi.org/10.1063/1.5127956.
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Lao, Lingling, Alexander Korotkov, Zhang Jiang, Wojciech Mruczkiewicz, Thomas E. O'Brien, and Dan E. Browne. "Software mitigation of coherent two-qubit gate errors." Quantum Science and Technology 7, no. 2 (2022): 025021. http://dx.doi.org/10.1088/2058-9565/ac57f1.
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Abstract Two-qubit gates are important components of quantum computing. However, unwanted interactions between qubits (so-called parasitic gates) can be particularly problematic and degrade the performance of quantum applications. In this work, we present two software methods to mitigate parasitic two-qubit gate errors. The first approach is built upon the Cartan’s KAK decomposition and keeps the original unitary decomposition for the error-free native two-qubit gate. It counteracts a parasitic two-qubit gate by only applying single-qubit rotations and therefore has no two-qubit gate overhead. We show the optimal choice of single-qubit mitigation gates. The second approach applies a numerical optimisation algorithm to re-compile a target unitary into the error-parasitic two-qubit gate plus single-qubit gates. We demonstrate these approaches on the CPhase-parasitic iSWAP-like gates. The KAK-based approach helps decrease unitary infidelity by a factor of 3 compared to the noisy implementation without error mitigation. When arbitrary single-qubit rotations are allowed, recompilation could completely mitigate the effect of parasitic errors but may require more native gates than the KAK-based approach. We also compare their average gate fidelity under realistic noise models, including relaxation and depolarising errors. Numerical results suggest that different approaches are advantageous in different error regimes, providing error mitigation guidance for near-term quantum computers.
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Abu-Nada, Ali. "Quantum computing simulation of the hydrogen molecular ground-state energies with limited resources." Open Physics 19, no. 1 (2021): 628–33. http://dx.doi.org/10.1515/phys-2021-0071.
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Abstract In this article, the hydrogen molecular ground-state energies using our algorithm based on quantum variational principle are calculated. They are calculated through a simulator since the system of the present study (i.e., the hydrogen molecule) is relatively small and hence the ground-state energies for this molecule are efficiently classically simulable using a simulator. Complete details of this algorithm are elucidated. For this, a full description on the fermions–qubits and the molecular Hamiltonian–qubit Hamiltonian transformations, is given. The authors search for qubit system parameters ( θ 0 {\theta }_{0} and θ 1 {\theta }_{1} ) that yield the minimum energies for the system and also study the ground state energies as a function of the molecular bond length. Proposed circuit is humble and does not include many parameters compared with that of Kandala et al., the authors control only two parameters ( θ 0 {\theta }_{0} and θ 1 {\theta }_{1} ).
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Simoni, Mario, Giovanni Amedeo Cirillo, Giovanna Turvani, Mariagrazia Graziano, and Maurizio Zamboni. "Towards Compact Modeling of Noisy Quantum Computers: A Molecular-Spin-Qubit Case of Study." ACM Journal on Emerging Technologies in Computing Systems 18, no. 1 (2022): 1–26. http://dx.doi.org/10.1145/3474223.
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Classical simulation of Noisy Intermediate Scale Quantum computers is a crucial task for testing the expected performance of real hardware. The standard approach, based on solving Schrödinger and Lindblad equations, is demanding when scaling the number of qubits in terms of both execution time and memory. In this article, attempts in defining compact models for the simulation of quantum hardware are proposed, ensuring results close to those obtained with standard formalism. Molecular Nuclear Magnetic Resonance quantum hardware is the target technology, where three non-ideality phenomena—common to other quantum technologies—are taken into account: decoherence, off-resonance qubit evolution, and undesired qubit-qubit residual interaction. A model for each non-ideality phenomenon is embedded into a MATLAB simulation infrastructure of noisy quantum computers. The accuracy of the models is tested on a benchmark of quantum circuits, in the expected operating ranges of quantum hardware. The corresponding outcomes are compared with those obtained via numeric integration of the Schrödinger equation and the Qiskit’s QASMSimulator. The achieved results give evidence that this work is a step forward towards the definition of compact models able to provide fast results close to those obtained with the traditional physical simulation strategies, thus paving the way for their integration into a classical simulator of quantum computers.
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Chernega, Vladimir N., and Vladimir I. Man’ko. "Qubit portrait of qudit states and Bell inequalities." Journal of Russian Laser Research 28, no. 2 (2007): 103–24. http://dx.doi.org/10.1007/s10946-007-0005-8.
