PRX Quantum

Journal: PRX Quantum

Publisher

American Physical Society

ISSN

2691-3399

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Publications 1 - 10 of 33

Quantum Clocks are More Accurate Than Classical Ones
Woods, Mischa P.; Silva, Ralph; Pütz, Gilles; et al. (2022)
PRX Quantum
A clock is, from an information-theoretic perspective, a system that emits information about time. One may therefore ask whether the theory of information imposes any constraints on the maximum precision of clocks. Here we show a quantum-over-classical advantage for clocks or, more precisely, the task of generating information about what time it is. The argument is based on information-theoretic considerations: we analyze how the accuracy of a clock scales with its size, measured in terms of the number of bits that could be stored in it. We find that a quantum clock can achieve a quadratically improved accuracy compared to a purely classical one of the same size.
Tailoring Three-Dimensional Topological Codes for Biased Noise
Huang, Eric; Pesah, Arthur; Chubb, Christopher; et al. (2023)
PRX Quantum
Tailored topological stabilizer codes in two dimensions have been shown to exhibit high-storagethreshold error rates and improved subthreshold performance under biased Pauli noise. Three-dimensional (3D) topological codes can allow for several advantages including a transversal implementation of nonClifford logical gates, single-shot decoding strategies, and parallelized decoding in the case of fracton codes, as well as construction of fractal-lattice codes. Motivated by this, we tailor 3D topological codes for enhanced storage performance under biased Pauli noise. We present Clifford deformations of various 3D topological codes, such that they exhibit a threshold error rate of 50% under infinitely biased Pauli noise. Our examples include the 3D surface code on the cubic lattice, the 3D surface code on a checkerboard lattice that lends itself to a subsystem code with a single-shot decoder, and the 3D color code, as well as fracton models such as the X-cube model, the Sierpinski model, and the Haah code. We use the belief propagation with ordered statistics decoder (BP OSD) to study threshold error rates at finite bias. We also present a rotated layout for the 3D surface code, which uses roughly half the number of physical qubits for the same code distance under appropriate boundary conditions. Imposing coprime periodic dimensions on this rotated layout leads to logical operators of weight O(n) at infinite bias and a corresponding exp[-O(n)] subthreshold scaling of the logical failure rate, where n is the number of physical qubits in the code. Even though this scaling is unstable due to the existence of logical representations with O(1) low-rate and O(n2/3) high-rate Pauli errors, the number of such representations scales only polynomially for the Clifford-deformed code, leading to an enhanced effective distance.
Embedding Overhead Scaling of Optimization Problems in Quantum Annealing
Könz, Mario S.; Lechner, Wolfgang; Katzgraber, Helmut G.; et al. (2021)
PRX Quantum
In order to treat all-to-all-connected quadratic binary optimization problems (QUBOs) with hard- ware quantum annealers, an embedding of the original problem is required due to the sparsity of the topology of the hardware. The embedding of fully connected graphs-typically found in industrial applications incurs a quadratic space overhead and thus a significant overhead in the time to solution. Here, we investigate this embedding penalty of established planar embedding schemes such as square-lattice embedding, embedding on a chimera lattice, and the Lechner-Hauke-Zoller scheme, using simulated quantum annealing on classical hardware. Large-scale quantum Monte Carlo simulation suggests a polynomial time-to-solution overhead. Our results demonstrate that standard analog quantum annealing hardware is at a disadvantage in comparison to classical digital annealers, as well as gate-model quantum annealers, and could also serve as a benchmark for improvements of the standard quantum annealing protocol.
Experimental Bayesian Calibration of Trapped-Ion Entangling Operations
Gerster, Lukas; Martínez-García, Fernando; Hrmo, Pavel; et al. (2022)
PRX Quantum
