Journal articles: 'Quantum Gate Fidelity'

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Magesan, Easwar. "Depolarizing behavior of quantum channels in higher dimensions." Quantum Information and Computation 11, no. 5&6 (May 2011): 466–84. http://dx.doi.org/10.26421/qic11.5-6-8.

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The paper analyzes the behavior of quantum channels, particularly in large dimensions, by proving various properties of the quantum gate fidelity. Many of these properties are of independent interest in the theory of distance measures on quantum operations. A non-uniqueness result for the gate fidelity is proven, a consequence of which is the existence of non-depolarizing channels that produce a constant gate fidelity on pure states. Asymptotically, the gate fidelity associated with any quantum channel is shown to converge to that of a depolarizing channel. Methods for estimating the minimum of the gate fidelity are also presented.

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Li, Ran, and Frank Gaitan. "High-fidelity universal quantum gates." Quantum Information and Computation 10, no. 11&12 (November 2010): 936–46. http://dx.doi.org/10.26421/qic10.11-12-4.

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Twisted rapid passage is a type of non-adiabatic rapid passage that generates controllable quantum interference effects that were first observed experimentally in $2003$. It is shown that twisted rapid passage sweeps can be used to implement a universal set of quantum gates $\calGU$ that operate with high-fidelity. The gate set $\calGU$ consists of the Hadamard and NOT gates, together with variants of the phase, $\pi /8$, and controlled-phase gates. For each gate $g$ in $\calGU$, sweep parameter values are provided which simulations indicate will produce a unitary operation that approximates $g$ with error probability$P_{e} < 10^{-4}$. Note that \textit{all\/} gates in $\calGU$ are implemented using a \textit{single family\/} of control-field, and the error probability for each gate falls below the rough-and-ready estimate for the accuracy threshold $P_{a}\sim 10^{-4}$.

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Zhao, Jie, Kui Liu, Hao Jeng, Mile Gu, Jayne Thompson, Ping Koy Lam, and Syed M. Assad. "A high-fidelity heralded quantum squeezing gate." Nature Photonics 14, no. 5 (February 17, 2020): 306–9. http://dx.doi.org/10.1038/s41566-020-0592-2.

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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, no. 10 (November 1, 2021): 100301. http://dx.doi.org/10.1088/0256-307x/38/10/100301.

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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.

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Xue, Xiao, Maximilian Russ, Nodar Samkharadze, Brennan Undseth, Amir Sammak, Giordano Scappucci, and Lieven M. K. Vandersypen. "Quantum logic with spin qubits crossing the surface code threshold." Nature 601, no. 7893 (January 19, 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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Liu, Teng, Peng-Fei Lu, Bi-Ying Hu, Hao Wu, Qi-Feng Lao, Ji Bian, Yang Liu, Feng Zhu, and Le Luo. "Phonon-mediated many-body quantum entanglement and logic gates in ion traps." Acta Physica Sinica 71, no. 8 (2022): 1. http://dx.doi.org/10.7498/aps.71.20220360.

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The implementation of high-fidelity multi-ion entangled states and quantum gates are the basis for ion trap quantum computing. There are developed quantum gate experimental schemes for realizing multi-ion entanglement and quantum gate, such as Mølmer-Sørensen Gate and Cirac-Zoller Gate; In recent years, there are also ultrafast entanglement gates that operate outside the Lamb-Dicke regime by designing ultrafast pulse sequences. In this typical many-body quantum system, these entanglement gate schemes all couple the spin states between ions by driving the phonon energy level or motion state of the ion chain. To improve the fidelity of quantum gates, they all use modulated laser pulses or appropriately designed pulse sequences to decouple the multi-mode motion states. In this review, we summarize and analyze the essential aspects of the realization of these entanglement gate schemes from the theories and experiments, and we also reveal the basic physical process of realizing quantum gates through nonlinear interactions in non-equilibrium processes by driving ion chain motion states.

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Johnston, Nathaniel, and David W. Kribs. "Quantum gate fidelity in terms of Choi matrices." Journal of Physics A: Mathematical and Theoretical 44, no. 49 (November 18, 2011): 495303. http://dx.doi.org/10.1088/1751-8113/44/49/495303.

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Ni, Kang-Kuen, Till Rosenband, and David D. Grimes. "Dipolar exchange quantum logic gate with polar molecules." Chemical Science 9, no. 33 (2018): 6830–38. http://dx.doi.org/10.1039/c8sc02355g.

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Patel, Raj B., Joseph Ho, Franck Ferreyrol, Timothy C. Ralph, and Geoff J. Pryde. "A quantum Fredkin gate." Science Advances 2, no. 3 (March 2016): e1501531. http://dx.doi.org/10.1126/sciadv.1501531.

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Minimizing the resources required to build logic gates into useful processing circuits is key to realizing quantum computers. Although the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems have made more complex operations intractable. This is exemplified in the classical Fredkin (controlled-SWAP) gate for which, despite theoretical proposals, no quantum analog has been realized. By adding control to the SWAP unitary, we use photonic qubit logic to demonstrate the first quantum Fredkin gate, which promises many applications in quantum information and measurement. We implement example algorithms and generate the highest-fidelity three-photon Greenberger-Horne-Zeilinger states to date. The technique we use allows one to add a control operation to a black-box unitary, something that is impossible in the standard circuit model. Our experiment represents the first use of this technique to control a two-qubit operation and paves the way for larger controlled circuits to be realized efficiently.