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Groszkowski, Peter, and Jens Koch. "Scqubits: a Python package for superconducting qubits." Quantum 5 (November 17, 2021): 583. http://dx.doi.org/10.22331/q-2021-11-17-583.
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scqubits is an open-source Python package for simulating and analyzing superconducting circuits. It provides convenient routines to obtain energy spectra of common superconducting qubits, such as the transmon, fluxonium, flux, cos(2ϕ) and the 0-π qubit. scqubits also features a number of options for visualizing the computed spectral data, including plots of energy levels as a function of external parameters, display of matrix elements of various operators as well as means to easily plot qubit wavefunctions. Many of these tools are not limited to single qubits, but extend to composite Hilbert spaces consisting of coupled superconducting qubits and harmonic (or weakly anharmonic) modes. The library provides an extensive suite of methods for estimating qubit coherence times due to a variety of commonly considered noise channels. While all functionality of scqubits can be accessed programatically, the package also implements GUI-like widgets that, with a few clicks can help users both create relevant Python objects, as well as explore their properties through various plots. When applicable, the library harnesses the computing power of multiple cores via multiprocessing. scqubits further exposes a direct interface to the Quantum Toolbox in Python (QuTiP) package, allowing the user to efficiently leverage QuTiP's proven capabilities for simulating time evolution.
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Horiuchi, Noriaki. "Flying qubit carrying a spin qubit." Nature Photonics 7, no. 4 (2013): 336. http://dx.doi.org/10.1038/nphoton.2013.78.
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Drahi, David, Demid V. Sychev, Khurram K. Pirov, et al. "Entangled resource for interfacing single- and dual-rail optical qubits." Quantum 5 (March 23, 2021): 416. http://dx.doi.org/10.22331/q-2021-03-23-416.
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Today's most widely used method of encoding quantum information in optical qubits is the dual-rail basis, often carried out through the polarisation of a single photon. On the other hand, many stationary carriers of quantum information – such as atoms – couple to light via the single-rail encoding in which the qubit is encoded in the number of photons. As such, interconversion between the two encodings is paramount in order to achieve cohesive quantum networks. In this paper, we demonstrate this by generating an entangled resource between the two encodings and using it to teleport a dual-rail qubit onto its single-rail counterpart. This work completes the set of tools necessary for the interconversion between the three primary encodings of the qubit in the optical field: single-rail, dual-rail and continuous-variable.
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Paini, Marco, Amir Kalev, Dan Padilha, and Brendan Ruck. "Estimating expectation values using approximate quantum states." Quantum 5 (March 16, 2021): 413. http://dx.doi.org/10.22331/q-2021-03-16-413.
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We introduce an approximate description of an N-qubit state, which contains sufficient information to estimate the expectation value of any observable to a precision that is upper bounded by the ratio of a suitably-defined seminorm of the observable to the square root of the number of the system's identical preparations M, with no explicit dependence on N. We describe an operational procedure for constructing the approximate description of the state that requires, besides the quantum state preparation, only single-qubit rotations followed by single-qubit measurements. We show that following this procedure, the cardinality of the resulting description of the state grows as 3MN. We test the proposed method on Rigetti's quantum processor unit with 12, 16 and 25 qubits for random states and random observables, and find an excellent agreement with the theory, despite experimental errors.
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Labib, Farrokh. "Stabilizer rank and higher-order Fourier analysis." Quantum 6 (February 9, 2022): 645. http://dx.doi.org/10.22331/q-2022-02-09-645.
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We establish a link between stabilizer states, stabilizer rank, and higher-order Fourier analysis – a still-developing area of mathematics that grew out of Gowers's celebrated Fourier-analytic proof of Szemerédi's theorem \cite{gowers1998new}. We observe that n-qudit stabilizer states are so-called nonclassical quadratic phase functions (defined on affine subspaces of Fpn where p is the dimension of the qudit) which are fundamental objects in higher-order Fourier analysis. This allows us to import tools from this theory to analyze the stabilizer rank of quantum states. Quite recently, in \cite{peleg2021lower} it was shown that the n-qubit magic state has stabilizer rank &#x03A9;(n). Here we show that the qudit analog of the n-qubit magic state has stabilizer rank &#x03A9;(n), generalizing their result to qudits of any prime dimension. Our proof techniques use explicitly tools from higher-order Fourier analysis. We believe this example motivates the further exploration of applications of higher-order Fourier analysis in quantum information theory.
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Hastings, Matthew B., and Jeongwan Haah. "Dynamically Generated Logical Qubits." Quantum 5 (October 19, 2021): 564. http://dx.doi.org/10.22331/q-2021-10-19-564.
