University of California Team Demonstrates Fast Mid-Circuit Measurement for Neutral Atom Arrays

Tsai-Chen Lee and colleagues at University of California, in collaboration with Columbia University, Lawrence Berkeley National Laboratory, and California 94720 2Challenge Institute, have achieved a sharp reduction in cycle time for mid-circuit measurement and feedforward techniques in a neutral-atom array. The work addresses a key limitation of current neutral-atom platforms, where measurement-and-feedforward cycles typically exceed 1ms, reaching speeds below s. Coupling atomic qubits to a high-finesse optical cavity and implementing a new measurement scheme minimises coherence loss, reducing the impact on unmeasured qubits to less than 2% infidelity. This enables real-time error correction and adaptive circuit design, paving the way for faster and more complex quantum computations. Neutral-atom qubits, utilising the internal electronic states of individual atoms as quantum bits, are a leading modality in the development of scalable quantum processors due to their long coherence times and potential for strong interactions. However, the practical realisation of complex quantum algorithms requires the ability to not only manipulate these qubits but also to measure their state mid-computation and use that information to adjust subsequent operations, a process known as mid-circuit measurement and feedforward.

Resonance tuning and Purcell enhancement enable rapid qubit measurement

A high-finesse optical cavity, a highly reflective container for light characterised by a finesse exceeding several thousand, was employed to amplify interactions with the qubits, much like a megaphone focuses sound. These cavities are constructed with highly reflective mirrors, trapping photons within their volume and increasing the effective interaction time between light and the qubits. This cavity actively shaped the measurement process by tuning individual qubits slightly off-resonance with the cavity mode, effectively shielding them from spurious electromagnetic disturbances during measurement of other qubits in the array. This off-resonance detuning minimises unwanted interactions and preserves the quantum coherence of the unmeasured qubits. A selected qubit was then probed with light at a resonant frequency, and the emitted photons, carrying information about the qubit’s state, were collected using a process called Purcell enhancement. Purcell enhancement is a phenomenon where the spontaneous emission rate of a qubit is increased when placed within an optical cavity, analogous to concentrating a light source to accelerate detection. The cavity mode acts as an artificial atom, enhancing the interaction between the qubit and the electromagnetic field, thereby increasing the rate at which photons are emitted and collected.

An array of four atomic qubits coupled to this cavity achieved fast mid-circuit measurement and real-time feedforward. Gate fidelity, a measure of the accuracy of quantum operations, increased to sub percent infidelity, while coherence of a fifth, unmeasured qubit decreased by less than two percent. Reducing the measurement-and-feedforward cycle time to under 100 microseconds represented a sharp improvement over typical times exceeding one millisecond, achieved by shielding qubits and enhancing photon collection. The reduction in cycle time is critical because it directly impacts the speed at which quantum algorithms can be executed. Furthermore, an adaptive gating method, which dynamically adjusts the duration and intensity of laser pulses used to control the qubits, reduced atom loss and detection time, improving measurement fidelity. This adaptive approach compensates for variations in atomic properties and environmental conditions, ensuring consistent and reliable measurement outcomes.

Rapid Measurement and Low-Disturbance Control Enable High-Fidelity Adaptive Quantum Computation

A measurement-and-feedforward cycle time of 45 microseconds was achieved, over twenty times faster than previously established neutral-atom array approaches exceeding 1 millisecond. This speed is crucial, as slower cycles previously limited the complexity and scale of quantum computations, now making real-time error correction and adaptive circuit design viable. Real-time error correction involves detecting and correcting errors that occur during quantum computation, while adaptive circuit design allows the quantum algorithm to be modified based on measurement results, optimising performance and robustness. Coupling atomic qubits to a high-finesse optical cavity minimised coherence loss, impacting unmeasured qubits by less than 2% infidelity. Maintaining qubit coherence is paramount, as any loss of coherence leads to errors in the computation. The low infidelity observed in the unmeasured qubits demonstrates the effectiveness of the cavity-based measurement scheme in preserving quantum information.

This combination of rapid measurement and minimal disturbance establishes optical cavities as a promising route towards faster control of neutral-atom quantum systems and more intricate quantum processing. Researchers at California, Berkeley have demonstrated adaptive quantum state discrimination with 81% accuracy, exceeding the 69% and 73% achieved using non-adaptive measurement schemes. Quantum state discrimination is the task of distinguishing between different quantum states, and adaptive measurement schemes improve the accuracy of this task by dynamically adjusting the measurement settings. Real-time feedforward control, where measurement outcomes on initial qubits directly influence the subsequent manipulation of other qubits within the neutral-atom array, drives this improvement; specifically, measurement angles were dynamically adjusted to optimise results. Conditional coherent state preparation, creating a desired quantum state on a third qubit, reached 85% success with adaptive measurement, compared to 72% and 78% using fixed approaches. This demonstrates the ability to use measurement outcomes to actively control and manipulate the quantum state of other qubits, enabling more complex quantum operations.

Reducing measurement times in superconducting qubit systems via optical cavities

The relentless pursuit of scalable quantum computation demands ever-more precise control over individual qubits, and this work offers a compelling advance in that direction. A key question remains regarding how the benefits accumulate as the system grows, given that the current demonstration focuses on minimising disturbance to a single, unmeasured qubit within the array. Further investigation will be needed to assess the impact on coherence across a larger, fully interconnected network of qubits, potentially revealing unforeseen challenges in maintaining quantum information as complexity increases. Scaling to larger qubit arrays will require careful consideration of factors such as cavity mode overlap and crosstalk between qubits.

Acknowledging that scaling up this system presents challenges with maintaining coherence across a larger qubit network is sensible. Optical cavities were used to control neutral atoms, significantly reducing measurement cycle times to below 100 microseconds. This speed enables real-time feedback and adaptive circuits for improved quantum state control and preparation, despite some disturbance to unmeasured qubits. This demonstration of sub-100 microsecond measurement and feedforward cycles represents a substantial advance for neutral-atom quantum computing, exceeding the performance of earlier free-space methods which typically required over one millisecond. Importantly, qubit coherence was maintained during these rapid measurements, limiting disturbance to unmeasured qubits to less than two percent infidelity, vital for complex calculations. The demonstrated techniques could potentially be extended to other qubit platforms, such as superconducting qubits, offering a pathway towards faster and more efficient quantum computation across diverse architectures.

Researchers successfully reduced the time needed to measure and respond to individual qubits in a neutral-atom array to less than 100 microseconds. This speed is significantly faster than previous methods, which typically took over one millisecond, and allows for real-time feedback during quantum calculations. The team demonstrated this by measuring four qubits with minimal disturbance, less than two percent infidelity, to a fifth, unmeasured qubit. This advance establishes optical cavities as a promising route to faster control of neutral-atom quantum systems and could potentially be applied to other qubit technologies.