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Royer, Baptiste, Arne L. Grimsmo, Nicolas Didier, and Alexandre Blais. "Fast and high-fidelity entangling gate through parametrically modulated longitudinal coupling." Quantum 1 (May 11, 2017): 11. http://dx.doi.org/10.22331/q-2017-05-11-11.

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We investigate an approach to universal quantum computation based on the modulation of longitudinal qubit-oscillator coupling. We show how to realize a controlled-phase gate by simultaneously modulating the longitudinal coupling of two qubits to a common oscillator mode. In contrast to the more familiar transversal qubit-oscillator coupling, the magnitude of the effective qubit-qubit interaction does not rely on a small perturbative parameter. As a result, this effective interaction strength can be made large, leading to short gate times and high gate fidelities. We moreover show how the gate infidelity can be exponentially suppressed with squeezing and how the entangling gate can be generalized to qubits coupled to separate oscillators. Our proposal can be realized in multiple physical platforms for quantum computing, including superconducting and spin qubits.

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Liu, Kui, Jiaming Li, Rongguo Yang, and Shuqin Zhai. "High-fidelity heralded quantum squeezing gate based on entanglement." Optics Express 28, no. 16 (July 24, 2020): 23628. http://dx.doi.org/10.1364/oe.398096.

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Rudnicki, Łukasz, Zbigniew Puchała, and Karol Zyczkowski. "Gauge invariant information concerning quantum channels." Quantum 2 (April 11, 2018): 60. http://dx.doi.org/10.22331/q-2018-04-11-60.

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Motivated by the gate set tomography we study quantum channels from the perspective of information which is invariant with respect to the gauge realized through similarity of matrices representing channel superoperators. We thus use the complex spectrum of the superoperator to provide necessary conditions relevant for complete positivity of qubit channels and to express various metrics such as average gate fidelity.

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TRINDADE, M. A. S., L. M. SILVA FILHO, L. C. SANTOS, M. GRAÇAS R. MARTINS, and J. D. M. VIANNA. "QUANTUM INFORMATION, THERMOFIELD DYNAMICS AND THERMALIZED BOSONIC OSCILLATOR." International Journal of Modern Physics B 27, no. 24 (September 11, 2013): 1350133. http://dx.doi.org/10.1142/s0217979213501336.

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We show through Thermofield Dynamics approach that the action of the thermalized quantum logic gate on the thermalized state is equivalent to thermalization of the state that arise from the application of the nonthermalized quantum logic gate. In particular, we study the effect of temperature on a mixed state associated to a system capable of implementing a controlled-NOT (CNOT) quantum logic gate. According to a proposal in the literature, a way of implementing such a logic gate is by using a representation of the qubit states as elements of the Fock space of a bosonic system. We consider such a proposal and use the Thermofield Dynamics to determine the thermalized qubit states. The temperature acts as a quantum noise on pure states, making them a statistical mixture. In this context, we analyze the fidelity as a function of the temperature and using the Mandel parameter, we determine temperature ranges for which the statistics of the system becomes subpoissonian, poissonian and superpoissonian. Finally, we calculate the Wigner function, allowing an analysis of the thermal state in phase space, and we obtain that the increase of temperature decreases nonclassical properties of the system. The temperature range where one has a subpoissonian statistics and high fidelity is determined.

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LIU, CHUAN-LONG, YAN-WEI WANG, and YI-ZHUANG ZHENG. "IMPLEMENTATION OF NON-LOCAL TOFFOLI GATE VIA CAVITY QUANTUM ELECTRODYNAMICS." International Journal of Quantum Information 07, no. 03 (April 2009): 669–80. http://dx.doi.org/10.1142/s0219749909003329.

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A scheme for realizing the non-local Toffoli gate among three spatially separated nodes through cavity quantum electrodynamics (C-QED) is presented. The scheme can obtain high fidelity in the current C-QED system. With entangled qubits as quantum channels, the operation is resistive to actual environment noise.

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Kawakami, Erika, Thibaut Jullien, Pasquale Scarlino, Daniel R. Ward, Donald E. Savage, Max G. Lagally, Viatcheslav V. Dobrovitski, et al. "Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet." Proceedings of the National Academy of Sciences 113, no. 42 (October 3, 2016): 11738–43. http://dx.doi.org/10.1073/pnas.1603251113.

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The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ∼99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ∼400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. This work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification.

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Iradat, R. D., M. A. Majidi, and R. S. Said. "Quantum Dynamics of Two Nitrogen-Vacancy Center Ensembles Coupled to a Driven Superconducting Quantum Circuit." Journal of Physics: Conference Series 2377, no. 1 (November 1, 2022): 012050. http://dx.doi.org/10.1088/1742-6596/2377/1/012050.