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We present a quantum error correcting code with dynamically generated logical qubits. When viewed as a subsystem code, the code has no logical qubits. Nevertheless, our measurement patterns generate logical qubits, allowing the code to act as a fault-tolerant quantum memory. Our particular code gives a model very similar to the two-dimensional toric code, but each measurement is a two-qubit Pauli measurement.
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Bravyi, Sergey, Ruslan Shaydulin, Shaohan Hu, and Dmitri Maslov. "Clifford Circuit Optimization with Templates and Symbolic Pauli Gates." Quantum 5 (November 16, 2021): 580. http://dx.doi.org/10.22331/q-2021-11-16-580.
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The Clifford group is a finite subgroup of the unitary group generated by the Hadamard, the CNOT, and the Phase gates. This group plays a prominent role in quantum error correction, randomized benchmarking protocols, and the study of entanglement. Here we consider the problem of finding a short quantum circuit implementing a given Clifford group element. Our methods aim to minimize the entangling gate count assuming all-to-all qubit connectivity. First, we consider circuit optimization based on template matching and design Clifford-specific templates that leverage the ability to factor out Pauli and SWAP gates. Second, we introduce a symbolic peephole optimization method. It works by projecting the full circuit onto a small subset of qubits and optimally recompiling the projected subcircuit via dynamic programming. CNOT gates coupling the chosen subset of qubits with the remaining qubits are expressed using symbolic Pauli gates. Software implementation of these methods finds circuits that are only 0.2% away from optimal for 6 qubits and reduces the two-qubit gate count in circuits with up to 64 qubits by 64.7% on average, compared with the Aaronson-Gottesman canonical form.
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McKemmish, Laura K., David J. Kedziora, Graham R. White, Noel S. Hush, and Jeffrey R. Reimers. "Frequency-based Quantum Computers from a Chemist's Perspective." Australian Journal of Chemistry 65, no. 5 (2012): 512. http://dx.doi.org/10.1071/ch12053.
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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.
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Pal, Amit Kumar, and Indrani Bose. "Entanglement in a molecular three-qubit system." Journal of Physics: Condensed Matter 22, no. 1 (2009): 016004. http://dx.doi.org/10.1088/0953-8984/22/1/016004.
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Picó-Cortés, Jordi, and Gloria Platero. "Dynamical second-order noise sweetspots in resonantly driven spin qubits." Quantum 5 (December 23, 2021): 607. http://dx.doi.org/10.22331/q-2021-12-23-607.
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Quantum dot-based quantum computation employs extensively the exchange interaction between nearby electronic spins in order to manipulate and couple different qubits. The exchange interaction, however, couples the qubit states to charge noise, which reduces the fidelity of the quantum gates that employ it. The effect of charge noise can be mitigated by working at noise sweetspots in which the sensitivity to charge variations is reduced. In this work we study the response to charge noise of a double quantum dot based qubit in the presence of ac gates, with arbitrary driving amplitudes, applied either to the dot levels or to the tunneling barrier. Tuning with an ac driving allows to manipulate the sign and strength of the exchange interaction as well as its coupling to environmental electric noise. Moreover, we show the possibility of inducing a second-order sweetspot in the resonant spin-triplet qubit in which the dephasing time is significantly increased.
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Plachta, Stephen Z. D., Markus Hiekkamäki, Abuzer Yakaryılmaz, and Robert Fickler. "Quantum advantage using high-dimensional twisted photons as quantum finite automata." Quantum 6 (June 30, 2022): 752. http://dx.doi.org/10.22331/q-2022-06-30-752.
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Quantum finite automata (QFA) are basic computational devices that make binary decisions using quantum operations. They are known to be exponentially memory efficient compared to their classical counterparts. Here, we demonstrate an experimental implementation of multi-qubit QFAs using the orbital angular momentum (OAM) of single photons. We implement different high-dimensional QFAs encoded on a single photon, where multiple qubits operate in parallel without the need for complicated multi-partite operations. Using two to eight OAM quantum states to implement up to four parallel qubits, we show that a high-dimensional QFA is able to detect the prime numbers 5 and 11 while outperforming classical finite automata in terms of the required memory. Our work benefits from the ease of encoding, manipulating, and deciphering multi-qubit states encoded in the OAM degree of freedom of single photons, demonstrating the advantages structured photons provide for complex quantum information tasks.
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Hussain, Riaz, Giuseppe Allodi, Alessandro Chiesa, et al. "Coherent Manipulation of a Molecular Ln-Based Nuclear Qudit Coupled to an Electron Qubit." Journal of the American Chemical Society 140, no. 31 (2018): 9814–18. http://dx.doi.org/10.1021/jacs.8b05934.
