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1. The Power of Two Bases: Robust and Copy-Optimal Certification of Nearly All Quantum States with Few-Qubit Measurements

   https://doi.org/10.1145/3798129.3800884  

   Coladangelo, Andrea; Li, Jerry; Slote, Joseph; Wu, Ellen (June 2026, ACM)

   A central task in quantum information science is state certification: testing whether an unknown state is 𝜖1-close to a fixed target state, or 𝜖2-far. Recent work has shown that surprisingly simple measurement protocols—comprising only single-qubit measurements—suffice to certify arbitrary 𝑛-qubit states [14, 18]. However, these certification protocols are not robust: rather than allowing constant 𝜖1, they can only positively certify states within 𝜖1 = 𝑂(1/𝑛) trace distance of the target. In many experimental settings, the appropriate error tolerance is constant as the system size grows, so this lack of robustness renders existing tests inapplicable at scale, no matter how many times the test is repeated. Here we present robust certification protocols based on few-qubit measurements that apply to all but a 𝑂(2−𝑛)-fraction of pure targetstates. Our first protocol achieves constant robustness, i.e. 𝜖1 = Θ(1),using a single 𝑂(log𝑛)-qubit measurement along with single-qubit measurements in the 𝑍 or 𝑋 basis on the other qubits. As a corollary of its robustness, this protocol also achieves constant (in 𝑛) copy complexity, which is optimal. Our second protocol uses exclusively single-qubit measurements and is nearly robust: 𝜖1 = Ω(1/log𝑛). Our tests are based on a new uncertainty principle for conditional fidelities which may be of independent interest. 

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2. A universal quantum gate set for transmon qubits with strong ZZ interactions

   Junling Long, Tongyu Zhao (March 2021, ArXivorg)

   null (Ed.)

   High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time tg=53.8 ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography. 

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3. Enhanced measurement of neutral atom qubits with machine learning

   Phuttitarn, Linipun; Becker, Brooke; Chinnarasu, Ravikumar; Graham, Trent; Saffman, Mark (June 2024, Bulletin of American Physical Society, 55th DAMOP 2024)

   The ability to make high-fidelity qubit measurements with minimal collateral disruption to the system is not only relevant to initialization and final read-out -- it is also essential to achieving quantum error correction on a universal quantum computation. Qubit state measurements in a neutral atom array are achieved by probing the array with light detuned from a cycling transition and capturing resulting fluorescence with a high quantum efficiency imaging device, producing a greyscale image of the neutral atom array. Conventionally, to achieve a fidelity above 99%, the typical probing period is several ms. This is a significant delay, given that the longest gate operation only takes several micros. In this poster, we demonstrate qubit state measurements assisted by a supervised convolutional neural network (CNN) in a neutral atom quantum processor. We present two CNN architectures for analyzing neutral atom qubit readout data: a compact 5-layer single-qubit CNN architecture and a 6-layer multi-qubit CNN architecture. We benchmark both architectures against a conventional Gaussian threshold analysis method. We demonstrate up to 56% reduction of measurement infidelity using the CNN compared to a conventional analysis method. This work presents a proof of concept for a CNN network to be implemented as a real-time readout processing method on a neutral atom quantum computer, enabling faster readout time and improved fidelity. 

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4. Optimizing Initial State of Detector Sensors in Quantum Sensor Networks

   https://doi.org/10.1145/3655028  

   Zhan, Caitao; Gupta, Himanshu; Hillery, Mark (June 2024, ACM Transactions on Quantum Computing)

   none (Ed.)

   In this article, we consider a network of quantum sensors, where each sensor is a qubit detector that “fires,” i.e., its state changes when an event occurs close by. The change in state due to the firing of a detector is given by a unitary operator, which is the same for all sensors in the network. Such a network of detectors can be used to localize an event, using a protocol to determine the firing sensor, presumably the one closest to the event. The determination of the firing sensor can be posed as aQuantum State Discriminationproblem, which incurs a probability of error depending on the initial state and the measurement operators used. In this article, we address the problem of determining the optimal initial global state of a network of detectors that incur a minimum probability of error in determining the firing sensor. For this problem, we derive necessary and sufficient conditions for the existence of an initial state that allows for perfect discrimination, i.e., zero probability of error. Using insights from this result, we derive a conjectured optimal solution for the initial state, provide a pathway to prove the conjecture, and validate the conjecture empirically using multiple search heuristics that seem to perform near-optimally. 

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5. Experimentally Probing Entropy Reduction via Iterative Quantum Information Transfer

   https://doi.org/10.1103/5lp2-9sps  

   Yada, Toshihiro; Stas, Pieter-Jan; Suleymanzade, Aziza; Knall, Erik N; Yoshioka, Nobuyuki; Sagawa, Takahiro; Lukin, Mikhail D (August 2025, Physical Review X)

   Thermodynamic principles governing energy and information are important tools for a deeper understanding and better control of quantum systems. In this work, we experimentally investigate the interplay of the thermodynamic costs and information flow in a quantum system undergoing iterative quantum measurement and feedback. Our study employs a state stabilization protocol involving repeated measurement and feedback on an electronic spin qubit associated with a silicon-vacancy center in diamond, which is strongly coupled to a diamond nanocavity. This setup allows us to verify the fundamental laws of nonequilibrium quantum thermodynamics, including the second law and the fluctuation theorem, both of which incorporate measures of quantum information flow induced by iterative measurement and feedback. We further assess the reducible entropy based on the feedback’s causal structure and quantitatively demonstrate the thermodynamic advantages of non-Markovian feedback over Markovian feedback. For that purpose, we extend the theoretical framework of quantum thermodynamics to include the causal structure of the applied feedback protocol. Our work lays the foundation for investigating the entropic and energetic costs of real-time quantum control in various quantum systems. 

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