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We theoretically model and simulate the dynamics of a hybrid quantum system consisting of two non-local ensembles of nitrogen-vacancy center and a superconducting transmon qubit mediated by two transmission line resonators. We apply a time-dependent external field to enhance this system’s speed and fidelity to function as a controlled-phase gate. Our simulation result shows that a high-fidelity entangled state of two non-local NV spins is 92%. It is achievable under realistic parameter regimes within a timescale of 1.1 nanoseconds. Our result paves the way to improving potential quantum computing and sensing applications.

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Wan, Yong, Daniel Kienzler, Stephen D. Erickson, Karl H. Mayer, Ting Rei Tan, Jenny J. Wu, Hilma M. Vasconcelos, et al. "Quantum gate teleportation between separated qubits in a trapped-ion processor." Science 364, no. 6443 (May 30, 2019): 875–78. http://dx.doi.org/10.1126/science.aaw9415.

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Large-scale quantum computers will require quantum gate operations between widely separated qubits. A method for implementing such operations, known as quantum gate teleportation (QGT), requires only local operations, classical communication, and shared entanglement. We demonstrate QGT in a scalable architecture by deterministically teleporting a controlled-NOT (CNOT) gate between two qubits in spatially separated locations in an ion trap. The entanglement fidelity of our teleported CNOT is in the interval (0.845, 0.872) at the 95% confidence level. The implementation combines ion shuttling with individually addressed single-qubit rotations and detections, same- and mixed-species two-qubit gates, and real-time conditional operations, thereby demonstrating essential tools for scaling trapped-ion quantum computers combined in a single device.

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Sanders, Yuval R., Joel J. Wallman, and Barry C. Sanders. "Bounding quantum gate error rate based on reported average fidelity." New Journal of Physics 18, no. 1 (December 21, 2015): 012002. http://dx.doi.org/10.1088/1367-2630/18/1/012002.

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Chen, Z. Q., J. Q. Wang, X. L. Li, Y. H. Ji, B. R. Zhang, Y. Y. Jiang, and Z. S. Wang. "Avoiding Loss of Fidelity for Universal Entangling Geometric Quantum Gate." International Journal of Theoretical Physics 48, no. 10 (July 9, 2009): 2904–15. http://dx.doi.org/10.1007/s10773-009-0082-2.

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Figueroa-Romero, Pedro, Kavan Modi, and Min-Hsiu Hsieh. "Towards a general framework of Randomized Benchmarking incorporating non-Markovian Noise." Quantum 6 (December 1, 2022): 868. http://dx.doi.org/10.22331/q-2022-12-01-868.

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The rapid progress in the development of quantum devices is in large part due to the availability of a wide range of characterization techniques allowing to probe, test and adjust them. Nevertheless, these methods often make use of approximations that hold in rather simplistic circumstances. In particular, assuming that error mechanisms stay constant in time and have no dependence in the past, is something that will be impossible to do as quantum processors continue scaling up in depth and size. We establish a theoretical framework for the Randomized Benchmarking protocol encompassing temporally-correlated, so-called non-Markovian noise, at the gate level, for any gate set belonging to a wide class of finite groups. We obtain a general expression for the Average Sequence Fidelity (ASF) and propose a way to obtain average gate fidelities of full non-Markovian noise processes. Moreover, we obtain conditions that are fulfilled when an ASF displays authentic non-Markovian deviations. Finally, we show that even though gate-dependence does not translate into a perturbative term within the ASF, as in the Markovian case, the non-Markovian sequence fidelity nevertheless remains stable under small gate-dependent perturbations.

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ROMITO, ALESSANDRO, GABRIELE DE CHIARA, DAVIDE ROSSINI, and SIMONE MONTANGERO. "IMPLEMENTATION OF QUANTUM COMMUNICATION PROTOCOLS IN JOSEPHSON JUNCTION ARRAYS." International Journal of Quantum Information 04, no. 03 (June 2006): 519–29. http://dx.doi.org/10.1142/s0219749906001967.

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We discuss simple protocols to realize state transfer with high fidelity in Josephson junction arrays. We consider both the cases of constant and properly modulated couplings between the site of the chain. We investigate the influence of static disorder both in the Josephson energies and in the coupling to the background gate charges and we show that they can lead to a fractal fidelity. We also analyze the readout process, and its backaction on the state transfer.

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GUZMÁN, ANGELA M., and MARCO A. DUEÑAS E. "QUANTUM INFORMATION IN OPTICAL LATTICES." International Journal of Modern Physics B 24, no. 25n26 (October 20, 2010): 5105–14. http://dx.doi.org/10.1142/s0217979210057249.

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Experimental realizations of a two-qubit quantum logic gate based on cold atom collisions have been elusive mainly due to the decoherence effects introduced during the quantum gate operation, which cause transitions out of the two-qubit space and lead to a decreased gate operation fidelity. This type of decoherence effects, due to the non closeness of the interacting two-qubit system, are characteristic of the electromagnetic interaction, since the electromagnetic vacuum acts as a reservoir whose eigenmodes might become active during the gate operation. To describe the cold-atom collision we consider the quantum non-Hermitian dipole-dipole interaction instead of the less realistic s-scattering approach widely used in the literature. By adding an ancillary qubit, we take advantage of the spatial modulation of the non-Hermitian part of the interaction potential to obtain a "resonant" condition that should be satisfied to achieve lossless operation of a specific two-qubit quantum phase-gate. We demonstrate that careful engineering of the collision is required to obtain a specific truth table and to suppress the effects inherent in the openness of the system arising from the electromagnetic interaction.