The performance of quantum gate operations is experimentally determined by how correctly operational parameters can be determined and set, and how stable these parameters can be maintained. In addition, gates acting on different sets of qubits require unique sets of control parameters. Thus, an efficient multidimensional parameter estimation procedure is crucial to calibrate even medium-sized quantum processors. Here, we develop and characterize an efficient calibration protocol to automatically estimate and adjust experimental parameters of the widely used two-qubit Molmer-Sorensen entangling gate operation in a trapped-ion quantum information processor. The protocol exploits Bayesian parameter estimation methods that include a stopping criterion based on a desired gate infidelity. We experimentally demonstrate a tune-up procedure that leads to a residual median gate infidelity due to miscalibration of 1.3(1) x 10(-3), requiring 1200 +/- 500 experimental cycles, while completing the entire gate calibration procedure in less than one minute, which provides a significant speedup over commonly used manual tune-up routines. This approach is applicable to other quantum information processor architectures with known or sufficiently characterized theoretical models.
High-Fidelity, Multiqubit Generalized Measurements with Dynamic Circuits
Ivashkov, Petr; Uchehara, Gideon; Jiang, Liang; et al. (2024)
PRX Quantum
Generalized measurements, also called positive operator-valued measures (POVMs), can offer advantages over projective measurements in various quantum information tasks. Here, we realize a generalized measurement of one and two superconducting qubits with high fidelity and in a single experimental setting. To do so, we propose a hybrid method, the "Naimark-terminated binary tree," based on a hybridization of Naimark's dilation and binary tree techniques that leverages emerging hardware capabilities for midcircuit measurements and feed-forward control. Furthermore, we showcase a highly effective use of approximate compiling to enhance POVM fidelity in noisy conditions. We argue that our hybrid method scales better toward larger system sizes than its constituent methods and demonstrate its advantage by performing detector tomography of symmetric, informationally complete POVM (SIC POVM). Detector fidelity is further improved through a composite error-mitigation strategy that incorporates twirling and a newly devised conditional readout error mitigation. Looking forward, we expect improvements in approximate compilation and hardware noise for dynamic circuits to enable generalized measurements of larger multiqubit POVMs on superconducting qubits.
Few-Electron Single and Double Quantum Dots in an In As Two-Dimensional Electron Gas
Mittag, Christopher; Koski, Jonne V.; Karalic, Matija; et al. (2021)
PRX Quantum
Most proof-of-principle experiments for spin qubits have been performed with GaAs-based quantum dots because of the excellent control they offer over tunneling barriers and the orbital and spin degrees of freedom. Here we present the first realization of high-quality single and double quantum dots hosted in an InAs two-dimensional electron gas, demonstrating accurate control down to the few-electron regime, where we observe a clear Kondo effect and singlet-triplet spin blockade. We measure an electronic g factor of 16 and a typical magnitude of the random hyperfine fields on the quantum dots of approximately 0.6mT. We estimate the spin-orbit length in the system to be approximately 5−10μm (which is almost 2 orders of magnitude longer than typically measured in InAs nanostructures), achieved by a very symmetric design of the quantum well. These favorable properties put the InAs two-dimensional electron gas on the map as a compelling host for studying fundamental aspects of spin qubits. Furthermore, having weak spin-orbit coupling in a material with a large Rashba coefficient potentially opens up avenues for engineering structures with spin-orbit coupling that can be controlled locally in space and/or time.
Error-Detected State Transfer and Entanglement in a Superconducting Quantum Network
Burkhart, Luke D.; Teoh, James D.; Zhang, Yaxing; et al. (2021)
PRX Quantum
Microwave photons are used to wire up modular quantum processors, but mitigating the effects of loss between modules remains a crucial challenge. We use a low-loss bus resonator to couple bosonic qubits across a superconducting network with protocols made robust to photon loss in the bus. We transfer a multiphoton qubit and track loss events, improving the fidelity to the break-even point with respect to the best uncorrectable encoding. We also demonstrate a entanglement protocol using Hong-Ou-Mandel interference and error detection to prepare a two-photon Bell state with fidelity 94% and success probability 0.79, halving the error obtained with a single photon. This network link also presents new opportunities for resource-efficient direct gates between modules.
Improving the Performance of Deep Quantum Optimization Algorithms with Continuous Gate Sets
Lacroix, Nathan; Hellings, Christoph; Andersen, Christian Kraglund; et al. (2020)
