arXiv subjects

Nathanaël Cottet

Publications and source records attributed to Nathanaël Cottet.

6 recordsLinked to original sources

arXiv2026

Observation of a power transfer controlled by the phase of a quantum superposition

A driven qubit exchanges energy with the propagating modes that drive it. When two spatially separated modes drive a single qubit with opposite amplitudes, their net action on the qubit cancels. Yet the qubit can still transfer power from one mode to the other through stimulated emission. The directionality originates from opposite stimulated emission powers into each line. We realize this situation with a superconducting transmon qubit coupled to two transmission lines and show that the direction of the power flow is set by the phase $ϕ$ of the qubit superposition between its ground and excited states, rather than by any classical control parameter. From a time-resolved measurement of the output power in one line, we observe a transfer that varies as $\cosϕ$ and hence changes direction between $ϕ=0$ and $ϕ=π$. The directionality of the total power flow is limited by the phase independent contributions of the reflected drive and of spontaneous emission, which sets a routing efficiency that we measure as a function of the input power. For an equal superposition of ground and excited qubit states (maximal coherence), the efficiency reaches $63\%$, close to the bound of $69\%$ expected from the measured qubit coherence times.

quant-ph↗

arXiv2024

Autoparametric resonance extending the bit-flip time of a cat qubit up to 0.3 s

Cat qubits, for which logical $|0\rangle$ and $|1\rangle$ are coherent states $|\pmα\rangle$ of a harmonic mode, offer a promising route towards quantum error correction. Using dissipation to our advantage so that photon pairs of the harmonic mode are exchanged with single photons of its environment, it is possible to stabilize the logical states and exponentially increase the bit-flip time of the cat qubit with the photon number $|α|^2$. Large two-photon dissipation rate $κ_2$ ensures fast qubit manipulation and short error correction cycles, which are instrumental to correct the remaining phase-flip errors in a repetition code of cat qubits. Here we introduce and operate an autoparametric superconducting circuit that couples a mode containing the cat qubit to a lossy mode whose frequency is set at twice that of the cat mode. This passive coupling does not require a parametric pump and reaches a rate $κ_2/2π\approx 2~\mathrm{MHz}$. With such a strong two-photon dissipation, bit-flip errors of the autoparametric cat qubit are prevented for a characteristic time up to 0.3~s with only a mild impact on phase-flip errors. Besides, we illustrate how the phase of a quantum superposition between $|α\rangle$ and $|-α\rangle$ can be arbitrarily changed by driving the harmonic mode while keeping the engineered dissipation active.

quant-ph↗

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.

quant-ph↗

arXiv2021

Multiplexed photon number measurement

When a two-level system -- a qubit -- is used as a probe of a larger system, it naturally leads to answering a single yes-no question about the system state. Here we propose a method where a single qubit is able to extract, not a single, but many bits of information about the photon number of a microwave resonator using continuous measurement. We realize a proof-of-principle experiment by recording the fluorescence emitted by a superconducting qubit reflecting a frequency comb, thus implementing multiplexed photon counting where the information about each Fock state -- from 0 to 8 -- is simultaneously encoded in independent measurement channels. Direct Wigner tomography of the quantum state of the resonator evidences the back-action of the measurement as well as the optimal information extraction parameters. Our experiment unleashes the full potential of quantum meters by replacing a sequential quantum measurements with simultaneous and continuous measurements separated in the frequency domain.

quant-ph↗

arXiv2020

Electron shelving of a superconducting artificial atom

Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom in the form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit transition. Originally termed "electron shelving", such a conditional fluorescence phenomenon is the cornerstone of many quantum information platforms, including trapped ions, solid state defects, and semiconductor quantum dots. Here we apply the shelving idea to circuit atoms and demonstrate a conditional fluorescence readout of fluxonium qubit placed inside a matched one-dimensional waveguide. Cycling the non-computational transition between ground and third excited states produces a microwave photon every 91 ns conditioned on the qubit ground state, while the qubit coherence time exceeds 50 us. The readout has a built-in quantum non-demolition property, allowing over 100 fluorescence cycles in agreement with a four-level optical pumping model. Our result introduces a resource-efficient alternative to cQED. It also adds a state-of-the-art quantum memory to the growing toolbox of waveguide QED.

cond-mat.mes-hall↗

arXiv2019

Maxwell's demon in superconducting circuits

This paper provides an overview of the first experimental realizations of quantum-mechanical Maxwell's demons based on superconducting circuits. The principal results of these experiments are recalled and put into context. We highlight the versatility offered by superconducting circuits for studying quantum thermodynamics.

quant-ph↗