As we approach the era of quantum advantage, when quantum computers (QCs) can outperform any classical computer on particular tasks, there remains the difficult challenge of how to validate their performance. While algorithmic success can be easily verified in some instances such as number factoring or oracular algorithms, these approaches only provide pass/fail information for a single QC. On the other hand, a comparison between different QCs on the same arbitrary circuit provides a lower-bound for generic validation: a quantum computation is only as valid as the agreement between the results produced on different QCs. Such an approach is also at the heart of evaluating metrological standards such as disparate atomic clocks. In this paper, we report a cross-platform QC comparison using randomized and correlated measurements that results in a wealth of information on the QC systems. We execute several quantum circuits on widely different physical QC platforms and analyze the cross-platform fidelities.

UR - https://arxiv.org/abs/2107.11387 ER - TY - JOUR T1 - Interactive Protocols for Classically-Verifiable Quantum Advantage Y1 - 2021 A1 - Daiwei Zhu A1 - Gregory D. Kahanamoku-Meyer A1 - Laura Lewis A1 - Crystal Noel A1 - Or Katz A1 - Bahaa Harraz A1 - Qingfeng Wang A1 - Andrew Risinger A1 - Lei Feng A1 - Debopriyo Biswas A1 - Laird Egan A1 - Alexandru Gheorghiu A1 - Yunseong Nam A1 - Thomas Vidick A1 - Umesh Vazirani A1 - Norman Y. Yao A1 - Marko Cetina A1 - Christopher Monroe AB -Achieving quantum computational advantage requires solving a classically intractable problem on a quantum device. Natural proposals rely upon the intrinsic hardness of classically simulating quantum mechanics; however, verifying the output is itself classically intractable. On the other hand, certain quantum algorithms (e.g. prime factorization via Shor's algorithm) are efficiently verifiable, but require more resources than what is available on near-term devices. One way to bridge the gap between verifiability and implementation is to use "interactions" between a prover and a verifier. By leveraging cryptographic functions, such protocols enable the classical verifier to enforce consistency in a quantum prover's responses across multiple rounds of interaction. In this work, we demonstrate the first implementation of an interactive quantum advantage protocol, using an ion trap quantum computer. We execute two complementary protocols -- one based upon the learning with errors problem and another where the cryptographic construction implements a computational Bell test. To perform multiple rounds of interaction, we implement mid-circuit measurements on a subset of trapped ion qubits, with subsequent coherent evolution. For both protocols, the performance exceeds the asymptotic bound for classical behavior; maintaining this fidelity at scale would conclusively demonstrate verifiable quantum advantage.

UR - https://arxiv.org/abs/2112.05156 ER - TY - JOUR T1 - Observation of measurement-induced quantum phases in a trapped-ion quantum computer Y1 - 2021 A1 - Crystal Noel A1 - Pradeep Niroula A1 - Daiwei Zhu A1 - Andrew Risinger A1 - Laird Egan A1 - Debopriyo Biswas A1 - Marko Cetina A1 - Alexey V. Gorshkov A1 - Michael Gullans A1 - David A. Huse A1 - Christopher Monroe AB -Many-body open quantum systems balance internal dynamics against decoherence from interactions with an environment. Here, we explore this balance via random quantum circuits implemented on a trapped ion quantum computer, where the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a fault-tolerent threshold. We probe the "pure" phase, where the system is rapidly projected to a deterministic state conditioned on the measurement outcomes, and the "mixed" or "coding" phase, where the initial state becomes partially encoded into a quantum error correcting codespace. We find convincing evidence of the two phases and show numerically that, with modest system scaling, critical properties of the transition clearly emerge.

UR - https://arxiv.org/abs/2106.05881 ER - TY - JOUR T1 - Fault-Tolerant Operation of a Quantum Error-Correction Code Y1 - 2020 A1 - Laird Egan A1 - Dripto M. Debroy A1 - Crystal Noel A1 - Andrew Risinger A1 - Daiwei Zhu A1 - Debopriyo Biswas A1 - Michael Newman A1 - Muyuan Li A1 - Kenneth R. Brown A1 - Marko Cetina A1 - Christopher Monroe AB -Quantum error correction protects fragile quantum information by encoding it in a larger quantum system whose extra degrees of freedom enable the detection and correction of errors. An encoded logical qubit thus carries increased complexity compared to a bare physical qubit. Fault-tolerant protocols contain the spread of errors and are essential for realizing error suppression with an error-corrected logical qubit. Here we experimentally demonstrate fault-tolerant preparation, rotation, error syndrome extraction, and measurement on a logical qubit encoded in the 9-qubit Bacon-Shor code. For the logical qubit, we measure an average fault-tolerant preparation and measurement error of 0.6% and a transversal Clifford gate with an error of 0.3% after error correction. The result is an encoded logical qubit whose logical fidelity exceeds the fidelity of the entangling operations used to create it. We compare these operations with non-fault-tolerant protocols capable of generating arbitrary logical states, and observe the expected increase in error. We directly measure the four Bacon-Shor stabilizer generators and are able to detect single qubit Pauli errors. These results show that fault-tolerant quantum systems are currently capable of logical primitives with error rates lower than their constituent parts. With the future addition of intermediate measurements, the full power of scalable quantum error-correction can be achieved.

UR - https://arxiv.org/abs/2009.11482 ER -