Quantum control of a cat-qubit with bit-flip times exceeding ten seconds

Binary classical information is routinely encoded in the two metastable states of a dynamical system. Since these states may exhibit macroscopic lifetimes, the encoded information inherits a strong protection against bit-flips. A recent qubit - the cat-qubit - is encoded in the manifold of metastable states of a quantum dynamical system, thereby acquiring bit-flip protection. An outstanding challenge is to gain quantum control over such a system without breaking its protection. If this challenge is met, significant shortcuts in hardware overhead are forecast for quantum computing. In this experiment, we implement a cat-qubit with bit-flip times exceeding ten seconds. This is a four order of magnitude improvement over previous cat-qubit implementations, and six orders of magnitude enhancement over the single photon lifetime that compose this dynamical qubit. This was achieved by introducing a quantum tomography protocol that does not break bit-flip protection. We prepare and image quantum superposition states, and measure phase-flip times above 490 nanoseconds. Most importantly, we control the phase of these superpositions while maintaining the bit-flip time above ten seconds. This work demonstrates quantum operations that preserve macroscopic bit-flip times, a necessary step to scale these dynamical qubits into fully protected hardware-efficient architectures

Mesure de la fluorescence de spin avec un compteur de photons à micro-ondes

Magnetic resonance is a branch of science that aims to detect spins via their absorption and emission of electromagnetic radiation. Two areas can be distinguished: nuclear magnetic resonance (NMR) which aims at detecting the spins of nuclei, and electron spin resonance (ESR) that concerns the detection of the spin of unpaired electrons. In this thesis, we introduce a new method for ESR spectroscopy, consisting of the detection of the incoherent microwave fluorescence signal emitted by an ensemble spins relaxing to their ground state after an excitation pulse. In order to demonstrate this method, we use the electronic spins belonging to bismuth donors in silicon. Emission of the fluorescence signal is favored by the Purcell effect, due to the coupling of the spin ensemble to a superconducting resonator with small mode volume and low losses. We connect the output port of the spin-resonator system to the input of a newly-developed frequency-tunable single microwave photon detector (SMPD), based on four-wave mixing with a superconducting qubit. After an excitation pulse, the spins relax to their ground state emitting a stream of incoherent photons which constitutes the fluorescence signal and is detected with the SMPD. We show that the fluorescence signal can be used to perform spectroscopy of the ensemble and measure relevant properties. We compare this technique to standard echo detection and discuss its increased sensitivity for small numbers of spins.La résonance magnétique est une branche de la science qui vise à détecter les spins via leur absorption et émission de rayonnement électromagnétique. On distingue deux domaines: la résonance magnétique nucléaire (RMN) qui vise à détecter les spins des noyaux, et la résonance de spin électronique (ESR) qui concerne la détection du spin des électrons non appariés. Dans cette thèse, nous introduisons une nouvelle méthode pour la spectroscopie ESR, consistant en la détection du signal de fluorescence micro-onde incohérent émis par un ensemble de spins relaxant vers leur état fondamental après avoir été excités. Afin de démontrer cette méthode, nous utilisons les spins électroniques appartenant aux donneurs de bismuth dans le silicium. L’émission du signal de fluorescence est favorisée par l’effet Purcell, dû au couplage de l’ensemble de spins à un résonateur supraconducteur ayant un petit volume de mode et de faibles pertes. Nous connectons le port de sortie du système spin-résonateur à l’entrée d’un détecteur de photons micro-ondes accordable en fréquence (SMPD) récemment développé, basé sur le mélange à quatre ondes avec un qubit supraconducteur. Après une impulsion d’excitation, les spins relaxent vers leur état fondamental en émettant un flux de photons incohérents qui constitue le signal de fluorescence et qui est détecté par le SMPD. Nous montrons que le signal de fluorescence peut être utilisé pour effectuer la spectroscopie de l’ensemble et mesurer ses propriétés. Nous comparons cette technique à la détection par écho et discutons l’avantage en sensibilité pour un petit nombre de spins

Irreversible Qubit-Photon Coupling for the Detection of Itinerant Microwave Photons