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Madden, Liam, and Andrea Simonetto. "Best Approximate Quantum Compiling Problems." ACM Transactions on Quantum Computing 3, no. 2 (June 30, 2022): 1–29. http://dx.doi.org/10.1145/3505181.

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We study the problem of finding the best approximate circuit that is the closest (in some pertinent metric) to a target circuit, and which satisfies a number of hardware constraints, like gate alphabet and connectivity. We look at the problem in the CNOT+rotation gate set from a mathematical programming standpoint, offering contributions both in terms of understanding the mathematics of the problem and its efficient solution. Among the results that we present, we are able to derive a 14-CNOT 4-qubit Toffoli decomposition from scratch, and show that the Quantum Shannon Decomposition can be compressed by a factor of two without practical loss of fidelity.

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Ding, Cheng-Yun, Li-Na Ji, Tao Chen, and Zheng-Yuan Xue. "Path-optimized nonadiabatic geometric quantum computation on superconducting qubits." Quantum Science and Technology 7, no. 1 (November 22, 2021): 015012. http://dx.doi.org/10.1088/2058-9565/ac3621.

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Abstract Quantum computation based on nonadiabatic geometric phases has attracted a broad range of interests, due to its fast manipulation and inherent noise resistance. However, it is limited to some special evolution paths, and the gate-times are typically longer than conventional dynamical gates, resulting in weakening of robustness and more infidelities of the implemented geometric gates. Here, we propose a path-optimized scheme for geometric quantum computation (GQC) on superconducting transmon qubits, where high-fidelity and robust universal nonadiabatic geometric gates can be implemented, based on conventional experimental setups. Specifically, we find that, by selecting appropriate evolution paths, the constructed geometric gates can be superior to their corresponding dynamical ones under different local errors. Numerical simulations show that the fidelities for single-qubit geometric phase, π/8 and Hadamard gates can be obtained as 99.93%, 99.95% and 99.95%, respectively. Remarkably, the fidelity for two-qubit control-phase gate can be as high as 99.87%. Therefore, our scheme provides a new perspective for GQC, making it more promising in the application of large-scale fault-tolerant quantum computation.

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Weidenfeller, Johannes, Lucia C. Valor, Julien Gacon, Caroline Tornow, Luciano Bello, Stefan Woerner, and Daniel J. Egger. "Scaling of the quantum approximate optimization algorithm on superconducting qubit based hardware." Quantum 6 (December 7, 2022): 870. http://dx.doi.org/10.22331/q-2022-12-07-870.

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Quantum computers may provide good solutions to combinatorial optimization problems by leveraging the Quantum Approximate Optimization Algorithm (QAOA). The QAOA is often presented as an algorithm for noisy hardware. However, hardware constraints limit its applicability to problem instances that closely match the connectivity of the qubits. Furthermore, the QAOA must outpace classical solvers. Here, we investigate swap strategies to map dense problems into linear, grid and heavy-hex coupling maps. A line-based swap strategy works best for linear and two-dimensional grid coupling maps. Heavy-hex coupling maps require an adaptation of the line swap strategy. By contrast, three-dimensional grid coupling maps benefit from a different swap strategy. Using known entropic arguments we find that the required gate fidelity for dense problems lies deep below the fault-tolerant threshold. We also provide a methodology to reason about the execution-time of QAOA. Finally, we present a QAOA Qiskit Runtime program and execute the closed-loop optimization on cloud-based quantum computers with transpiler settings optimized for QAOA. This work highlights some obstacles to improve to make QAOA competitive, such as gate fidelity, gate speed, and the large number of shots needed. The Qiskit Runtime program gives us a tool to investigate such issues at scale on noisy superconducting qubit hardware.

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Chen, Feng, Gang Ren, and Haijun Yu. "Transformation of coherent state in Hadamard gate via multi-photon catalysis." Laser Physics Letters 19, no. 11 (October 7, 2022): 115203. http://dx.doi.org/10.1088/1612-202x/ac9402.

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Abstract Based on two-mode continuous Hadamard gate (TCHG) and multi-photon catalysis, this paper presents a scheme to generate non-classical quantum states via coherent state. When considering the m-photon input and m-photon detection in a-mode, the output state is obtained for the input coherent in b-mode. An interesting finding is that as the amplitude of the output coherent state increases, both the detection efficiency and the fidelity of the output quantum state gradually approach zero. Thus, only certain low-intensity quantum light field states can pass through Hadamard gate. In addition, the quantum statistical distribution and squeezing properties of the output quantum states are also analyzed in detail. Our results show that TCHG is a powerful tool for generating non-classical quantum states under multi-photon catalysis.

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Zhang, Ziqiu, Xi Jiang, and Shiqing Tang. "Realization of Quantum Swap Gate and Generation of Entangled Coherent States." Symmetry 14, no. 9 (September 19, 2022): 1951. http://dx.doi.org/10.3390/sym14091951.

