Journal articles: 'Qubits'

1

Bluvstein, Dolev, Harry Levine, Giulia Semeghini, Tout T. Wang, Sepehr Ebadi, Marcin Kalinowski, Alexander Keesling, et al. "A quantum processor based on coherent transport of entangled atom arrays." Nature 604, no. 7906 (April 20, 2022): 451–56. http://dx.doi.org/10.1038/s41586-022-04592-6.

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AbstractThe ability to engineer parallel, programmable operations between desired qubits within a quantum processor is key for building scalable quantum information systems1,2. In most state-of-the-art approaches, qubits interact locally, constrained by the connectivity associated with their fixed spatial layout. Here we demonstrate a quantum processor with dynamic, non-local connectivity, in which entangled qubits are coherently transported in a highly parallel manner across two spatial dimensions, between layers of single- and two-qubit operations. Our approach makes use of neutral atom arrays trapped and transported by optical tweezers; hyperfine states are used for robust quantum information storage, and excitation into Rydberg states is used for entanglement generation3–5. We use this architecture to realize programmable generation of entangled graph states, such as cluster states and a seven-qubit Steane code state6,7. Furthermore, we shuttle entangled ancilla arrays to realize a surface code state with thirteen data and six ancillary qubits8 and a toric code state on a torus with sixteen data and eight ancillary qubits9. Finally, we use this architecture to realize a hybrid analogue–digital evolution2 and use it for measuring entanglement entropy in quantum simulations10–12, experimentally observing non-monotonic entanglement dynamics associated with quantum many-body scars13,14. Realizing a long-standing goal, these results provide a route towards scalable quantum processing and enable applications ranging from simulation to metrology.

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2

Yuan, Wei-Ping, Zhi-Cheng He, Sai Li, and Zheng-Yuan Xue. "Fast Reset Protocol for Superconducting Transmon Qubits." Applied Sciences 13, no. 2 (January 6, 2023): 817. http://dx.doi.org/10.3390/app13020817.

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For larger-scale quantum information processing, qubit reset plays an important role, as the coherent times for qubits are limited. However, previous schemes require either long reset times or a complex pulse calibration technique, leading to low efficiency in qubit reset. Here, we propose a fast and simple reset protocol for superconducting transmon qubits based on the coupler-coupled qubits architecture. In this setup, a mixing pulse is used to transfer the qubit excitation to the combined excitation of a low-qulity coupler and readout resonator, which will quickly decay to their respectively ground states, leading to efficient qubit reset to the ground state. Our numerical results show that the residual population of the qubit’s excited state can be suppressed to 0.04% within 28 ns; the reset time will be 283 ns if photon depletion of the readout resonator is required. Thus, our protocol provides a promising way for the high-efficiency superconducting qubit reset.

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Bhattacharyya, Shaman, and Somnath Bhattacharyya. "Demonstration of the Holonomically Controlled Non-Abelian Geometric Phase in a Three-Qubit System of a Nitrogen Vacancy Center." Entropy 24, no. 11 (November 2, 2022): 1593. http://dx.doi.org/10.3390/e24111593.

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The holonomic approach to controlling (nitrogen-vacancy) NV-center qubits provides an elegant way of theoretically devising universal quantum gates that operate on qubits via calculable microwave pulses. There is, however, a lack of simulated results from the theory of holonomic control of quantum registers with more than two qubits describing the transition between the dark states. Considering this, we have been experimenting with the IBM Quantum Experience technology to determine the capabilities of simulating holonomic control of NV-centers for three qubits describing an eight-level system that produces a non-Abelian geometric phase. The tunability of the geometric phase via the detuning frequency is demonstrated through the high fidelity (~85%) of three-qubit off-resonant holonomic gates over the on-resonant ones. The transition between the dark states shows the alignment of the gate’s dark state with the qubit’s initial state hence decoherence of the multi-qubit system is well-controlled through a π/3 rotation.

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4

Takeda, Kenta, Akito Noiri, Takashi Nakajima, Takashi Kobayashi, and Seigo Tarucha. "Quantum error correction with silicon spin qubits." Nature 608, no. 7924 (August 24, 2022): 682–86. http://dx.doi.org/10.1038/s41586-022-04986-6.

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AbstractFuture large-scale quantum computers will rely on quantum error correction (QEC) to protect the fragile quantum information during computation1,2. Among the possible candidate platforms for realizing quantum computing devices, the compatibility with mature nanofabrication technologies of silicon-based spin qubits offers promise to overcome the challenges in scaling up device sizes from the prototypes of today to large-scale computers3–5. Recent advances in silicon-based qubits have enabled the implementations of high-quality one-qubit and two-qubit systems6–8. However, the demonstration of QEC, which requires three or more coupled qubits1, and involves a three-qubit gate9–11 or measurement-based feedback, remains an open challenge. Here we demonstrate a three-qubit phase-correcting code in silicon, in which an encoded three-qubit state is protected against any phase-flip error on one of the three qubits. The correction to this encoded state is performed by a three-qubit conditional rotation, which we implement by an efficient single-step resonantly driven iToffoli gate. As expected, the error correction mitigates the errors owing to one-qubit phase-flip, as well as the intrinsic dephasing mainly owing to quasi-static phase noise. These results show successful implementation of QEC and the potential of a silicon-based platform for large-scale quantum computing.

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5

Zakharov, R. K., and E. K. Bashkirov. "Entanglement of two dipole-coupled qubits induced by a thermal field of one-mode lossless cavity with Kerr medium." Journal of Physics: Conference Series 2086, no. 1 (December 1, 2021): 012216. http://dx.doi.org/10.1088/1742-6596/2086/1/012216.

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Abstract We studied the dynamics of two qubits interacting with one-mode thermal quantum electromagnetic field of microwave cavity with Kerr medium. Using the exact solution for considered model we derived the qubit-qubit negativity for separa coherent initial qubits states. We showed that initial qubits coherencee interaction can greatly enhance the degree of qubits entanglement in the presence of the Kerr nonlinearity and dipole-dipole interactionyeven for high thermal field intensities.

