Audrey Bienfait

Spins in solids could provide long-lived memories for superconducting quantum computers, but practical devices need a microwave interface that remains tunable in a magnetic field. This study develops a superconducting resonator with adjustable frequency and bandwidth to address bismuth spins in silicon at a clock transition, where they are less sensitive to magnetic noise. The authors measure quantum coherence lasting nearly half a second and show that tuning the resonator can suppress the spins' microwave echoes. These capabilities provide building blocks for controlling the storage and retrieval of quantum information in spin-based memories.

Résonance magnétique avec des champs micro-ondes quantiques: Dans une expérience classique de résonance paramagnétique électronique (RPE), le couplage entre les spins et leur environnement électromagnétique est faible, limitant considérablement la sensibilité de la mesure. Grâce à l’utilisation combinée d'un amplificateur paramétrique Josephson et de micro-résonateurs supraconductuers de hauts facteurs de qualité refroidis à quelques millikelvins, ce travail rapporte la conception et la mise en œuvre d’un spectromètre RPE dont la sensibilité de détection est limitée par les fluctuations quantiques du champ électromagnétique au lieu d’un bruit d’origine thermique ou technique. Des mesures de RPE pulsée sur un ensemble de doneurs Bismuth dans le silicium permettent de démontrer une sensibilité de 1700 spins détectés par écho de Hahn avec un signal-sur-bruit unitaire. La sensibilité est encore améliorée en générant un état de vide comprimé dans le guide d'onde de détection, ce qui réduit les fluctuations quantiques au-delà de la limite quantique. Les hauts facteurs de qualité et le petit volume de mode du résonateur supraconducteur développés pour une sensibilité accrue accroit également le couplage spin-résonateur jusqu'au point où les fluctuations quantiques ont un effet dramatique sur la dynamique des spins. En effet, l’émission spontanée de photons dans le résonateur micro-onde est considérablement renforcée par l'effet Purcell, ce qui en fait le mécanisme de relaxation de spin dominant. Le taux de relaxation est augmenté de trois ordres de grandeur lorsque les spins sont accordés à résonance, démontrant que la relaxation de spin peut-être contrôlée sur demande. Nos résultats fournissent une méthode nouvelle et universelle pour initialiser des systèmes de spin dans leur état fondamental, avec des applications en résonance magnétique et en information quantique.

Recent progress in manipulating individual quantum systems enables the exploration of engines exploiting nonclassical resources. One of the most appealing is the energy provided by the inherent backaction of quantum measurements. While a handful of experiments have investigated the inner dynamics of engines fueled by measurement backaction, powering a task by such an engine is missing. Here, we demonstrate the amplification of microwave signals by an engine fueled by repeated quantum measurements of a superconducting transmon qubit. Using feedback, the engine acts as a quantum Maxwell demon operating without a hot thermal source. Measuring the gain of this amplification constitutes a direct probing of the work output of the engine, in contrast to inferring the work by measuring the qubit state along its evolution. Observing a good agreement between both work estimation methods, our experiment validates the accuracy of the indirect method. We characterize the long-term stability of the engine as well as its robustness to transmon decoherence, loss, and drifts. Our experiment exemplifies the use of energy brought by quantum measurement backaction.

Scaling up quantum computing devices requires solving ever more complex quantum control tasks. Machine learning has been proposed as a promising approach to tackle the resulting challenges. However, experimental implementations are still scarce. In this work, we demonstrate experimentally a neural-network-based preparation of Schrödinger cat states in a cavity coupled dispersively to a qubit. We show that it is possible to teach a neural network to output optimized control pulses for a whole family of quantum states. After being trained in simulations, the network takes a description of the target quantum state as input and rapidly produces the pulse shape for the experiment, without any need for time-consuming additional optimization or retraining for different states. Our experimental results demonstrate more generally how deep neural networks and transfer learning can produce efficient simultaneous solutions to a range of quantum control tasks, which will benefit not only state preparation but also parametrized quantum gates.

The number of excitations in a large quantum system (harmonic oscillator or qudit) can be measured in a quantum nondemolition manner using a dispersively coupled qubit. It typically requires a series of qubit pulses that encode various binary questions about the photon number. Recently, a method based on the fluorescence measurement of a qubit driven by a train of identical pulses was introduced to track the photon number in a cavity, hence simplifying its monitoring and raising interesting questions about the measurement backaction of this scheme. A first realization with superconducting circuits demonstrated how the average number of photons could be measured in this way. Here we present an experiment that reaches single-shot photocounting and number tracking owing to a cavity decay rate 4 orders of magnitude smaller than both the dispersive coupling rate and the qubit emission rate. An innovative notch filter and pogo-pin-based galvanic contact makes possible these seemingly incompatible features. The qubit dynamics under the pulse train is characterized. We observe quantum jumps by monitoring the photon number via the qubit fluorescence as photons leave the cavity one at a time. Additionally, we extract the measurement rate and induced dephasing rate and compare them to theoretical models. Our method could be applied to quantum error correction protocols on bosonic codes or qudits.

High-quality factor microwave resonators operating in a magnetic field are a necessity for some quantum sensing applications and hybrid platforms. Losses in microwave superconducting resonators can have several origins, including microscopic defects, usually known as two-level-systems. Here, we characterize the magnetic field response of NbTiN resonators patterned on sapphire and observe clear absorption lines occurring at specific magnetic fields. We identify the spin systems responsible for these features, including a yet unreported spin with g = 1.85 that we attribute to defects in the NbTiN thin film. We develop mitigation strategies involving, namely, an aluminum etch mask, resulting in maintaining quality factors above Q>2×105 in the range of 0–0.3 T.