Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session Y69: Progress in Practical FaultTolerant Quantum Error CorrectionInvited

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Sponsoring Units: DQI Chair: Baptiste Royer, Université de Sherbrooke Room: Room 421 
Friday, March 10, 2023 8:00AM  8:36AM 
Y69.00001: Suppressing quantum errors by scaling a surface code logical qubit Invited Speaker: Julian Kelly Practical quantum computing will require error rates that are well below what is achievable with physical qubits. Quantum error correction offers a path to algorithmicallyrelevant error rates by encoding logical qubits within many physical qubits, where 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 in order for logical performance to improve with increasing code size. Here, we report the measurement of logical qubit performance scaling across multiple code sizes, and demonstrate that our system of superconducting qubits has sufficient performance to overcome the additional errors from increasing qubit number. We find our distance5 surface code logical qubit modestly outperforms an ensemble of distance3 logical qubits on average, both in terms of logical error probability over 25 cycles and logical error per cycle (2.914%±0.016% compared to 3.028%±0.023%). To investigate damaging, lowprobability error sources, we run a distance25 repetition code and observe a 1.7×10−6 logical error per round floor set by a single highenergy event (1.6×10−7 when excluding this event). We are able to accurately model our experiment, and from this model we can extract error budgets that highlight the biggest challenges for future systems. These results mark the first experimental demonstration where quantum error correction begins to improve performance with increasing qubit number, illuminating the path to reaching the logical error rates required for computation. 
Friday, March 10, 2023 8:36AM  9:12AM 
Y69.00002: Distributed quantum computation with spins in diamond Invited Speaker: Tim Hugo Taminiau A potential path towards largescale quantum processors is to distribute computations over a quantum network [1]. By connecting many small modules through interconnects one can sidestep the challenge of placing dense arrays of qubits on a single chip of everincreasing complexity. Optically active spin qubits in diamond are a promising platform for such distributed quantum computations and simualtions, as they combine longlived qubits at high operating temperatures with a coherent interaction with photons that enables optical interconnects [2]. Ultimately, moving to largescale systems will require using errorcorrected logical qubits that are operated fault tolerantly, so that reliable computation becomes possible despite noisy operations. 
Friday, March 10, 2023 9:12AM  9:48AM 
Y69.00003: Tailored quantum error correction for largescale quantum computing under structured noise Invited Speaker: Jahan Claes Largescaling quantum computers will require error correction in order to reliably perform computations. However, the hardware overhead for error correction remains dauntingly large, with each logical qubit potentially requiring thousands of physical qubits for reliable operation. One promising approach to reducing the overheads of error correction is to tailor quantum error correcting codes to the dominant noise in the qubit hardware. 
Friday, March 10, 2023 9:48AM  10:24AM 
Y69.00004: The Kerr cat: spectral kissing and phaseflip robustness Invited Speaker: Nicholas E Frattini Schrödinger cat states, superpositions of coherent states in an oscillator, can be stabilized by a driven effective Hamiltonian thanks to the interplay between Kerr nonlinearity and singlemode squeezing [1]. The pair of resulting degenerate Kerrcat states form a qubit whose coherence along one Bloch sphere axis increases exponentially with the average photon number, while decreasing only linearly along the other axes. The qubit protection arises from the progressive pairwise kissing of consecutive levels as the average photon number is increaseda quantum manifestation of robust period doubling in this system. We experimentally observe the pairwise kissing via spectroscopy and the associated increase in the perioddoubled coherent state lifetime, which exceeds 1 ms, a factor of 440 improvement over the bare lifetime of the constituent SNAILtransmon circuit. Furthermore, we achieve quantum nondemolition readout fidelities > 99% and retain universal quantum control [2]. Our system experimentally realizes a hardwareefficient repitition code leaving the Kerrcat qubit poised for use in further quantum error correction schemes. 
Friday, March 10, 2023 10:24AM  11:00AM 
Y69.00005: Experimental Quantum Error Correction on Quantinuum Ion Trap Invited Speaker: Ciaran RyanAnderson While we are currently in the NISQ era, recent advances in quantum computing hardware and software have opened up opportunities to explore increasingly complex quantum error correction (QEC) primitives. In this talk, we will discuss our recent work in advancing the study of experimental QEC that is enabled by the unique capabilities of Quantinuum’s System Model H1 machines and software. In particular, we utilize our ability to do midcircuit measurements with low crosstalk, effective alltoall connectivity, feedforward, conditional quantum and classical operations, and hybrid classical/quantum compute to implement QEC primitives needed to implement universal quantum computing and both determine and apply corrections during computation on upto two distancethree logical qubits. The capabilities further provide the flexibility to explore different QEC codes, protocols, and decoding schemes. Thus, this work highlights the maturing of current quantum technology towards the goal of largescale faulttolerant quantum computing. 
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