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6

FUJII, TOSHIYUKI, MUNEHIRO NISHIDA, SATOSHI TANDA, and NORIYUKI HATAKENAKA. "TALKING BREATHER QUBITS." International Journal of Modern Physics B 23, no. 20n21 (August 20, 2009): 4352–64. http://dx.doi.org/10.1142/s0217979209063511.

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Breather is an elementary excitation regarded as a bound state of a fluxon and an antifluxon in a long Josephson junction. In quantum-mechanical regime, the breather energy is quantized so that the breather can be considered as an artificial moving atom. We propose a new type of fluxon qubit that is constructed by quantum-mechanical superposition of the breather's states. We describe quantum logic gates of breather qubit required for constructing quantum computer. In addition, our qubit can move in the system so that transfer of quntum information is possible between mobile qubits as well as stationary qubits. Our talking qubits support the global information sharing in quantum information networks.

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7

Bultrini, Daniel, Samson Wang, Piotr Czarnik, Max Hunter Gordon, M. Cerezo, Patrick J. Coles, and Lukasz Cincio. "The battle of clean and dirty qubits in the era of partial error correction." Quantum 7 (July 13, 2023): 1060. http://dx.doi.org/10.22331/q-2023-07-13-1060.

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When error correction becomes possible it will be necessary to dedicate a large number of physical qubits to each logical qubit. Error correction allows for deeper circuits to be run, but each additional physical qubit can potentially contribute an exponential increase in computational space, so there is a trade-off between using qubits for error correction or using them as noisy qubits. In this work we look at the effects of using noisy qubits in conjunction with noiseless qubits (an idealized model for error-corrected qubits), which we call the "clean and dirty" setup. We employ analytical models and numerical simulations to characterize this setup. Numerically we show the appearance of Noise-Induced Barren Plateaus (NIBPs), i.e., an exponential concentration of observables caused by noise, in an Ising model Hamiltonian variational ansatz circuit. We observe this even if only a single qubit is noisy and given a deep enough circuit, suggesting that NIBPs cannot be fully overcome simply by error-correcting a subset of the qubits. On the positive side, we find that for every noiseless qubit in the circuit, there is an exponential suppression in concentration of gradient observables, showing the benefit of partial error correction. Finally, our analytical models corroborate these findings by showing that observables concentrate with a scaling in the exponent related to the ratio of dirty-to-total qubits.

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8

Chao, Rui, Michael E. Beverland, Nicolas Delfosse, and Jeongwan Haah. "Optimization of the surface code design for Majorana-based qubits." Quantum 4 (October 28, 2020): 352. http://dx.doi.org/10.22331/q-2020-10-28-352.

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The surface code is a prominent topological error-correcting code exhibiting high fault-tolerance accuracy thresholds. Conventional schemes for error correction with the surface code place qubits on a planar grid and assume native CNOT gates between the data qubits with nearest-neighbor ancilla qubits.Here, we present surface code error-correction schemes using only Pauli measurements on single qubits and on pairs of nearest-neighbor qubits. In particular, we provide several qubit layouts that offer favorable trade-offs between qubit overhead, circuit depth and connectivity degree. We also develop minimized measurement sequences for syndrome extraction, enabling reduced logical error rates and improved fault-tolerance thresholds.Our work applies to topologically protected qubits realized with Majorana zero modes and to similar systems in which multi-qubit Pauli measurements rather than CNOT gates are the native operations.

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9

LIU, YANG, GUI LU LONG, and YANG SUN. "ANALYTIC ONE-BIT AND CNOT GATE CONSTRUCTIONS OF GENERAL n-QUBIT CONTROLLED GATES." International Journal of Quantum Information 06, no. 03 (June 2008): 447–62. http://dx.doi.org/10.1142/s0219749908003621.

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General n-qubit controlled unitary gates are frequently used in quantum information processing tasks. Barenco, Bennett, Cleve, Di Vincenzo, Margolus and Shor [Phys. Rev. A52 (1995) 3457] have given the general construction methods, and explicit results for up-to-four-qubits controlled unitary gates. We extended their calculation and gave two analytic expressions for the construction of general n-qubit controlled unitary gates in terms of one-qubit and two-qubit CNOT gates. There are two expressions – one is exponential in the qubit number which is efficient for up to ten qubits, and the other is polynomial in the qubit number, which is efficient for more than ten qubits.

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10

Militello, Benedetto, and Anna Napoli. "Synchronizing Two Superconducting Qubits through a Dissipating Resonator." Entropy 23, no. 8 (July 31, 2021): 998. http://dx.doi.org/10.3390/e23080998.

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A system consisting of two qubits and a resonator is considered in the presence of different sources of noise, bringing to light the possibility of making the two qubits evolve in a synchronized way. A direct qubit–qubit interaction turns out to be a crucial ingredient, as well as the dissipation processes involving the resonator. The detrimental role of the local dephasing of the qubits is also taken into account.

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11

Kumar, Preethika, and Steven R. Skinner. "Universal quantum computing in linear nearest neighbor architectures." Quantum Information and Computation 11, no. 3&4 (March 2011): 300–312. http://dx.doi.org/10.26421/qic11.3-4-8.

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We introduce a scheme for realizing universal quantum computing in a linear nearest neighbor architecture with fixed couplings. We first show how to realize a controlled-NOT gate operation between two adjacent qubits without having to isolate the two qubits from qubits adjacent to them. The gate operation is implemented by applying two consecutive pulses of equal duration, but varying amplitudes, on the target qubit. Since only a single control parameter is required in implementing our scheme, it is very efficient. We next show how our scheme can be used to realize single qubit rotations and two-qubit controlled-unitary operations. As most proposals for solid state implementations of a quantum computer use a one-dimensional line of qubits, the schemes presented here will be extremely useful.

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12

Acharya, Rajeev, Igor Aleiner, Richard Allen, Trond I. Andersen, Markus Ansmann, Frank Arute, Kunal Arya, et al. "Suppressing quantum errors by scaling a surface code logical qubit." Nature 614, no. 7949 (February 22, 2023): 676–81. http://dx.doi.org/10.1038/s41586-022-05434-1.