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Musfeldt, Janice L., Zhenxian Liu, Diego López-Alcalá, et al. "Vibronic Relaxation Pathways in Molecular Spin Qubit Na9[Ho(W5O18)2]·35H2O under Pressure." Magnetochemistry 9, no. 2 (2023): 53. http://dx.doi.org/10.3390/magnetochemistry9020053.
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In order to explore how spectral sparsity and vibronic decoherence pathways can be controlled in a model qubit system with atomic clock transitions, we combined diamond anvil cell techniques with synchrotron-based far infrared spectroscopy and first-principles calculations to reveal the vibrational response of Na9[Ho(W5O18)2]·35H2O under compression. Because the hole in the phonon density of states acts to reduce the overlap between the phonons and f manifold excitations in this system, we postulated that pressure might move the HoO4 rocking, bending, and asymmetric stretching modes that couple with the MJ = ±5, ±2, and ±7 levels out of resonance, reducing their interactions and minimizing decoherence processes, while a potentially beneficial strategy for some molecular qubits, pressure slightly hardens the phonons in Na9[Ho(W5O18)2]·35H2O and systematically fills in the transparency window in the phonon response. The net result is that the vibrational spectrum becomes less sparse and the overlap with the various MJ levels of the Ho3+ ion actually increases. These findings suggest that negative pressure, achieved using chemical means or elongational strain, could further open the transparency window in this rare earth-containing spin qubit system, thus paving the way for the use of device surfaces and interface elongational/compressive strains to better manage decoherence pathways.
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Koiller, Belita, Xuedong Hu, Rodrigo B. Capaz, Adriano S. Martins, and Sankar Das Sarma. "Silicon-based spin and charge quantum computation." Anais da Academia Brasileira de Ciências 77, no. 2 (2005): 201–22. http://dx.doi.org/10.1590/s0001-37652005000200002.
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Silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals due to the relatively long spin coherence times. For these spin qubits, donor electron charge manipulation by external gates is a key ingredient for control and read-out of single-qubit operations, while shallow donor exchange gates are frequently invoked to perform two-qubit operations. More recently, charge qubits based on tunnel coupling in P+2 substitutional molecular ions in Si have also been proposed. We discuss the feasibility of the building blocks involved in shallow donor quantum computation in silicon, taking into account the peculiarities of silicon electronic structure, in particular the six degenerate states at the conduction band edge. We show that quantum interference among these states does not significantly affect operations involving a single donor, but leads to fast oscillations in electron exchange coupling and on tunnel-coupling strength when the donor pair relative position is changed on a lattice-parameter scale. These studies illustrate the considerable potential as well as the tremendous challenges posed by donor spin and charge as candidates for qubits in silicon.
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Altintas, Azmi Ali, Fatih Ozaydin, Cihan Bayindir, and Veysel Bayrakci. "Prisoners’ Dilemma in a Spatially Separated System Based on Spin–Photon Interactions." Photonics 9, no. 9 (2022): 617. http://dx.doi.org/10.3390/photonics9090617.
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Having access to ideal quantum mechanical resources, the prisoners’ dilemma can be ceased Here, we propose a distributed quantum circuit to allow spatially separated prisoners to play the prisoners’ dilemma game. Decomposing the circuit into controlled-Z and single-qubit gates only, we design a corresponding spin–photon-interaction-based physical setup within the reach of current technology. In our setup, spins are considered to be the players’ logical qubits, which can be realized via nitrogen-vacancy centers in diamond or quantum dots coupled to optical cavities, and the game is played via a flying photon realizing logic operations by interacting with the spatially separated optical cavities to which the spin qubits are coupled. We also analyze the effect of the imperfect realization of two-qubit gates on the game, and discuss the revival of the dilemma and the emergence of new Nash equilibria.
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Hilaire, Paul, Edwin Barnes, and Sophia E. Economou. "Resource requirements for efficient quantum communication using all-photonic graph states generated from a few matter qubits." Quantum 5 (February 15, 2021): 397. http://dx.doi.org/10.22331/q-2021-02-15-397.