PRX Quantum
Variational quantum algorithms are believed to be promising for solving computationally hard problems on noisy intermediate-scale quantum (NISQ) systems. Gaining computational power from these algorithms critically relies on the mitigation of errors during their execution, which for coherence-limited operations is achievable by reducing the gate count. Here, we demonstrate an improvement of up to a factor of 3 in algorithmic performance for the quantum approximate optimization algorithm (QAOA) as measured by the success probability, by implementing a continuous hardware-efficient gate set using superconducting quantum circuits. This gate set allows us to perform the phase separation step in QAOA with a single physical gate for each pair of qubits instead of decomposing it into two CZ gates and single-qubit gates. With this reduced number of physical gates, which scales with the number of layers employed in the algorithm, we experimentally investigate the circuit-depth-dependent performance of QAOA applied to exact-cover problem instances mapped onto three and seven qubits, using up to a total of 399 operations and up to nine layers. Our results demonstrate that the use of continuous gate sets may be a key component in extending the impact of near-term quantum computers.
Laser-Painted Cavity-Mediated Interactions in a Quantum Gas
Bonifacio, Mariano; Piazza, Francesco; Donner, Tobias Ulrik (2024)
PRX Quantum
Experimental platforms based on ultracold atomic gases have significantly advanced the quantum simulation of complex systems, yet the exploration of phenomena driven by long-range interactions remains a formidable challenge. Currently available methods utilizing dipolar quantum gases or multimode cavities allow us to implement long-range interactions with a 1/𝑟�3 character or with a spatial profile fixed by the mode structure of the vacuum electromagnetic field surrounding the atoms, respectively. Here, we propose an experimental scheme employing laser-painted cavity-mediated interactions, which enables the realization of atom-atom interactions that are fully tunable in range, shape, and sign. Our approach combines the versatility of cavity quantum electrodynamics with the precision of laser manipulation, thus providing a highly flexible platform for simulating and understanding long-range interactions in quantum many-body systems. Our analytical predictions are supported by numerical simulations describing the full dynamics of the atoms, the laser, and the cavity. These reveal the self-organization of domains in the density of the atomic cloud, confirming the finite-range nature of the induced interactions. We demonstrate that there is a wide and experimentally accessible parameter regime in which our protocol works robustly, with negligible heating of the quantum gas. The methodology not only paves the way for exploring new territories in quantum simulation but also enhances the understanding of fundamental physics, potentially leading to the discovery of novel quantum states and phases.
Flip-Chip-Based Fast Inductive Parity Readout of a Planar Superconducting Island
Hinderling, M.; ten Kate, S.C.; Haxell, D.Z.; et al. (2024)
PRX Quantum
The properties of superconducting devices depend sensitively on the parity (even or odd) of the quasiparticles that they contain. Encoding quantum information in the parity degree of freedom is central in several emerging solid-state qubit architectures, including in hybrid superconductor-semiconductor devices. In the latter case, accurate, nondestructive, and time-resolved parity measurements are a challenging issue. Here, we report on control and real-time parity measurement in a superconducting island embedded in a superconducting loop and realized in a hybrid two-dimensional heterostructure using a microwave resonator. To avoid microwave losses impeding time-resolved measurements, the device and readout resonator are located on separate chips, connected via flip-chip bonding, and couple inductively through vacuum. The superconducting resonator detects the parity-dependent circuit inductance, allowing for fast parity readout. We have resolved even- and odd-parity states with a signal-to-noise ratio of SNR≈3 for an integration time of 20μs and a detection fidelity exceeding 98%. The real-time parity measurement shows a state lifetime extending into the millisecond range. Our approach will lead to a better understanding of coherence-limiting mechanisms in superconducting quantum hardware and help to advance inductive-readout schemes for hybrid qubits.

Publications 1 - 10 of 33

Journal: PRX Quantum

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