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AbstractPractical quantum computing will require error rates well below those achievable with physical qubits. Quantum error correction1,2 offers a path to algorithmically relevant error rates by encoding logical qubits within many physical qubits, for which increasing the number of physical qubits enhances protection against physical errors. However, introducing more qubits also increases the number of error sources, so the density of errors must be sufficiently low for logical performance to improve with increasing code size. Here we report the measurement of logical qubit performance scaling across several code sizes, and demonstrate that our system of superconducting qubits has sufficient performance to overcome the additional errors from increasing qubit number. We find that our distance-5 surface code logical qubit modestly outperforms an ensemble of distance-3 logical qubits on average, in terms of both logical error probability over 25 cycles and logical error per cycle ((2.914 ± 0.016)% compared to (3.028 ± 0.023)%). To investigate damaging, low-probability error sources, we run a distance-25 repetition code and observe a 1.7 × 10−6 logical error per cycle floor set by a single high-energy event (1.6 × 10−7 excluding this event). We accurately model our experiment, extracting error budgets that highlight the biggest challenges for future systems. These results mark an experimental demonstration in which quantum error correction begins to improve performance with increasing qubit number, illuminating the path to reaching the logical error rates required for computation.

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13

HUO, WEN YI, and GUI LU LONG. "GENERATING QUANTUM ENTANGLEMENT IN SCALABLE SUPERCONDUCTING CHARGE QUBITS." International Journal of Quantum Information 05, no. 06 (December 2007): 829–36. http://dx.doi.org/10.1142/s0219749907003286.

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We propose an efficient scheme to generate quantum entanglement in scalable superconducting charge qubits with a one-dimensional superconducting transmission line resonator (STLR) as data bus. The coupling between qubit and data bus may be turned on and off by controlling the DC gate voltage and externally applied flux of the superconducting charge qubit. In our proposal, the entanglement between two arbitrary qubits and W states of three qubits can be generated quickly and easily.

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14

Childs, Andrew M., Debbie Leung, Laura Mancinska, and Maris Ozols. "Characterization of universal two-qubit Hamiltonians." Quantum Information and Computation 11, no. 1&2 (January 2011): 19–39. http://dx.doi.org/10.26421/qic11.1-2-3.

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Suppose we can apply a given 2-qubit Hamiltonian H to any (ordered) pair of qubits. We say H is n-universal if it can be used to approximate any unitary operation on n qubits. While it is well known that almost any 2-qubit Hamiltonian is 2-universal, an explicit characterization of the set of non-universal 2-qubit Hamiltonians has been elusive. Our main result is a complete characterization of 2-non-universal 2-qubit Hamiltonians. In particular, there are three ways that a 2-qubit Hamiltonian $H$ can fail to be universal: (1) H shares an eigenvector with the gate that swaps two qubits, (2) H acts on the two qubits independently (in any of a certain family of bases), or (3) H has zero trace (with the third condition relevant only when the global phase of the unitary matters). A 2-non-universal 2-qubit Hamiltonian can still be n-universal for some n \geq 3. We give some partial results on 3-universality.

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15

Cai, J. M., Z. W. Zhou, and G. C. Guo. "Fully multi-qubit entangled states." Quantum Information and Computation 7, no. 8 (November 2007): 766–74. http://dx.doi.org/10.26421/qic7.8-6.

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We investigate the properties of different levels of entanglement in graph states which correspond to connected graphs. Combining the operational definition of graph states and the postulates of entanglement measures, we prove that in connected graph states of $N$ qubits there is no genuine $k$-qubit entanglement, $2\leq k\leq N-1$, among every $k$ qubits. These results about connected graph states naturally lead to the definition of fully multi-qubit entangled states. We also find that the connected graph states of four qubits is one but not the only one class of fully four-qubit entangled states.

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16

Ahsan, Muhammad, and Syed Abbas Zilqurnain Naqvi. "Performance of topological quantum error correction in the presence of correlated noise." Quantum Information and Computation 18, no. 9&10 (August 2018): 743–78. http://dx.doi.org/10.26421/qic18.9-10-2.

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We investigate the efficacy of topological quantum error-correction in correlated noise model which permits collective coupling of all the codeword qubits to the same non-Markovian environment. In this noise model, the probability distribution over set of phase-flipped qubits, decays sub-exponentially in the size of the set and carries non-trivial likelihood of the occurring large numbers of qubits errors. We find that in the presence of noise correlation, one cannot guarantee arbitrary high computational accuracy simply by incrementing the codeword size while retaining constant noise level per qubit operation. However, if instead, per-operation qubit error probability in an n-qubits long codeword is reduced O(\sqrt{n}) times below the accuracy threshold, arbitrarily accurate quantum computation becomes feasible with acceptable scaling of the codeword size. Our results suggest that progressively reducing noise level in qubits and gates is as important as continuously integrating more qubits to realize scalable and reliable quantum computer.

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17

Javed, Shamiya, Ranjana Prakash, and Hari Prakash. "High success perfect transmission of 1-qubit information using purposefully delayed sharing of non-maximally entangled 2-qubit resource and repeated generalized Bell-state measurements." International Journal of Quantum Information 19, no. 02 (March 2021): 2150015. http://dx.doi.org/10.1142/s0219749921500155.

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We propose a new scheme in which perfect transmission of 1-qubit information is achieved with high success using purposefully delayed sharing of non-maximally entangled 2-qubit resource and repeated generalized Bell-state measurements (GBSM). Alice possesses initially all qubits and she makes repeated GBSM on the pair of qubits, consisting of (1) the qubit of information state and (2) one of the two entangled resource qubits (taken alternately) until transmission with perfect fidelity is indicated. Alice then sends to Bob, the qubit not used in the last GBSM and also the result of this GBSM and Bob applies a suitable unitary transformation to replicate exactly the information state. Continued probabilistic transmission with unit fidelity is achieved by changing continuously the generalized Bell basis and also the pair of measured qubits of the collapsed states. We calculate the success probability up to the third repeated attempt of GBSM and plot it with concurrence of the entangled resource state. We also discuss the maximal average fidelity.