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Quantum communication technologies show great promise for applications ranging from the secure transmission of secret messages to distributed quantum computing. Due to fiber losses, long-distance quantum communication requires the use of quantum repeaters, for which there exist quantum memory-based schemes and all-photonic schemes. While all-photonic approaches based on graph states generated from linear optics avoid coherence time issues associated with memories, they outperform repeater-less protocols only at the expense of a prohibitively large overhead in resources. Here, we consider using matter qubits to produce the photonic graph states and analyze in detail the trade-off between resources and performance, as characterized by the achievable secret key rate per matter qubit. We show that fast two-qubit entangling gates between matter qubits and high photon collection and detection efficiencies are the main ingredients needed for the all-photonic protocol to outperform both repeater-less and memory-based schemes.
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Sabín, Carlos. "Digital Quantum Simulation of Linear and Nonlinear Optical Elements." Quantum Reports 2, no. 1 (2020): 208–20. http://dx.doi.org/10.3390/quantum2010013.
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We provide a recipe for the digitalization of linear and nonlinear quantum optics in networks of superconducting qubits. By combining digital techniques with boson-qubit mappings, we address relevant problems that are typically considered in analog simulators, such as the dynamical Casimir effect or molecular force fields, including nonlinearities. In this way, the benefits of digitalization are extended in principle to a new realm of physical problems. We present preliminary examples launched in IBM Q 5 Tenerife.
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Grzesiak, Nikodem, Andrii Maksymov, Pradeep Niroula, and Yunseong Nam. "Efficient quantum programming using EASE gates on a trapped-ion quantum computer." Quantum 6 (January 27, 2022): 634. http://dx.doi.org/10.22331/q-2022-01-27-634.
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Parallel operations in conventional computing have proven to be an essential tool for efficient and practical computation, and the story is not different for quantum computing. Indeed, there exists a large body of works that study advantages of parallel implementations of quantum gates for efficient quantum circuit implementations. Here, we focus on the recently invented efficient, arbitrary, simultaneously entangling (EASE) gates, available on a trapped-ion quantum computer. Leveraging its flexibility in selecting arbitrary pairs of qubits to be coupled with any degrees of entanglement, all in parallel, we show an n-qubit Clifford circuit can be implemented using 6log(n) EASE gates, an n-qubit multiply-controlled NOT gate can be implemented using 3n/2 EASE gates, and an n-qubit permutation can be implemented using six EASE gates. We discuss their implications to near-term quantum chemistry simulations and the state of the art pattern matching algorithm. Given Clifford + multiply-controlled NOT gates form a universal gate set for quantum computing, our results imply efficient quantum computation by EASE gates, in general.
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Lowe, Angus, Matija Medvidović, Anthony Hayes, et al. "Fast quantum circuit cutting with randomized measurements." Quantum 7 (March 2, 2023): 934. http://dx.doi.org/10.22331/q-2023-03-02-934.
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We propose a new method to extend the size of a quantum computation beyond the number of physical qubits available on a single device. This is accomplished by randomly inserting measure-and-prepare channels to express the output state of a large circuit as a separable state across distinct devices. Our method employs randomized measurements, resulting in a sample overhead that is O&#x007E;(4k/&#x03B5;2), where &#x03B5; is the accuracy of the computation and k the number of parallel wires that are "cut" to obtain smaller sub-circuits. We also show an information-theoretic lower bound of &#x03A9;(2k/&#x03B5;2) for any comparable procedure. We use our techniques to show that circuits in the Quantum Approximate Optimization Algorithm (QAOA) with p entangling layers can be simulated by circuits on a fraction of the original number of qubits with an overhead that is roughly 2O(p&#x03BA;), where &#x03BA; is the size of a known balanced vertex separator of the graph which encodes the optimization problem. We obtain numerical evidence of practical speedups using our method applied to the QAOA, compared to prior work. Finally, we investigate the practical feasibility of applying the circuit cutting procedure to large-scale QAOA problems on clustered graphs by using a 30-qubit simulator to evaluate the variational energy of a 129-qubit problem as well as carry out a 62-qubit optimization.
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Ullah, Aman, José J. Baldoví, Alejandro Gaita-Ariño, and Eugenio Coronado. "Insights on the coupling between vibronically active molecular vibrations and lattice phonons in molecular nanomagnets." Dalton Transactions 50, no. 32 (2021): 11071–76. http://dx.doi.org/10.1039/d1dt01832a.
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Jing-Min, Hou, Tian Li-Jun, and Ge Mo-Lin. "Two-Qubit Quantum Logic Gate in Molecular Magnets." Chinese Physics Letters 22, no. 9 (2005): 2147–50. http://dx.doi.org/10.1088/0256-307x/22/9/002.