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The cross fusion of quantum mechanics and information science forms quantum information science. Quantum logic gates and quantum entanglement are very important building blocks in quantum information processing. In this paper, we propose one-step schemes for realizing quantum swap gates and generating two-mode entangled coherent states via circuit QED. In our scheme, due to the adiabatic elimination of the excited state of the qutrit under the condition of large detuning, the decoherence of the spontaneous emission of the qutrit can be ignored. The fidelity of the quantum swap gate remains at a very high level. In addition, we also explore the nonclassical properties of two-mode entangled coherent states prepared in our scheme by addressing the second-order correlation function and intermodal squeezing. In particular, two classes of entangled coherent states demonstrate distinct entanglement and nonclassical behavior.

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Li, Meng, Qian Zhang, Yang Chen, Xifeng Ren, Qihuang Gong, and Yan Li. "Femtosecond Laser Direct Writing of Integrated Photonic Quantum Chips for Generating Path-Encoded Bell States." Micromachines 11, no. 12 (December 15, 2020): 1111. http://dx.doi.org/10.3390/mi11121111.

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Integrated photonic quantum chip provides a promising platform to perform quantum computation, quantum simulation, quantum metrology and quantum communication. Femtosecond laser direct writing (FLDW) is a potential technique to fabricate various integrated photonic quantum chips in glass. Several quantum logic gates fabricated by FLDW have been reported, such as polarization and path encoded quantum controlled-NOT (CNOT) gates. By combining several single qubit gates and two qubit gates, the quantum circuit can realize different functions, such as generating quantum entangled states and performing quantum computation algorithms. Here we demonstrate the FLDW of integrated photonic quantum chips composed of one Hadamard gate and one CNOT gate for generating all four path-encoded Bell states. The experimental results show that the average fidelity of the reconstructed truth table reaches as high as 98.8 ± 0.3%. Our work is of great importance to be widely applied in many quantum circuits, therefore this technique would offer great potential to fabricate more complex circuits to realize more advanced functions.

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Li, Shaowei, Daojin Fan, Ming Gong, Yangsen Ye, Xiawei Chen, Yulin Wu, Huijie Guan, et al. "Realization of Fast All-Microwave Controlled-Z Gates with a Tunable Coupler." Chinese Physics Letters 39, no. 3 (February 1, 2022): 030302. http://dx.doi.org/10.1088/0256-307x/39/3/030302.

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The development of high-fidelity two-qubit quantum gates is essential for digital quantum computing. Here, we propose and realize an all-microwave parametric controlled-Z (CZ) gates by coupling strength modulation in a superconducting Transmon qubit system with tunable couplers. After optimizing the design of the tunable coupler together with the control pulse numerically, we experimentally realized a 100 ns CZ gate with high fidelity of 99.38% ± 0.34% and the control error being 0.1%. We note that our CZ gates are not affected by pulse distortion and do not need pulse correction, providing a solution for the real-time pulse generation in a dynamic quantum feedback circuit. With the expectation of utilizing our all-microwave control scheme to reduce the number of control lines through frequency multiplexing in the future, our scheme draws a blueprint for the high-integrable quantum hardware design.

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Polloreno, Anthony M., and Kevin C. Young. "Robustly decorrelating errors with mixed quantum gates." Quantum Science and Technology 7, no. 2 (January 10, 2022): 025004. http://dx.doi.org/10.1088/2058-9565/ac4423.

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Abstract Coherent errors in quantum operations are ubiquitous. Whether arising from spurious environmental couplings or errors in control fields, such errors can accumulate rapidly and degrade the performance of a quantum circuit significantly more than an average gate fidelity may indicate. As Hastings (2017 Quantum Inf. Comput. 17 488) and Campbell (2017 Phys. Rev. A 95 042306) have recently shown, by replacing the deterministic implementation of a quantum gate with a randomized ensemble of implementations, one can dramatically suppress coherent errors. Our work begins by reformulating the results of Hastings and Campbell as a quantum optimal control problem. We then discuss a family of convex programs able to solve this problem, as well as a set of secondary objectives designed to improve the performance, implementability, and robustness of the resulting mixed quantum gates. Finally, we implement these mixed quantum gates on a superconducting qubit and discuss randomized benchmarking results consistent with a marked reduction in the coherent error.

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Castellanos, Maria A., and Adam P. Willard. "Designing excitonic circuits for the Deutsch–Jozsa algorithm: mitigating fidelity loss by merging gate operations." Physical Chemistry Chemical Physics 23, no. 28 (2021): 15196–208. http://dx.doi.org/10.1039/d1cp01643a.

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Precisely arranged sets of dye molecules can utilized as elementary quantum computing elements. Here, we consider two different strategies for designing these excitonic circuits for a 2-qubit multi-step quantum algorithm.

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Zhu, Qianyu, Cheng Lü, Jin-Lei Wu, and Yan Li. "Gaussian soft control for controlled-Z gate on superconducting qubits with unilateral external driving." Laser Physics Letters 19, no. 9 (August 4, 2022): 095206. http://dx.doi.org/10.1088/1612-202x/ac83c2.