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18

Kjaergaard, Morten, Mollie E. Schwartz, Jochen Braumüller, Philip Krantz, Joel I. J. Wang, Simon Gustavsson, and William D. Oliver. "Superconducting Qubits: Current State of Play." Annual Review of Condensed Matter Physics 11, no. 1 (March 10, 2020): 369–95. http://dx.doi.org/10.1146/annurev-conmatphys-031119-050605.

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Superconducting qubits are leading candidates in the race to build a quantum computer capable of realizing computations beyond the reach of modern supercomputers. The superconducting qubit modality has been used to demonstrate prototype algorithms in the noisy intermediate-scale quantum (NISQ) technology era, in which non-error-corrected qubits are used to implement quantum simulations and quantum algorithms. With the recent demonstrations of multiple high-fidelity, two-qubit gates as well as operations on logical qubits in extensible superconducting qubit systems, this modality also holds promise for the longer-term goal of building larger-scale error-corrected quantum computers. In this brief review, we discuss several of the recent experimental advances in qubit hardware, gate implementations, readout capabilities, early NISQ algorithm implementations, and quantum error correction using superconducting qubits. Although continued work on many aspects of this technology is certainly necessary, the pace of both conceptual and technical progress in recent years has been impressive, and here we hope to convey the excitement stemming from this progress.

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19

TAKAHASHI, YASUHIRO. "AN APPROXIMATELY UNIVERSAL SET CONSISTING OF TWO OBSERVABLES." International Journal of Quantum Information 09, no. 06 (September 2011): 1393–412. http://dx.doi.org/10.1142/s021974991100809x.

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We consider the problem of minimizing the resources required for approximate universality in measurement-only quantum computation. This problem is important not only for realizing a quantum computer, but also for understanding the computational power of quantum computation. The resources we focus on are observables, which describe projective measurements, and ancillary qubits. We show that, if we are allowed to use two ancillary qubits, the set of observables { cos (π/8)X - sin (π/8)Y ,Z ⊗ X} is approximately universal for quantum computation. This is the first construction of an approximately universal set consisting only of one one-qubit observable and one two-qubit observable. Using the proof of the approximate universality, we also show that, if we are allowed to use two initialized ancillary qubits, one two-qubit observable is sufficient for graph state preparation. The use of only one two-qubit observable is optimal in terms of the number of observables available and the number of qubits to be measured jointly.

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20

Essammouni, K., A. Chouikh, T. Said, and M. Bennai. "niSWAP and NTCP gates realized in a circuit QED system." International Journal of Geometric Methods in Modern Physics 14, no. 07 (March 7, 2017): 1750100. http://dx.doi.org/10.1142/s0219887817501006.

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Based on superconducting qubit coupled to a resonator driven by a strong microwave field, we propose a method to implement two quantum logic gates ([Formula: see text]SWAP and NTCP gates) of one qubit simultaneously controlling [Formula: see text] qubits selected from [Formula: see text] qubits in a circuit QED [Formula: see text] by introducing qubit–qubit interaction. The interaction between the qubits and the circuit QED can be achieved by tuning the gate voltage and the external flux. The operation times of the logic gates are much smaller than the decoherence time and dephasing time. Moreover, the numerical simulation under the influence of the gates operations shows that the scheme could be achieved efficiently with presently available techniques.

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21

Merkli, M., G. P. Berman, F. Borgonovi, and V. I. Tsifrinovich. "Creation of Two-Particle Entanglement in Open Macroscopic Quantum Systems." Advances in Mathematical Physics 2012 (2012): 1–15. http://dx.doi.org/10.1155/2012/375182.

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We consider an open quantum system ofNnot directly interacting spins (qubits) in contact with both local and collective thermal environments. The qubit-environment interactions are energy conserving. We trace out the variables of the thermal environments andN−2qubits to obtain the time-dependent reduced density matrix for two arbitrary qubits. We numerically simulate the reduced dynamics and the creation of entanglement (concurrence) as a function of the parameters of the thermal environments and the number of qubits,N. Our results demonstrate that the two-qubit entanglement generally decreases asNincreases. We show analytically that, in the limitN→∞, no entanglement can be created. This indicates that collective thermal environments cannot create two-qubit entanglement when many qubits are located within a region of the size of the environment coherence length. We discuss possible relevance of our consideration to recent quantum information devices and biosystems.

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Daoud, M., R. Ahl Laamara, and S. Seddik. "A recursive approach for geometric quantifiers of quantum correlations in multiqubit Schrödinger cat states." International Journal of Modern Physics B 29, no. 19 (July 21, 2015): 1550124. http://dx.doi.org/10.1142/s0217979215501246.

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A recursive approach to determine the Hilbert–Schmidt measure of pairwise quantum discord in a special class of symmetric states of k qubits is presented. We especially focus on the reduced states of k qubits obtained from a balanced superposition of symmetric n-qubit states (multiqubit Schrödinger cat states) by tracing out n-k particles (k = 2, 3, …, n-1). Two pairing schemes are considered. In the first one, the geometric discord measuring the correlation between one qubit and the parity grouping (k-1) qubits is explicitly derived. This uses recursive relations between the Fano–Bloch correlation matrices associated with subsystems comprising k, k-1, … and two particles. A detailed analysis is given for two-, three- and four-qubit systems. In the second scheme, the subsystem comprising the (k-1) qubits is mapped into a system of two logical qubits. We show that these two bipartition schemes are equivalents in evaluating the pairwise correlation in multiqubits systems. The explicit expressions of classical states presenting zero discord are derived.

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Li, Xiangrong, and Dafa Li. "Rank-based SLOCC classification for odd $n$ qubits." Quantum Information and Computation 11, no. 7&8 (July 2011): 695–705. http://dx.doi.org/10.26421/qic11.7-8-10.

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We study the entanglement classification under stochastic local operations and classical communication (SLOCC) for odd n-qubit pure states. For this purpose, we introduce the rank with respect to qubit i for an odd n-qubit state. The ranks with respect to qubits 1,2, ... n give rise to the classification of the space of odd $n$ qubits into 3^n families.

