arXiv subjects

Rémy Dassonneville

Publications and source records attributed to Rémy Dassonneville.

7 recordsLinked to original sources

arXiv2026

Absence of Charge Offset Drift in a Transmon Qubit

Superconducting quantum circuits are sensitive to their electrostatic environment: uncontrolled charges accumulating on the electrodes of a Josephson junction shift the energy levels of a qubit, perturbing its operation and restricting their design. This effect is captured by a single parameter - the charge offset - whose slow, unpredictable drift has proven difficult to eliminate in practice. Here, we report a tantalum-based transmon qubit in which the charge offset remains pinned at zero over nearly three months of measurements, including two thermal cycles, with no observable compromise to the qubit lifetime. This exceptional stability disappears in later cooldowns, indicating a fragile mechanism at play. We attribute it to the inductance of a thin superconducting layer inadvertently formed in parallel with the Josephson junction during fabrication. X-ray surface spectroscopy suggests this layer arises from an incomplete wet-etch of tantalum on sapphire. Deliberately engineering such a layer offers a route to eliminating charge-offset drift in superconducting circuits more broadly.

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arXiv2026

Amplifying microwave pulses with a single qubit engine fueled by quantum measurements

Recent progress in manipulating individual quantum systems enables the exploration of engines exploiting non-classical 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 with 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.

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arXiv2025

Observation and mitigation of microwave echoes from dielectric defects in Josephson traveling wave amplifiers

Amplifying microwave signals with a noise close to the minimum imposed by quantum mechanics is now routinely performed with superconducting quantum devices. In particular, Josephson-based Traveling Wave Parametric Amplifiers (JTWPA) have shown record bandwidth with added noise close to the quantum limit. In this work, we report the appearance of echo signals emitted by JTWPAs driven by trains of high-power pulses exceeding their dynamical range. we explore the case of weak signals generated through high power pulses. By sending a train of such high-power pulses, beyond the 1 dB compression point of such amplifiers, we observe the appearance of echoes, solely due to the JTWPA. These echoes have micro-second coherence and we attribute their origin to microscopic defects in the amplifier dielectric layer. By analyzing the power and the coherence of the echo signal as a function of temperature, we estimate the dielectric loss brought by these defects, and their impact on the JTWPA quantum efficiency. We introduce a mitigation technique (BLAST) to prevent the appearance of these echoes, consisting in an additional high-power tone sent concurrently with each pulse. We demonstrate that it suppresses the spurious defect signals and we recover the typical gain and noise figure within 95% of their low-power values in 300 ns. These results can help to extend the use of JTWPAs in experiments where fast high-power sequences are necessary to generate weak microwave responses from the system under study, and also provide a path towards characterizing in-situ the dielectric losses of these devices.

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arXiv2023

Cloaking a qubit in a cavity

Cavity quantum electrodynamics (QED) uses a cavity to engineer the mode structure of the vacuum electromagnetic field such as to enhance the interaction between light and matter. Exploiting these ideas in solid-state systems has lead to circuit QED which has emerged as a valuable tool to explore the rich physics of quantum optics and as a platform for quantum computation. Here we introduce a simple approach to further engineer the light-matter interaction in a driven cavity by controllably decoupling a qubit from the cavity's photon population, effectively cloaking the qubit from the cavity. This is realized by driving the qubit with an external tone tailored to destructively interfere with the cavity field, leaving the qubit to interact with a cavity which appears to be in the vacuum state. Our experiment demonstrates how qubit cloaking can be exploited to cancel ac-Stark shift and measurement-induced dephasing, and to accelerate qubit readout.

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arXiv2023

Demonstration of Quantum Advantage in Microwave Quantum Radar

While quantum entanglement can enhance the performance of several technologies such as computing, sensing and cryptography, its widespread use is hindered by its sensitivity to noise and losses. Interestingly, even when entanglement has been destroyed, some tasks still exhibit a quantum advantage $Q$, defined by a $Q$-time speedup, over any classical strategies. A prominent example is the quantum radar, which enhances the detection of the presence of a target in noisy surroundings. To beat all classical strategies, Lloyd proposed to use a probe initially entangled with an idler that can be recombined and measured with the reflected probe. Observing any quantum advantage requires exploiting the quantum correlations between the probe and the idler. It involves their joint measurement or at least adapting the idler detection to the outcome of the probe measurement. In addition to successful demonstrations of such quantum illumination protocols at optical frequencies, the proposal of a microwave radar, closer to conventional radars, gathered a lot of interest. However, previous microwave implementations have not demonstrated any quantum advantage as probe and idler were always measured independently. In this work, we implement a joint measurement using a superconducting circuit and demonstrate a quantum advantage $Q>1$ for microwave radar. Storing the idler mitigates the detrimental impact of microwave loss on the quantum advantage, and the purity of the initial entangled state emerges as the next limit. While the experiment is a proof-of-principle performed inside a dilution refrigerator, it exhibits some of the inherent difficulties in implementing quantum radars such as the limited range of parameters where a quantum advantage can be observed or the requirement for very low probe and idler temperatures.

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arXiv2022

Energetics of a Single Qubit Gate

Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit and the controller that implements the gate? Here we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows. We measure the energy change in the drive field conditioned on the outcome of a projective qubit measurement. We demonstrate that the drive's energy change associated with the measurement backaction can exceed by far the energy that can be extracted by the qubit. This can be understood by considering the qubit as a weak measurement apparatus of the driving field.

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arXiv2020

Number-resolved photocounter for propagating microwave mode

Detectors of propagating microwave photons have recently been realized using superconducting circuits. However a number-resolved photocounter is still missing. In this letter, we demonstrate a single-shot counter for propagating microwave photons that can resolve up to $3$ photons. It is based on a pumped Josephson Ring Modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of $0.96 \pm 0.04$, and a dark count probability of $0.030 \pm 0.002$ for an average dead time of $4.5~\mathrm{μs}$. To maximize its performance, the device is first used as an \emph{in situ} waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from $(54 \pm 2)\%$ to $99\%$.

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