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Abstract Soft quantum control is a valid technique for highly selective interactions recently illustrated in Haase et al (2018 Phys. Rev. Lett. 121 050402), holding efficient resonant couplings among target levels while largely suppressing unwanted off-resonant contributions. Here we present a model for implementing a controlled-Z (CZ) gate in superconducting circuit quantum electrodynamics (QED) with two qubits being coupled to a microwave cavity. An external classical field that drives only one qubit, combined with the strong single-mode quantized cavity field dressing both qubits, is employed to induce the CZ gate between two qubits, and is also further tailored as a Gaussian soft control (GSC) to improve gate performances in various aspects. By contrast, we show that, with the same gate time, the CZ gate based on GSC can hold a higher fidelity, greater resilience to parameter errors, and stronger robustness against decoherence of system than that based on a rectangular pulse.

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Slimen, Iyed Ben, Amor Gueddana, and Vasudevan Lakshminarayanan. "Discrete-time quantum walk on circular graph: Simulations and effect of gate depth and errors." International Journal of Quantum Information 19, no. 02 (March 2021): 2150008. http://dx.doi.org/10.1142/s0219749921500088.

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We investigate the counterparts of random walks in universal quantum computing and their implementation using standard quantum circuits. Quantum walks have been recently well investigated for traversing graphs with certain oracles. We focus our study on traversing a 1D graph, namely a circle, and show how to implement a discrete-time quantum walk in quantum circuits built with universal CNOT and single qubit gates. We review elementary quantum gates and circuit decomposition techniques and propose a generalized version of all CNOT-based circuits of the quantum walk. We simulated these circuits on five different qubits IBM-Q quantum devices. This quantum computer has nonperfect gates based on superconducting qubits, and, therefore, we analyzed the impact of the CNOT errors and CNOT-depth on the fidelity of the circuit.

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34

Porter, Max D., and Ilon Joseph. "Observability of fidelity decay at the Lyapunov rate in few-qubit quantum simulations." Quantum 6 (September 8, 2022): 799. http://dx.doi.org/10.22331/q-2022-09-08-799.

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In certain regimes, the fidelity of quantum states will decay at a rate set by the classical Lyapunov exponent. This serves both as one of the most important examples of the quantum-classical correspondence principle and as an accurate test for the presence of chaos. While detecting this phenomenon is one of the first useful calculations that noisy quantum computers without error correction can perform [G. Benenti et al., Phys. Rev. E 65, 066205 (2001)], a thorough study of the quantum sawtooth map reveals that observing the Lyapunov regime is just beyond the reach of present-day devices. We prove that there are three bounds on the ability of any device to observe the Lyapunov regime and give the first quantitatively accurate description of these bounds: (1) the Fermi golden rule decay rate must be larger than the Lyapunov rate, (2) the quantum dynamics must be diffusive rather than localized, and (3) the initial decay rate must be slow enough for Lyapunov decay to be observable. This last bound, which has not been recognized previously, places a limit on the maximum amount of noise that can be tolerated. The theory implies that an absolute minimum of 6 qubits is required. Recent experiments on IBM-Q and IonQ imply that some combination of a noise reduction by up to 100× per gate and large increases in connectivity and gate parallelization are also necessary. Finally, scaling arguments are given that quantify the ability of future devices to observe the Lyapunov regime based on trade-offs between hardware architecture and performance.

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35

Li, R., M. Hoover, and F. Gaitan. "High fidelity universal set of quantum gates using non-adiabatic rapid passage." Quantum Information and Computation 9, no. 3&4 (March 2009): 290–316. http://dx.doi.org/10.26421/qic9.3-4-7.

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Numerical simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps first realized experimentally in 1991 should be capable of implementing a universal set of quantum gates \uniset\ that operate with high fidelity. The gates constituting \uniset\ are the Hadamard and NOT gates, together with variants of the phase, $\pi /8$, and controlled-phase gates. The universality of \uniset\ is established by showing that it can construct the universal set consisting of Hadamard, phase, $\pi /8$, and controlled-NOT gates. Sweep parameter values are provided which simulations indicate will produce the different gates in \uniset , and for which the gate error probability $P_{e}$ satisfies: (i)~$P_{e}<10^{-4}$ for the one-qubit gates; and (ii)~$P_{e}<1.27\times 10^{-3}$ for the modified controlled-phase gate. The sweeps in this class are non-composite and generate controllable quantum interference effects that allow the gates in \uniset\ to operate non-adiabatically while maintaining high fidelity. These interference effects have been observed using NMR, and it has previously been shown how these rapid passage sweeps can be applied to atomic systems using electric fields. Here we show how these sweeps can be applied to both superconducting charge and flux qubit systems. The simulations suggest that the universal set of gates \uniset\ produced by these rapid passage sweeps shows promise as possible elements of a fault-tolerant scheme for quantum computing.

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36

Wood, Christopher J., Jacob D. Biamonte, and David G. Cory. "Tensor networks and graphical calculus for open quantum systems." Quantum Information and Computation 15, no. 9&10 (July 2015): 759–811. http://dx.doi.org/10.26421/qic15.9-10-3.