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24

Ma, Sheng-li, Jing Zhang, Xin-ke Li, Ya-long Ren, Ji-kun Xie, Ming-tao Cao, and Fu-li Li. "Coupling-modulation–mediated generation of stable entanglement of superconducting qubits via dissipation." EPL (Europhysics Letters) 135, no. 6 (September 1, 2021): 63001. http://dx.doi.org/10.1209/0295-5075/ac2b5c.

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Abstract We propose an experimentally feasible scheme for the dissipative generation of stable entanglement of superconducting qubits via coupling modulation without applying any drives on qubits and resonator. Firstly, we study the circuit of one superconducting transmission line resonator coupled to two separated qubits via two superconducting quantum interference devices (SUQIDs). By modulating the inductance of the SQUIDs via external fluxes, we can tailor an appropriate qubit-resonator coupling with both red- and blue-sideband interactions. Combined with the photon loss of the resonator, the two qubits can be autonomously steered into a long-lived entangled state with high fidelity. Moreover, we extend the model to one resonator coupled to two separated qubit chains, each of which contains N linearly coupled superconducting qubits. We show that the lossy resonator can drive the whole system into a unique dark state, i.e., a series of N entangled pairs of qubits across the chains can be stabilized at the stationary state. So, the present work enables the preparation of a stable long-range entangled state between the two qubits in the end sites of the chains, which plays an important role for implementing scalable quantum computation and quantum communication.

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Gennaro, Giuseppe. "Relaxation Due to the Partial Swap Collisions with a Random Reservoir." Open Systems & Information Dynamics 18, no. 04 (December 2011): 353–62. http://dx.doi.org/10.1142/s1230161211000248.

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We analyze the dynamics of the system consisting of a qubit sequentially interacting with a chain of qubits that are initially individually in a pure random state. Each pairwise collision has been modeled as a partial swap transformation. The relaxation to equilibrium of the purity of the system qubit, averaged over all the initial states of the environment, is analytically computed. In particular, we show that the steady state depends on the parameter η of the partial swap transformation. Finally, we investigate aspects of the entanglement dynamics for qubits and show that such process can create typical multipartite entanglement between the system qubit and the qubits of the chain.

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XU, PENG, LIE WU, and NIAN-QUAN JIANG. "REALIZATION OF 1 → n CONTROLLED PHASE GATE IN CAVITY QED." International Journal of Quantum Information 09, no. 02 (March 2011): 773–78. http://dx.doi.org/10.1142/s021974991100771x.

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A scheme for realizing a multi-qubit phase gate with one control qubit simultaneously controlling n target qubits in a cavity QED system is proposed. The operation time of the gate is independent of the number n of qubits. The realizability of the gate with current technology is also discussed.

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Said, Taoufik, Abdelhaq Chouikh, Karima Essammouni, and Mohamed Bennai. "Realizing an N-two-qubit quantum logic gate in a cavity QED with nearest qubit--qubit interaction." Quantum Information and Computation 16, no. 5&6 (April 2016): 465–82. http://dx.doi.org/10.26421/qic16.5-6-4.

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We propose an effective way for realizing a three quantum logic gates (NTCP gate, NTCP-NOT gate and NTQ-NOT gate) of one qubit simultaneously controlling N target qubits based on the qubit-qubit interaction. We use the superconducting qubits in a cavity QED driven by a strong microwave field. In our scheme, the operation time of these gates is independent of the number N of qubits involved in the gate operation. These gates are insensitive to the initial state of the cavity QED and can be used to produce an analogous CNOT gate simultaneously acting on N qubits. The quantum phase gate can be realized in a time (nanosecond-scale) much smaller than decoherence time and dephasing time (microsecond-scale) in cavity QED. Numerical simulation under the influence of the gate operations shows that the scheme could be achieved efficiently within current state-of-the-art technology.

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Dykman, M. I., L. F. Santos, M. Shapiro, and F. M. Izrailev. "On-site localization of excitations." Quantum Information and Computation 5, no. 4&5 (July 2005): 335–49. http://dx.doi.org/10.26421/qic5.45-5.

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We demonstrate that, in a quantum computer with perpetually coupled qubits, all excitations can be confined to their sites (qubits) even without refocusing. The on-site localization is obtained by constructing a sequence of qubit energies that efficiently suppresses resonant hopping. The time during which a many-excitation state remains strongly localized in an infinite chain can exceed the reciprocal hopping frequency by $\agt 10^5$ already for a moderate bandwidth of qubit energies. The proposed energy sequence is also convenient for performing quantum operations on the qubits.

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Bashkirov, Evgeny K. "Entanglement of two superconducting qubits induced by a thermal noise of a cavity with Kerr medium taking into account the atomic coherence." Physics of Wave Processes and Radio Systems 25, no. 1 (March 29, 2022): 7–15. http://dx.doi.org/10.18469/1810-3189.2022.25.1.7-15.

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The system consisting of two identical artificial atoms (qubits), resonantly interacting with the mode of quantum field of an ideal microwave cavity in the presence of Kerr nonlinearity, is considered. For the considered model, an exact solution of the quantum Liouville equation for the full density matrix of the system two qubits + resonator field mode is obtained. To solve the quantum equation of evolution, the representation of dressed states, that is, the eigenfunctions of the Hamiltonian, was used. A complete set of dressed states of the considered model is found. With its help, the solution of the evolution equation was initially found for coherent initial states of qubits and Fock states of the field, that is, states with a certain number of photons in the resonator mode. Then, the above solution was generalized to the case of the thermal state of the resonator field. A reduced density matrix of two qubits is found by averaging over the field variables. The two-qubit density matrix is used to calculate the parameter of qubit entanglement in the analytical form. Concurrence was chosen as a quantitative criterion for qubit entanglement. A numerical simulation of the time dependence of the consistency of qubits for various parameters of the model and the initial states of qubits was carried out. The most interesting result seems to be that taking into account the initial coherence of qubits in the model with Kerr nonlinearity leads to a significant increase in the maximum degree of entanglement of qubits induced by the thermal field, even in the case of high intensities of the resonator field.