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Porfyrakis, Kyriakos. "(Invited) N@C60 and N@C70 for Quantum Information Processing: Beyond Qubits." ECS Meeting Abstracts MA2022-01, no. 11 (2022): 817. http://dx.doi.org/10.1149/ma2022-0111817mtgabs.
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Endohedral fullerenes such as N@C60, where a single atomic nitrogen is trapped inside the fullerene cage, have been proposed as qubit architectures due to the remarkably long relaxation times of their p-electron spins (T1 = 0.375 ms, T2 = 0.25 ms). Molecular quantum computers are still at the fringes of the field as recent developments have focused on other implementations such as superconducting qubits. However, molecular approaches present some advantages such as the ability to use chemical functionalization for scaling up qubit architectures. This, combined with continuous progress on miniaturization of electrodes via e-beam lithography and other techniques, means that molecular approaches will continue to be of interest. In this talk, I will review the field of fullerene-based quantum information processing. I will present progress on the synthesis, chemical functionalization and alignment of N@C60 and N@C70 in different matrices. Recently, we were able to align N@C60 and N@C70 derivatives in a liquid crystal matrix with ordering parameter Ozz = 0.61. With the aligned samples, we were able to achieve addressability of the available 4-electron spin levels in endohedral nitrogen by coherent manipulations. Furthermore, these functionalized molecules give rise to endohedral fullerene qudits: multi-level computational units alternative to the conventional 2-level qubits. Qudits offer a larger state space for encoding information and thus can offer enhancement of quantum algorithm efficiency. Indeed, we were able to demonstrate the first ever geometric phase using pulsed EPR; something that was first proposed over 30 years ago!
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Huerga, Daniel. "Variational Quantum Simulation of Valence-Bond Solids." Quantum 6 (December 13, 2022): 874. http://dx.doi.org/10.22331/q-2022-12-13-874.
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We introduce a hybrid quantum-classical variational algorithm to simulate ground-state phase diagrams of frustrated quantum spin models in the thermodynamic limit. The method is based on a cluster-Gutzwiller ansatz where the wave function of the cluster is provided by a parameterized quantum circuit whose key ingredient is a two-qubit real XY gate allowing to efficiently generate valence-bonds on nearest-neighbor qubits. Additional tunable single-qubit Z- and two-qubit ZZ-rotation gates allow the description of magnetically ordered and paramagnetic phases while restricting the variational optimization to the U(1) subspace. We benchmark the method against the J1&#x2212;J2 Heisenberg model on the square lattice and uncover its phase diagram, which hosts long-range ordered Neel and columnar anti-ferromagnetic phases, as well as an intermediate valence-bond solid phase characterized by a periodic pattern of 2×2 strongly-correlated plaquettes. Our results show that the convergence of the algorithm is guided by the onset of long-range order, opening a promising route to synthetically realize frustrated quantum magnets and their quantum phase transition to paramagnetic valence-bond solids with currently developed superconducting circuit devices.
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Issah, Ibrahim, Mohsin Habib, and Humeyra Caglayan. "Long-range qubit entanglement via rolled-up zero-index waveguide." Nanophotonics 10, no. 18 (2021): 4579–89. http://dx.doi.org/10.1515/nanoph-2021-0453.
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Abstract Preservation of an entangled state in a quantum system is one of the major goals in quantum technological applications. However, entanglement can be quickly lost into dissipation when the effective interaction among the qubits becomes smaller compared to the noise-injection from the environment. Thus, a medium that can sustain the entanglement of distantly spaced qubits is essential for practical implementations. This work introduces the fabrication of a rolled-up zero-index waveguide which can serve as a unique reservoir for the long-range qubit–qubit entanglement. We also present the numerical evaluation of the concurrence (entanglement measure) via Ansys Lumerical FDTD simulations using the parameters determined experimentally. The calculations demonstrate the feasibility and supremacy of the experimental method. We develop and fabricate this novel structure using cost-effective self-rolling techniques. The results of this study redefine the range of light-matter interactions and show the potential of the rolled-up zero-index waveguides for various classical and quantum applications such as quantum communication, quantum information processing, and superradiance.
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Mani, Tomoyasu. "Molecular qubits based on photogenerated spin-correlated radical pairs for quantum sensing." Chemical Physics Reviews 3, no. 2 (2022): 021301. http://dx.doi.org/10.1063/5.0084072.
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Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.
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Wang, Qingfeng, Ming Li, Christopher Monroe, and Yunseong Nam. "Resource-Optimized Fermionic Local-Hamiltonian Simulation on a Quantum Computer for Quantum Chemistry." Quantum 5 (July 26, 2021): 509. http://dx.doi.org/10.22331/q-2021-07-26-509.