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We describe a graphical calculus for completely positive maps and in doing so review the theory of open quantum systems and other fundamental primitives of quantum information theory using the language of tensor networks. In particular we demonstrate the construction of tensor networks to pictographically represent the Liouville-superoperator, Choi-matrix, process-matrix, Kraus, and system-environment representations for the evolution of quantum states, review how these representations interrelate, and illustrate how graphical manipulations of the tensor networks may be used to concisely transform between them. To further demonstrate the utility of the presented graphical calculus we include several examples where we provide arguably simpler graphical proofs of several useful quantities in quantum information theory including the composition and contraction of multipartite channels, a condition for whether an arbitrary bipartite state may be used for ancilla assisted process tomography, and the derivation of expressions for the average gate fidelity and entanglement fidelity of a channel in terms of each of the different representations of the channel.

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37

Yang, Bing, Hui Sun, Chun-Jiong Huang, Han-Yi Wang, Youjin Deng, Han-Ning Dai, Zhen-Sheng Yuan, and Jian-Wei Pan. "Cooling and entangling ultracold atoms in optical lattices." Science 369, no. 6503 (June 18, 2020): 550–53. http://dx.doi.org/10.1126/science.aaz6801.

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Scalable, coherent many-body systems can enable the realization of previously unexplored quantum phases and have the potential to exponentially speed up information processing. Thermal fluctuations are negligible and quantum effects govern the behavior of such systems with extremely low temperature. We report the cooling of a quantum simulator with 10,000 atoms and mass production of high-fidelity entangled pairs. In a two-dimensional plane, we cool Mott insulator samples by immersing them into removable superfluid reservoirs, achieving an entropy per particle of 1.9−0.4+1.7×10−3kB. The atoms are then rearranged into a two-dimensional lattice free of defects. We further demonstrate a two-qubit gate with a fidelity of 0.993 ± 0.001 for entangling 1250 atom pairs. Our results offer a setting for exploring low-energy many-body phases and may enable the creation of large-scale entanglement.

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38

Hansen, I., A. E. Seedhouse, K. W. Chan, F. E. Hudson, K. M. Itoh, A. Laucht, A. Saraiva, C. H. Yang, and A. S. Dzurak. "Implementation of an advanced dressing protocol for global qubit control in silicon." Applied Physics Reviews 9, no. 3 (September 2022): 031409. http://dx.doi.org/10.1063/5.0096467.

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Quantum computing based on solid state spins allows for densely packed arrays of quantum bits. However, the operation of large-scale quantum processors requires a shift in paradigm toward global control solutions. Here, we report a proof-of-principle demonstration of the SMART (sinusoidally modulated, always rotating, and tailored) qubit protocol. We resonantly drive a two-level system and add a tailored modulation to the dressing field to increase robustness to frequency detuning noise and microwave amplitude fluctuations. We measure a coherence time of 2 ms, corresponding to two orders of magnitude improvement compared to a bare spin, and an average Clifford gate fidelity exceeding 99%, despite the relatively long qubit gate times. We stress that the potential of this work lies in the scalability of the protocol and the relaxation of the engineering constraints for a large-scale quantum processor. This work shows that future scalable spin qubit arrays could be operated using global microwave control and local gate addressability, while increasing robustness to relevant experimental inhomogeneities.

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Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, Chris Stoughton, Collin Bradford, Sho Uemura, Silvia Zorzetti, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (April 1, 2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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40

Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, Chris Stoughton, Collin Bradford, Sho Uemura, Silvia Zorzetti, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (April 1, 2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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41

Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, Chris Stoughton, Collin Bradford, Sho Uemura, Silvia Zorzetti, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (April 1, 2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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Abstract:

We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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42

GAO, CHENG-YUAN, LEI MA, and JIN-MING LIU. "DYNAMIC PROPERTIES OF THE LARGE-DETUNING CAVITY QED SYSTEM IN THE PRESENCE OF CAVITY DECAY." Modern Physics Letters B 22, no. 26 (October 20, 2008): 2561–70. http://dx.doi.org/10.1142/s021798490801714x.

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We consider a physical process of two Λ-type three-level atoms interacting with a bimodal cavity including the influence of the cavity decay. We analyze the influence of cavity decay on several physical quantities of the process, such as atomic population probability, residual entanglement, concurrence of two atoms, average population inversion, average photon number, the fidelity for quantum phase gate, and the fidelity of generating atomic EPR state. It is found that all of these physical quantities decrease with the increase of cavity decay when the other relevant parameters are fixed.

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Jang, Wonho, Koji Terashi, Masahiko Saito, Christian W. Bauer, Benjamin Nachman, Yutaro Iiyama, Ryunosuke Okubo, and Ryu Sawada. "Initial-State Dependent Optimization of Controlled Gate Operations with Quantum Computer." Quantum 6 (September 8, 2022): 798. http://dx.doi.org/10.22331/q-2022-09-08-798.