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Yamamoto, Satoru, Shigeaki Nakazawa, Kenji Sugisaki, Kazunobu Sato, Kazuo Toyota, Daisuke Shiomi, and Takeji Takui. "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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Baßler, Pascal, Matthias Zipper, Christopher Cedzich, Markus Heinrich, Patrick H. Huber, Michael Johanning, and Martin Kliesch. "Synthesis of and compilation with time-optimal multi-qubit gates." Quantum 7 (April 20, 2023): 984. http://dx.doi.org/10.22331/q-2023-04-20-984.

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We develop a method to synthesize a class of entangling multi-qubit gates for a quantum computing platform with fixed Ising-type interaction with all-to-all connectivity. The only requirement on the flexibility of the interaction is that it can be switched on and off for individual qubits. Our method yields a time-optimal implementation of the multi-qubit gates. We numerically demonstrate that the total multi-qubit gate time scales approximately linear in the number of qubits. Using this gate synthesis as a subroutine, we provide compilation strategies for important use cases: (i) we show that any Clifford circuit on n qubits can be implemented using at most 2n multi-qubit gates without requiring ancilla qubits, (ii) we decompose the quantum Fourier transform in a similar fashion, (iii) we compile a simulation of molecular dynamics, and (iv) we propose a method for the compilation of diagonal unitaries with time-optimal multi-qubit gates, as a step towards general unitaries. As motivation, we provide a detailed discussion on a microwave controlled ion trap architecture with magnetic gradient induced coupling (MAGIC) for the generation of the Ising-type interactions.

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Aldeghi, Michele, Rolf Allenspach, and Gian Salis. "Modular nanomagnet design for spin qubits confined in a linear chain." Applied Physics Letters 122, no. 13 (March 27, 2023): 134003. http://dx.doi.org/10.1063/5.0139670.

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On-chip micromagnets enable electrically controlled quantum gates on electron spin qubits. Extending the concept to a large number of qubits is challenging in terms of providing large enough driving gradients and individual addressability. Here, we present a design aimed at driving spin qubits arranged in a linear chain and strongly confined in directions lateral to the chain. Nanomagnets are placed laterally to the one side of the qubit chain, one nanomagnet per two qubits. The individual magnets are “U”-shaped, such that the magnetic shape anisotropy orients the magnetization alternately toward and against the qubit chain even if an external magnetic field is applied along the qubit chain. The longitudinal and transversal stray field components serve as addressability and driving fields. Using micromagnetic simulations, we calculate driving and dephasing rates and the corresponding qubit quality factor. The concept is validated with spin-polarized scanning electron microscopy of Fe nanomagnets fabricated on silicon substrates, finding excellent agreement with micromagnetic simulations. Several features required for a scalable spin qubit design are met in our approach: strong driving and weak dephasing gradients, reduced crosstalk and operation at low external magnetic fields.

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Chien, Wei-Chen, Shun-Jhou Jhan, Kuei-Lin Chiu, Yu-xi Liu, Eric Kao, and Ching-Ray Chang. "Cryogenic Materials and Circuit Integration for Quantum Computers." Journal of Electronic Materials 49, no. 11 (September 28, 2020): 6844–58. http://dx.doi.org/10.1007/s11664-020-08442-x.

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Abstract Over the last decade, quantum computing has experienced significant changes and captured worldwide attention. In particular, superconducting qubits have become the leading candidates for scalable quantum computers, and a number of cryogenic materials have scientifically demonstrated their potential uses in constructing qubit chips. However, because of insufficient coherence time, establishing a robust and scalable quantum platform is still a long-term goal. Another consideration is the control circuits essential to initializing, operating and measuring the qubits. To keep noise low, control circuits in close proximity to the qubits require superior reliability in the cryogenic environment. The realization of the quantum advantage demands qubits with appropriate circuitry designs to maintain long coherence times and entanglement. In this work, we briefly summarize the current status of cryogenic materials for qubits and discuss typical cryogenic circuitry designs and integration techniques for qubit chips. In the end, we provide an assessment of the prospects of quantum computers and some other promising cryogenic materials.

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DOLL, ROLAND, MARTIJN WUBS, SIGMUND KOHLER, and PETER HÄNGGI. "FIDELITY AND ENTANGLEMENT OF A SPATIALLY EXTENDED LINEAR THREE-QUBIT REGISTER." International Journal of Quantum Information 06, supp01 (July 2008): 681–87. http://dx.doi.org/10.1142/s0219749908003955.

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We study decoherence of a three-qubit array coupled to substrate phonons. Assuming an initial three-qubit entangled state that would be decoherence-free for identical qubit positions, allows us to focus on non-Markovian effects of the inevitable spatial qubit separation. It turns out that the coherence is most affected when the qubits are regularly spaced. Moreover, we find that up to a constant scaling factor, two-qubit entanglement is not influenced by the presence of the third qubit, even though all qubits interact via the phonon field.

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35

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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36

Said, T., A. Chouikh, K. Essammouni, and M. Bennai. "Implementing N-quantum phase gate via circuit QED with qubit–qubit interaction." Modern Physics Letters B 30, no. 05 (February 20, 2016): 1650050. http://dx.doi.org/10.1142/s0217984916500500.

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We propose a method for realizing a quantum phase gate of one qubit simultaneously controlling [Formula: see text] target qubits based on the qubit–qubit interaction. We show how to implement the proposed gate with one transmon qubit simultaneously controlling [Formula: see text] transmon qubits in a circuit QED driven by a strong microwave field. In our scheme, the operation time of this phase gate is independent of the number [Formula: see text] of qubits. On the other hand, this gate can be realized in a time of nanosecond-scale much smaller than the decoherence time and dephasing time both being the time of microsecond-scale. Numerical simulation of the occupation probabilities of the second excited lever shows that the scheme could be achieved efficiently within current technology.

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Zhao, Peng, Yingshan Zhang, Guangming Xue, Yirong Jin, and Haifeng Yu. "Tunable coupling of widely separated superconducting qubits: A possible application toward a modular quantum device." Applied Physics Letters 121, no. 3 (July 18, 2022): 032601. http://dx.doi.org/10.1063/5.0097521.