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The ability to simulate a fermionic system on a quantum computer is expected to revolutionize chemical engineering, materials design, nuclear physics, to name a few. Thus, optimizing the simulation circuits is of significance in harnessing the power of quantum computers. Here, we address this problem in two aspects. In the fault-tolerant regime, we optimize the Rz and T gate counts along with the ancilla qubit counts required, assuming the use of a product-formula algorithm for implementation. We obtain a savings ratio of two in the gate counts and a savings ratio of eleven in the number of ancilla qubits required over the state of the art. In the pre-fault tolerant regime, we optimize the two-qubit gate counts, assuming the use of the variational quantum eigensolver (VQE) approach. Specific to the latter, we present a framework that enables bootstrapping the VQE progression towards the convergence of the ground-state energy of the fermionic system. This framework, based on perturbation theory, is capable of improving the energy estimate at each cycle of the VQE progression, by about a factor of three closer to the known ground-state energy compared to the standard VQE approach in the test-bed, classically-accessible system of the water molecule. The improved energy estimate in turn results in a commensurate level of savings of quantum resources, such as the number of qubits and quantum gates, required to be within a pre-specified tolerance from the known ground-state energy. We also explore a suite of generalized transformations of fermion to qubit operators and show that resource-requirement savings of up to more than 20%, in small instances, is possible.
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Yousefjani, Rozhin, and Abolfazl Bayat. "Parallel entangling gate operations and two-way quantum communication in spin chains." Quantum 5 (May 26, 2021): 460. http://dx.doi.org/10.22331/q-2021-05-26-460.
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The power of a quantum circuit is determined through the number of two-qubit entangling gates that can be performed within the coherence time of the system. In the absence of parallel quantum gate operations, this would make the quantum simulators limited to shallow circuits. Here, we propose a protocol to parallelize the implementation of two-qubit entangling gates between multiple users which are spatially separated, and use a commonly shared spin chain data-bus. Our protocol works through inducing effective interaction between each pair of qubits without disturbing the others, therefore, it increases the rate of gate operations without creating crosstalk. This is achieved by tuning the Hamiltonian parameters appropriately, described in the form of two different strategies. The tuning of the parameters makes different bilocalized eigenstates responsible for the realization of the entangling gates between different pairs of distant qubits. Remarkably, the performance of our protocol is robust against increasing the length of the data-bus and the number of users. Moreover, we show that this protocol can tolerate various types of disorders and is applicable in the context of superconductor-based systems. The proposed protocol can serve for realizing two-way quantum communication.
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Ardavan, Arzhang, Alice M. Bowen, Antonio Fernandez, et al. "Engineering coherent interactions in molecular nanomagnet dimers." npj Quantum Information 1, no. 1 (2015). http://dx.doi.org/10.1038/npjqi.2015.12.
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AbstractProposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr7Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates.
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Vepsäläinen, Antti, Roni Winik, Amir H. Karamlou, et al. "Improving qubit coherence using closed-loop feedback." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-29287-4.
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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.
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Noiri, Akito, Kenta Takeda, Takashi Nakajima, et al. "A shuttling-based two-qubit logic gate for linking distant silicon quantum processors." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-33453-z.
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AbstractControl of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qubits via coherent spin shuttling, a key technology for linking distant silicon quantum processors. Coherent shuttling of a spin qubit enables efficient switching of the exchange coupling with an on/off ratio exceeding 1000, while preserving the spin coherence by 99.6% for the single shuttling between neighboring dots. With this shuttling-mode exchange control, we demonstrate a two-qubit controlled-phase gate with a fidelity of 93%, assessed via randomized benchmarking. Combination of our technique and a phase coherent shuttling of a qubit across a large quantum dot array will provide feasible path toward a quantum link between distant silicon quantum processors, a key requirement for large-scale quantum computation.
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Landig, A. J., J. V. Koski, P. Scarlino, et al. "Virtual-photon-mediated spin-qubit–transmon coupling." Nature Communications 10, no. 1 (2019). http://dx.doi.org/10.1038/s41467-019-13000-z.
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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.
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Chicco, Simone, Alessandro Chiesa, Giuseppe Allodi, et al. "Controlled coherent dynamics of [VO(TPP)], a prototype molecular nuclear qudit with an electronic ancilla." Chemical Science, 2021. http://dx.doi.org/10.1039/d1sc01358k.
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By a combined theoretical and broadband nuclear magnetic resonance study, we show that [VOTPP] is a coupled electronic qubit-nuclear qudit system suitable to implement qudit-based quantum error correction and quantum simulation algorithms.