International audienceSingle photon detection is a key resource for sensing at the quantum limit and the enabling technologyfor measurement-based quantum computing. Photon detection at optical frequencies relies on irreversiblephotoassisted ionization of various natural materials. However, microwave photons have energies 5 ordersof magnitude lower than optical photons, and are therefore ineffective at triggering measurable phenomenaat macroscopic scales. Here, we report the observation of a new type of interaction between a single two-level system (qubit) and a microwave resonator. These two quantum systems do not interact coherently;instead, they share a common dissipative mechanism to a cold bath: the qubit irreversibly switches to itsexcited state if and only if a photon enters the resonator. We have used this highly correlated dissipationmechanism to detect itinerant photons impinging on the resonator. This scheme does not require any priorknowledge of the photon waveform nor its arrival time, and dominant decoherence mechanisms do nottrigger spurious detection events (dark counts). We demonstrate a detection efficiency of 58% and a recordlow dark count rate of 1.4 per millisecond. This work establishes engineered nonlinear dissipation as a keyenabling resource for a new class of low-noise nonlinear microwave detectors

Detecting spins by their fluorescence with a microwave photon counter

International audienc

Harnessing two-photon dissipation for enhanced quantum measurement and control

20 pages, appendix includedInternational audienceScaling 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 Schr\"odinger 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

Shaped pulses for transient compensation in quantum-limited electron spin resonance spectroscopy

International audienceIn high sensitivity inductive electron spin resonance spectroscopy, superconducting microwave resonators with large quality factors are employed. While they enhance the sensitivity, they also distort considerably the shape of the applied rectangular microwave control pulses, which limits the degree of control over the spin ensemble. Here, we employ shaped microwave pulses compensating the signal distortion to drive the spins faster than the resonator bandwidth. This translates into a shorter echo, with enhanced signal-to-noise ratio. The shaped pulses are also useful to minimize the dead-time of our spectrometer, which allows to reduce the wait time between successive drive pulses

Microwave fluorescence detection of spin echoes

Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been proposed as a sensitive method for electron paramagnetic resonance (EPR) spectroscopy, enabled by the development of operational Single Microwave Photon Detectors (SMPD) at millikelvin temperature. Here, we report the detection of spin echoes in the spin fluorescence signal. The echo manifests itself as a coherent modulation of the number of photons spontaneously emitted after a $\pi/2_X - \tau - \pi_Y - \tau - \pi/2_\Phi$ sequence, dependent on the relative phase $\Phi$. We demonstrate experimentally this detection method using an ensemble of $\mathrm{Er}^{3+}$ ion spins in a scheelite crystal of $\mathrm{CaWO}_4$. We use fluorescence-detected echoes to measure the erbium spin coherence time, as well as the echo envelope modulation due to the coupling to the $^{183}\mathrm{W}$ nuclear spins surrounding each ion. We finally compare the signal-to-noise ratio of inductively-detected and fluorescence-detected echoes, and show that it is larger with the fluorescence method

Microwave fluorescence detection of spin echoes

Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been proposed as a sensitive method for electron paramagnetic resonance (EPR) spectroscopy, enabled by the development of operational Single Microwave Photon Detectors (SMPD) at millikelvin temperature. Here, we report the detection of spin echoes in the spin fluorescence signal. The echo manifests itself as a coherent modulation of the number of photons spontaneously emitted after a $\pi/2_X - \tau - \pi_Y - \tau - \pi/2_\Phi$ sequence, dependent on the relative phase $\Phi$. We demonstrate experimentally this detection method using an ensemble of $\mathrm{Er}^{3+}$ ion spins in a scheelite crystal of $\mathrm{CaWO}_4$. We use fluorescence-detected echoes to measure the erbium spin coherence time, as well as the echo envelope modulation due to the coupling to the $^{183}\mathrm{W}$ nuclear spins surrounding each ion. We finally compare the signal-to-noise ratio of inductively-detected and fluorescence-detected echoes, and show that it is larger with the fluorescence method

Microwave fluorescence detection of spin echoes

Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been proposed as a sensitive method for electron paramagnetic resonance (EPR) spectroscopy, enabled by the development of operational Single Microwave Photon Detectors (SMPD) at millikelvin temperature. Here, we report the detection of spin echoes in the spin fluorescence signal. The echo manifests itself as a coherent modulation of the number of photons spontaneously emitted after a $\pi/2_X - \tau - \pi_Y - \tau - \pi/2_\Phi$ sequence, dependent on the relative phase $\Phi$. We demonstrate experimentally this detection method using an ensemble of $\mathrm{Er}^{3+}$ ion spins in a scheelite crystal of $\mathrm{CaWO}_4$. We use fluorescence-detected echoes to measure the erbium spin coherence time, as well as the echo envelope modulation due to the coupling to the $^{183}\mathrm{W}$ nuclear spins surrounding each ion. We finally compare the signal-to-noise ratio of inductively-detected and fluorescence-detected echoes, and show that it is larger with the fluorescence method