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In addition to striving to assemble more and more qubits in a single monolithic quantum device, taking a modular design strategy may mitigate numerous engineering challenges for achieving large-scalable quantum processors with superconducting qubits. Nevertheless, a major challenge in the modular quantum device is how to realize high-fidelity entanglement operations on qubits housed in different modules while preserving the desired isolation between modules. In this work, we propose a conceptual design of a modular quantum device, where nearby modules are spatially separated by centimeters. In principle, each module can contain tens of superconducting qubits and can be separately fabricated, characterized, packaged, and replaced. By introducing a bridge module between nearby qubit modules and taking the coupling scheme utilizing a tunable bus, tunable coupling of qubits that are housed in nearby qubit modules could be realized. Given physically reasonable assumptions, we expect that sub-100-ns two-qubit gates for qubits housed in nearby modules, which are spatially separated by more than two centimeters could be obtained. In this way, the inter-module gate operations are promising to be implemented with gate performance comparable with that of intra-module gate operations. Moreover, with the help of through-silicon vias technologies, this long-range coupling scheme may also allow one to implement inter-module couplers in a multi-chip stacked processor. Thus, the tunable longer-range coupling scheme and the proposed modular architecture may provide a promising foundation for solving challenges toward large-scale quantum information processing with superconducting qubits.

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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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Ivanyos, G., A. B. Nagy, and L. Ronyai. "Constructions for quantum computing with symmetrized gates." Quantum Information and Computation 8, no. 5 (May 2008): 411–29. http://dx.doi.org/10.26421/qic8.5-4.

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We investigate constructions for simulating quantum computers with a polynomial slowdown on ensembles composed of qubits on which symmetrized versions of one- and two-qubit gates can be performed. The simulation is based on taking Lie commutators of symmetrized Hamiltonians to extract Hamiltonians at desired local positions. During the simulation, only a part of the qubits can be used for storing information, the others are left unchanged by the commutators. We propose constructions for various symmetry groups where a pretty large fraction of the qubits can be used. As a few of the other qubits need to be set to one, our construction requires individual initialization of some of the qubits.

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40

Bashkirov, Eugene K. "Thermal entanglement in two-atom Tavis–Cummings model with taking into account the dipole-dipole interaction." Physics of Wave Processes and Radio Systems 26, no. 2 (July 1, 2023): 9–17. http://dx.doi.org/10.18469/1810-3189.2023.26.2.9-17.

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Background. Interest in the study of entangled states of systems of natural and artificial atoms (qubits) interacting with selected modes of microwave resonators is associated with their use as logic elements of quantum computers. At the same time, the most important task of the physics of quantum computing is the choice of the most effective mechanisms for manipulating and controlling the entangled states of qubits in such devices. Aim. The dynamics of the entanglement of two dipole-coupled superconducting Josephson qubits induced by a thermal noise of a coplanar resonator is studied for various initial states of the qubits. Methods. Based on the exact solution of the quantum Liouville equation for the whole density matrix of the system under consideration, the time behavior of the qubit entanglement parameter (negativity) is found for chaotic thermal, pure separable, and entangled initial states of qubits. Results. It is shown that the entanglement of qubits induced by the thermal noise of the resonator is possible for both the chaotic thermal states and separable states of qubits, except the case when both qubits are excited. It has also been found that, for small values of the dipole–dipole interaction parameter, taking this interaction into account leads to an increase in the degree of entanglement. For values of the dipole-dipole interaction parameter greater than some limit value, the opposite effect takes place. It is found that for entangled initial states of qubits, the inclusion of direct interaction has a small effect on the entanglement dynamics. It is shown that the initial coherence of qubit states can lead to a significant increase in the degree of their entanglement in the presence of a dipole–dipole interaction. Conclusion. The dipole-dipole interaction can be used as an effective mechanism for qubits entanglement manipulation and controlling.

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Kamjam, Nattawut, Poramet Pathumsoot, and Areeya Chantasri. "Random-telegraph noise mitigation and qubit decoherence in solid-state experiments." Journal of Physics: Conference Series 2431, no. 1 (January 1, 2023): 012102. http://dx.doi.org/10.1088/1742-6596/2431/1/012102.

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Abstract We investigate the recently proposed theoretical models and algorithms in Song et al. [1] for mitigating decoherence in solid-state qubit systems, where qubits are affected by charge (random telegraph) noises. The model includes a setup of a logical qubit (data qubit) and a spectator qubit, where the latter is used as a probe of the noise. The probe results can be used in correcting the phase error in order to improve the decoherence of the data qubit. In this work, we apply the proposed model with parameters extracted from recent solid-state qubit experiments. We extract parameters such as the noise switching rates, the qubit sensitivities to noise, and the measurement dead time. Using these parameters, we then numerically simulate the data qubit’s phase and the qubit decoherence. We also show that the proposed phase-correction technique using Bayesian estimation can improve the data qubit decoherence significantly.

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Bagrov, A. R., and E. K. Bashkirov. "ENTANGLEMNT IN NONLINEAR THREE-QUBITS JAYNES — CUMMINGS MODEL." Vestnik of Samara University. Natural Science Series 29, no. 1 (June 26, 2023): 89–101. http://dx.doi.org/10.18287/2541-7525-2023-29-1-89-101.

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In this paper, we investigated the dynamics of entanglement of pairs of qubits in a system of three identical qubits that interact non-resonantly with the selected mode of a microwave resonator without loss with the Kerr medium by means of single-photon transitions. We have found solutions to the quantum time Schrodinger equation for the total wave function of the system for the initial separable, biseparable and true entangled states of qubits and the Fock initial state of the resonator field. Based on these solutions, the criterion of entanglement of qubit pairs negativity is calculated. The results of numerical simulation of the negativity of qubit pairs have shown that the presence of disorder and Kerr nonlinearity in the case of an initial non-entangled state of a pair of qubits can lead to a significant increase in the degree of their entanglement. In the case of an initial entangled state of a pair of qubits, the disorder and the Kerr medium can lead to a significant stabilization of the initial entanglement

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LI, XI-HAN, BAO-KUI ZHAO, YU-BO SHENG, FU-GUO DENG, and HONG-YU ZHOU. "FAULT TOLERANT QUANTUM KEY DISTRIBUTION BASED ON QUANTUM DENSE CODING WITH COLLECTIVE NOISE." International Journal of Quantum Information 07, no. 08 (December 2009): 1479–89. http://dx.doi.org/10.1142/s021974990900595x.