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There is no unique way to encode a quantum algorithm into a quantum circuit. With limited qubit counts, connectivity, and coherence times, a quantum circuit optimization is essential to make the best use of near-term quantum devices. We introduce a new circuit optimizer called AQCEL, which aims to remove redundant controlled operations from controlled gates, depending on initial states of the circuit. Especially, the AQCEL can remove unnecessary qubit controls from multi-controlled gates in polynomial computational resources, even when all the relevant qubits are entangled, by identifying zero-amplitude computational basis states using a quantum computer. As a benchmark, the AQCEL is deployed on a quantum algorithm designed to model final state radiation in high energy physics. For this benchmark, we have demonstrated that the AQCEL-optimized circuit can produce equivalent final states with much smaller number of gates. Moreover, when deploying AQCEL with a noisy intermediate scale quantum computer, it efficiently produces a quantum circuit that approximates the original circuit with high fidelity by truncating low-amplitude computational basis states below certain thresholds. Our technique is useful for a wide variety of quantum algorithms, opening up new possibilities to further simplify quantum circuits to be more effective for real devices.

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KARASAWA, TOKISHIRO, MASANAO OZAWA, JULIO GEA-BANACLOCHE, and KAE NEMOTO. "QUANTUM PRECISION LIMITS FOR ANY IMPLEMENTATION OF SINGLE QUBIT GATES UNDER CONSERVATION LAWS." International Journal of Quantum Information 06, supp01 (July 2008): 701–6. http://dx.doi.org/10.1142/s0219749908003980.

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A quantum gate is implemented by control interactions between qubits and their ancilla system. It has been shown that the control interactions have possibilities to induce the dynamical decoherence on the qubits if an additive conservation law is assumed in the interactions and the ancilla system is finite. This decoherece put the precision limit on the gate, which cannot be removed from the qubit by optimizing the interaction and the initialization of the ancilla system. In this paper, we give the outline of investigating the precision limit which is formulated by the lower bound of the gate infidelity, one minus the squared fidelity, for an arbitrary self-adjoint gate on a single qubit. We show rigorous lower bounds in terms of the variance of the conserved quantity and a simple geometrical relation between the conservation law to be assumed and the gates to be implemented. We also comment on another approach to provide the precision limit for an arbitrary single qubit gate under a conservation law.

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45

Shi, Zhi-Cheng, Yan Xia, Jie Song, and He-Shan Song. "One-step implementation of the Fredkin gate via quantum Zeno dynamics." Quantum Information and Computation 12, no. 3&4 (March 2012): 215–30. http://dx.doi.org/10.26421/qic12.3-4-3.

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We study one-step implementation of the Fredkin gate in a bi-modal cavity under both resonant and large detuning conditions based on quantum Zeno dynamics, which reduces the complexity of experiment operations. The influence of cavity decay and atomic spontaneous emission is discussed by numerical calculation. The results demonstrate that the fidelity and the success probability are robust against cavity decay in both models and they are also insensitive to atomic spontaneous emission in the large detuning model. In addition, the interaction time is rather short in the resonant model compared to the large detuning model.

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46

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 (March 15, 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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47

Nielsen, Michael A. "A simple formula for the average gate fidelity of a quantum dynamical operation." Physics Letters A 303, no. 4 (October 2002): 249–52. http://dx.doi.org/10.1016/s0375-9601(02)01272-0.

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Xu, Lin, Gang Huang, Y. H. Ji, and Z. S. Wang. "Geometric Control for Unified Entangling Quantum Gate with High-Fidelity in Electric Circuit." International Journal of Theoretical Physics 49, no. 9 (May 26, 2010): 2002–15. http://dx.doi.org/10.1007/s10773-010-0384-4.

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Yin-Zhong, Wu, Shi Cui-Hua, Pan Tao, and Li Zhen-Ya. "Effects of Unequal Couplings on Fidelity of a Quantum Controlled-Controlled-Not Gate." Communications in Theoretical Physics 51, no. 6 (June 2009): 1033–36. http://dx.doi.org/10.1088/0253-6102/51/6/14.

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Wang, Ruixia, Peng Zhao, Yirong Jin, and Haifeng Yu. "Control and mitigation of microwave crosstalk effect with superconducting qubits." Applied Physics Letters 121, no. 15 (October 10, 2022): 152602. http://dx.doi.org/10.1063/5.0115393.

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Improving gate performance is vital for scalable quantum computing. Universal quantum computing also requires gate fidelity to reach a high level. For a superconducting quantum processor, which operates in the microwave band, the single-qubit gates are usually realized with microwave driving. The crosstalk between microwave pulses is a non-negligible error source. In this article, we propose an error mitigation scheme to address this crosstalk issue for single-qubit gates. There are three steps in our method. First, by controlling the detuning between qubits, the microwave induced classical crosstalk error can be constrained within the computational subspace. Second, by applying the general decomposition procedure, the arbitrary single-qubit gate can be decomposed as a sequence of [Formula: see text] and virtual Z gates. Finally, by optimizing the parameters in virtual Z gates, the error constrained in the computational space can be corrected. Using our method, no additional compensation signals are needed, arbitrary single-qubit gate time will not be prolonged, and the circuit depth containing simultaneous single-qubit gates will also not increase. The simulation results show that, in a specific regime of qubit–qubit detuning, the infidelities of simultaneous single-qubit gates can be as low as that without microwave crosstalk.

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Journal articles on the topic 'Quantum Gate Fidelity'

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