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1. Logical quantum processor based on reconfigurable atom arrays

   https://doi.org/10.1038/s41586-023-06927-3  

   Bluvstein, Dolev ; Evered, Simon J. ; Geim, Alexandra A. ; Li, Sophie H. ; Zhou, Hengyun ; Manovitz, Tom ; Ebadi, Sepehr ; Cain, Madelyn ; Kalinowski, Marcin ; Hangleiter, Dominik ; et al ( December 2023 , Nature)

   Suppressing errors is the central challenge for useful quantum computing¹, requiring quantum error correction (QEC)^2–6for large-scale processing. However, the overhead in the realization of error-corrected ‘logical’ qubits, in which information is encoded across many physical qubits for redundancy^2–4, poses substantial challenges to large-scale logical quantum computing. Here we report the realization of a programmable quantum processor based on encoded logical qubits operating with up to 280 physical qubits. Using logical-level control and a zoned architecture in reconfigurable neutral-atom arrays⁷, our system combines high two-qubit gate fidelities⁸, arbitrary connectivity^7,9, as well as fully programmable single-qubit rotations and mid-circuit readout^10–15. Operating this logical processor with various types of encoding, we demonstrate improvement of a two-qubit logic gate by scaling surface-code⁶distance fromd = 3 tod = 7, preparation of colour-code qubits with break-even fidelities⁵, fault-tolerant creation of logical Greenberger–Horne–Zeilinger (GHZ) states and feedforward entanglement teleportation, as well as operation of 40 colour-code qubits. Finally, using 3D [[8,3,2]] code blocks^16,17, we realize computationally complex sampling circuits¹⁸with up to 48 logical qubits entangled with hypercube connectivity¹⁹with 228 logical two-qubit gates and 48 logical CCZ gates²⁰. We find that this logical encoding substantially improves algorithmic performance with error detection, outperforming physical-qubit fidelities at both cross-entropy benchmarking and quantum simulations of fast scrambling^21,22. These results herald the advent of early error-corrected quantum computation and chart a path towards large-scale logical processors.

    

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2. Suppressing quantum errors by scaling a surface code logical qubit

   https://doi.org/10.1038/s41586-022-05434-1  

   Acharya, Rajeev ; Aleiner, Igor ; Allen, Richard ; Andersen, Trond I. ; Ansmann, Markus ; Arute, Frank ; Arya, Kunal ; Asfaw, Abraham ; Atalaya, Juan ; Babbush, Ryan ; et al ( February 2023 , Nature)

   Practical quantum computing will require error rates well below those achievable with physical qubits. Quantum error correction^1,2offers 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 × 10⁻⁶logical error per cycle floor set by a single high-energy event (1.6 × 10⁻⁷excluding 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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3. Quantum control and Berry phase of electron spins in rotating levitated diamonds in high vacuum

   https://doi.org/10.1038/s41467-024-49175-3  

   Jin, Yuanbin ; Shen, Kunhong ; Ju, Peng ; Gao, Xingyu ; Zu, Chong ; Grine, Alejandro_J ; Li, Tongcang ( June 2024 , Nature Communications)

   Levitated diamond particles in high vacuum with internal spin qubits have been proposed for exploring macroscopic quantum mechanics, quantum gravity, and precision measurements. The coupling between spins and particle rotation can be utilized to study quantum geometric phase, create gyroscopes and rotational matter-wave interferometers. However, previous efforts in levitated diamonds struggled with vacuum level or spin state readouts. To address these gaps, we fabricate an integrated surface ion trap with multiple stabilization electrodes. This facilitates on-chip levitation and, for the first time, optically detected magnetic resonance measurements of a nanodiamond levitated in high vacuum. The internal temperature of our levitated nanodiamond remains moderate at pressures below 10⁻⁵Torr. We have driven a nanodiamond to rotate up to 20 MHz (1.2 × 10⁹rpm), surpassing typical nitrogen-vacancy (NV) center electron spin dephasing rates. Using these NV spins, we observe the effect of the Berry phase arising from particle rotation. In addition, we demonstrate quantum control of spins in a rotating nanodiamond. These results mark an important development in interfacing mechanical rotation with spin qubits, expanding our capacity to study quantum phenomena.

    

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4. Fabrication Tolerant Inverse Design Grating Couplers for Scalable Trapped Ion Quantum Computing

   https://doi.org/10.1364/CLEO_AT.2022.JW3A.23  

   Klawson, Daniel ; Shirao, Mizuki ; Mouradian, Sara ; Wu, Ming C. ( January 2022 , Conference on Lasers and Electro-Optics)

   A fabrication tolerant Si grating coupler for 1.762 µm operation is optimized with inverse design, allowing for −30 dB crosstalk between a pair of¹³³Ba⁺trapped ion qubits within expected fabrication variation.

    

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5. Improving qubit coherence using closed-loop feedback

   https://doi.org/10.1038/s41467-022-29287-4  

   Vepsäläinen, Antti ; Winik, Roni ; Karamlou, Amir H. ; Braumüller, Jochen ; Paolo, Agustin Di ; Sung, Youngkyu ; Kannan, Bharath ; Kjaergaard, Morten ; Kim, David K. ; Melville, Alexander J. ; et al ( April 2022 , Nature Communications)

   Superconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10⁻⁴to (5.9 ± 0.7) × 10⁻⁴. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.

    

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