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We present two robust quantum key distribution protocols against two kinds of collective noise, following some ideas in quantum dense coding. Three-qubit entangled states are used as quantum information carriers, two of which form the logical qubit, which is invariant with a special type of collective noise. The information is encoded on logical qubits with four unitary operations, which can be read out faithfully with Bell-state analysis on two physical qubits and a single-photon measurement on the other physical qubit, not three-photon joint measurements. Two bits of information are exchanged faithfully and securely by transmitting two physical qubits through a noisy channel. When the losses in the noisy channel is low, these protocols can be used to transmit a secret message directly in principle.

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44

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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Golkar, S., and M. K. Tavassoly. "Atomic motion and dipole–dipole effects on the stability of atom–atom entanglement in Markovian/non-Markovian reservoir." Modern Physics Letters A 34, no. 10 (March 28, 2019): 1950077. http://dx.doi.org/10.1142/s0217732319500779.

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In this paper, we consider the entanglement dynamics of two identical qubits (two-level atoms) accompanied by dipole–dipole interaction within a common reservoir in the strong and weak coupling regimes. We suppose that the qubits move in the reservoir which is at zero temperature. Using the time-dependent Schrödinger equation, the state vector of the qubits-reservoir system is obtained by which we can evaluate the concurrence as a suitable measure of entanglement between the two qubits. The results show that by choosing special initial conditions for the qubits, a different dynamical behavior of entanglement is visible in such a way that entanglement protection occurs. Also, we find that the qubit motion in the absence of dipole–dipole interaction leads to preservation or at least more slowly decay of entanglement. However, in the presence of dipole–dipole interaction with the movement of qubits, different results can be observed which depend on the initial states of the qubits, i.e. entanglement may or may not be protected.

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Conrad, J., C. Chamberland, N. P. Breuckmann, and B. M. Terhal. "The small stellated dodecahedron code and friends." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 376, no. 2123 (May 28, 2018): 20170323. http://dx.doi.org/10.1098/rsta.2017.0323.

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We explore a distance-3 homological CSS quantum code, namely the small stellated dodecahedron code, for dense storage of quantum information and we compare its performance with the distance-3 surface code. The data and ancilla qubits of the small stellated dodecahedron code can be located on the edges respectively vertices of a small stellated dodecahedron, making this code suitable for three-dimensional connectivity. This code encodes eight logical qubits into 30 physical qubits (plus 22 ancilla qubits for parity check measurements) in contrast with one logical qubit into nine physical qubits (plus eight ancilla qubits) for the surface code. We develop fault-tolerant parity check circuits and a decoder for this code, allowing us to numerically assess the circuit-based pseudo-threshold. This article is part of a discussion meeting issue ‘Foundations of quantum mechanics and their impact on contemporary society’.

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Benenti, G., G. Casati, and S. Montangero. "Stability of Quantum Computing in the Presence of Imperfections." International Journal of Modern Physics B 17, no. 22n24 (September 30, 2003): 3932–46. http://dx.doi.org/10.1142/s0217979203021927.

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We model an isolated quantum computer as a two-dimensional lattice of qubits (spin halves) with fluctuations in individual qubit energies and residual short-range inter-qubit couplings. We show that above a critical inter-qubit coupling strength, quantum chaos sets in and this results in the interaction induced dynamical thermalization and occupation numbers well described by the Fermi–Dirac distribution. This thermalization destroys the noninteracting qubit structure and sets serious requirements for the quantum computer operability. We then construct a quantum algorithm which uses qubits in an optimal way and efficiently simulates a physical model with rich and complex dynamics. The numerical study of the effect of static imperfections in the quantum computer hardware shows that the main elements of the phase space structures are accurately reproduced up to a time scale which is polynomial in the number of qubits. The errors generated by these imperfections are more significant than the errors of random noise in gate operations.

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Ben-Aryeh, Y., and A. Mann. "Explicit constructions of all separable two-qubits density matrices and related problems for three-qubits systems." International Journal of Quantum Information 13, no. 08 (December 2015): 1550061. http://dx.doi.org/10.1142/s0219749915500616.

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Explicitly separable density matrices are constructed for all separable two-qubits states based on Hilbert–Schmidt (HS) decompositions. For density matrices which include only two-qubits correlations the number of HS parameters is reduced to 3 by using local rotations, and for two-qubits states which include single qubit measurements, the number of parameters is reduced to 4 by local Lorentz transformations. For both cases, we related the absolute values of the HS parameters to probabilities, and the outer products of various Pauli matrices were transformed to pure state density matrices products. We discuss related problems for three-qubits. For n-qubits correlation systems ([Formula: see text]) the sufficient condition for separability may be improved by local transformations, related to high order singular value decompositions (SVDs).

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Чуйкин, О. А., Я. С. Гринберг, and А. А. Штыгашев. "Затухание вакуумных осцилляций Раби в двухкубитной структуре в высокодобротном резонаторе." Физика твердого тела 62, no. 9 (2020): 1407. http://dx.doi.org/10.21883/ftt.2020.09.49762.13h.

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In this work, we study the damping of vacuum Rabi oscillations for a system of two superconducting solid-state qubits placed in a high-quality microwave resonator. Two different cases are considered: the first qubit is excited at the initial moment, and the initial state is an entangled symmetric and antisymmetric pair. The dependence of the damping on various parameters, primarily on the photon-qubit coupling and on the distance between qubits, is studied in detail. It is shown that for some parameters, the relaxation time of the excited qubit is significantly longer than that for a single qubit in the cavity.

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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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Journal articles on the topic 'Qubits'

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