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Jurcevic, Petar, and Luke C. G. Govia. "Effective qubit dephasing induced by spectator-qubit relaxation." Quantum Science and Technology, August 25, 2022. http://dx.doi.org/10.1088/2058-9565/ac8cad.
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Abstract In many leading architectures for quantum computing, it remains to be understood if we can equate single-qubit coherence times measured in isolation, to that of coherence times measured in multi-qubit devices. On a multi-qubit superconducting circuit platform we show an increase in the dephasing rate of a control qubit due to the spontaneous relaxation of spectator qubits coupled to the control qubit. We attribute this increased dephasing to random in time Z-phase kicks on the control qubit due to the interplay between spectator relaxation and the control-spectator ZZ- interaction. We measure the magnitude of this extra dephasing using Ramsey decay experiments, show how it can be corrected via dynamical decoupling pulse sequences, and demonstrate that randomized benchmarking is insensitive to the effect. Our experimental results are supported by a robust theoretical model that captures an arbitrary number of spectator qubits, and gives a simple, intuitive picture for the mechanism behind the enhanced dephasing.
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Yoneda, J., W. Huang, M. Feng, et al. "Coherent spin qubit transport in silicon." Nature Communications 12, no. 1 (2021). http://dx.doi.org/10.1038/s41467-021-24371-7.
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AbstractA fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.
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Hyyppä, Eric, Suman Kundu, Chun Fai Chan, et al. "Unimon qubit." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-34614-w.
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AbstractSuperconducting qubits seem promising for useful quantum computers, but the currently wide-spread qubit designs and techniques do not yet provide high enough performance. Here, we introduce a superconducting-qubit type, the unimon, which combines the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. In agreement with our quantum models, we measure the qubit frequency, ω01/(2π), and increased anharmonicity α/(2π) at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13 ns single-qubit gates on two qubits with (ω01, α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation seems to be dominated by dielectric losses. Thus, improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible useful quantum advantage with noisy systems.
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Place, Alexander P. M., Lila V. H. Rodgers, Pranav Mundada, et al. "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds." Nature Communications 12, no. 1 (2021). http://dx.doi.org/10.1038/s41467-021-22030-5.
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AbstractThe superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.
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Erhard, Alexander, Joel J. Wallman, Lukas Postler, et al. "Characterizing large-scale quantum computers via cycle benchmarking." Nature Communications 10, no. 1 (2019). http://dx.doi.org/10.1038/s41467-019-13068-7.
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AbstractQuantum computers promise to solve certain problems more efficiently than their digital counterparts. A major challenge towards practically useful quantum computing is characterizing and reducing the various errors that accumulate during an algorithm running on large-scale processors. Current characterization techniques are unable to adequately account for the exponentially large set of potential errors, including cross-talk and other correlated noise sources. Here we develop cycle benchmarking, a rigorous and practically scalable protocol for characterizing local and global errors across multi-qubit quantum processors. We experimentally demonstrate its practicality by quantifying such errors in non-entangling and entangling operations on an ion-trap quantum computer with up to 10 qubits, and total process fidelities for multi-qubit entangling gates ranging from $$99.6(1)\%$$99.6(1)% for 2 qubits to $$86(2)\%$$86(2)% for 10 qubits. Furthermore, cycle benchmarking data validates that the error rate per single-qubit gate and per two-qubit coupling does not increase with increasing system size.
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Tsai, Jeng-Yuan, Jinbo Pan, Hsin Lin, Arun Bansil, and Qimin Yan. "Antisite defect qubits in monolayer transition metal dichalcogenides." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-28133-x.
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AbstractBeing atomically thin and amenable to external controls, two-dimensional (2D) materials offer a new paradigm for the realization of patterned qubit fabrication and operation at room temperature for quantum information sciences applications. Here we show that the antisite defect in 2D transition metal dichalcogenides (TMDs) can provide a controllable solid-state spin qubit system. Using high-throughput atomistic simulations, we identify several neutral antisite defects in TMDs that lie deep in the bulk band gap and host a paramagnetic triplet ground state. Our in-depth analysis reveals the presence of optical transitions and triplet-singlet intersystem crossing processes for fingerprinting these defect qubits. As an illustrative example, we discuss the initialization and readout principles of an antisite qubit in WS2, which is expected to be stable against interlayer interactions in a multilayer structure for qubit isolation and protection in future qubit-based devices. Our study opens a new pathway for creating scalable, room-temperature spin qubits in 2D TMDs.