Flux Qubits with Long Coherence Times for Hybrid Quantum Circuits

We present measurements of superconducting flux qubits embedded in a three dimensional copper cavity. The qubits are fabricated on a sapphire substrate and are measured by coupling them inductively to an on-chip superconducting resonator located in the middle of the cavity. At their flux-insensitive point, all measured qubits reach an intrinsic energy relaxation time in the 6-20 microseconds range and a pure dephasing time comprised between 3 and 10 microseconds. This significant improvement over previous works opens the way to the coherent coupling of a flux-qubit to individual spins

Manipulating Fock states of a harmonic oscillator while preserving its linearity

We present a new scheme for controlling the quantum state of a harmonic oscillator by coupling it to an anharmonic multilevel system (MLS) with first to second excited state transition frequency on-resonance with the oscillator. In this scheme that we call "ef-resonant", the spurious oscillator Kerr non-linearity inherited from the MLS is very small, while its Fock states can still be selectively addressed via an MLS transition at a frequency that depends on the number of photons. We implement this concept in a circuit-QED setup with a microwave 3D cavity (the oscillator, with frequency 6.4 GHz and quality factor QO=2E-6) embedding a frequency tunable transmon qubit (the MLS). We characterize the system spectroscopically and demonstrate selective addressing of Fock states and a Kerr non-linearity below 350 Hz. At times much longer than the transmon coherence times, a non-linear cavity response with driving power is also observed and explained.Comment: 8 pages, 5 figure

Superconducting qubit as a probe of quantum fluctuations in a nonlinear resonator

International audienceIn addition to their central role in quantum information processing, qubits have proven to be useful tools in a range of other applications such as enhanced quantum sensing and as spectrometers of quantum noise. Here we show that a superconducting qubit strongly coupled to a nonlinear resonator can act as a probe of quantum fluctuations of the intra-resonator field. Building on previous work [M. Boissoneault et al. Phys. Rev. A 85, 022305 (2012)], we derive an effective master equation for the qubit which takes into account squeezing of the resonator field. We show how sidebands in the qubit excitation spectrum that are predicted by this model can reveal information about squeezing and quantum heating. The main results of this paper have already been successfully compared to experimental data [F. R. Ong et al. Phys. Rev. Lett. 110, 047001 (2013)] and we present here the details of the derivations

Practical Single Microwave Photon Counter with 10−2210^\mathrm{-22} W/Hz\mathrm{W/\sqrt{Hz}} sensitivity

Single photon detection played an important role in the development of quantum optics. Its implementation in the microwave domain is challenging because the photon energy is 5 orders of magnitude smaller. In recent years, significant progress has been made in developing single microwave photon detectors (SMPDs) based on superconducting quantum bits or bolometers. In this paper we present a new practical SMPD based on the irreversible transfer of an incoming photon to the excited state of a transmon qubit by a four-wave mixing process. This device achieves a detection efficiency $\eta = 0.43$ and an operational dark count rate $\alpha = 85$ $\mathrm{s^{-1}}$, mainly due to the out-of-equilibrium microwave photons in the input line. The corresponding power sensitivity is $\mathcal{S} = 10^{-22}$ $\mathrm{W/\sqrt{Hz}}$, one order of magnitude lower than the state of the art. The detector operates continuously over hour timescales with a duty cycle $\eta_\mathrm{D}=0.84$, and offers frequency tunability of $\sim 400$ MHz around 7 GHz

Single-shot qubit readout in circuit Quantum Electrodynamics

The future development of quantum information using superconducting circuits requires Josephson qubits [1] with long coherence times combined to a high-fidelity readout. Major progress in the control of coherence has recently been achieved using circuit quantum electrodynamics (cQED) architectures [2, 3], where the qubit is embedded in a coplanar waveguide resonator (CPWR) which both provides a well controlled electromagnetic environment and serves as qubit readout. In particular a new qubit design, the transmon, yields reproducibly long coherence times [4, 5]. However, a high-fidelity single-shot readout of the transmon, highly desirable for running simple quantum algorithms or measur- ing quantum correlations in multi-qubit experiments, is still lacking. In this work, we demonstrate a new transmon circuit where the CPWR is turned into a sample-and-hold detector, namely a Josephson Bifurcation Amplifer (JBA) [6, 7], which allows both fast measurement and single-shot discrimination of the qubit states. We report Rabi oscillations with a high visibility of 94% together with dephasing and relaxation times longer than 0:5 \mu\s. By performing two subsequent measurements, we also demonstrate that this new readout does not induce extra qubit relaxation.Comment: 14 pages including 4 figures, preprint forma

Electron-spin spectral diffusion in an erbium doped crystal at millikelvin temperatures

Erbium-doped crystals offer a versatile platform for hybrid quantum devices because they combine magnetically-sensitive electron-spin transitions with telecom-wavelength optical transitions. At the high doping concentrations necessary for many quantum applications, however, strong magnetic interactions of the electron-spin bath lead to excess spectral diffusion and rapid decoherence. Here we lithographically fabricate a 4.4 GHz superconducting planar micro-resonator on a $\text{CaWO}_{4}$ crystal doped with Er ions at a concentration of twenty parts per million relative to Ca. Using the microwave resonator, we characterize the spectral diffusion processes that limit the electron-spin coherence of Er ions at millikelvin temperatures by applying 2- and 3-pulse echo sequences. The coherence time shows a strong temperature dependence, reaching 1.3 ms at 23 mK for an electron-spin transition of $^{167}\text{Er}$.Comment: 10 pages, 5 figure

Experimental violation of a Bell's inequality in time with weak measurement

The violation of J. Bell's inequality with two entangled and spatially separated quantum two- level systems (TLS) is often considered as the most prominent demonstration that nature does not obey ?local realism?. Under different but related assumptions of "macrorealism", plausible for macroscopic systems, Leggett and Garg derived a similar inequality for a single degree of freedom undergoing coherent oscillations and being measured at successive times. Such a "Bell's inequality in time", which should be violated by a quantum TLS, is tested here. In this work, the TLS is a superconducting quantum circuit whose Rabi oscillations are continuously driven while it is continuously and weakly measured. The time correlations present at the detector output agree with quantum-mechanical predictions and violate the inequality by 5 standard deviations.Comment: 26 pages including 10 figures, preprint forma

Démonstration de l accélération quantique avec un processeur quantique à deux Transmons

Cette thèse a pour sujet la caractérisation d un processeur quantique utilisant deux qubits supraconducteurs de type Transmon couplés capacitivement. Chacun des deux qubits peut être manipulé individuellement et lu en une seule fois de façon non destructive. Avec ce système, il est possible de réaliser une porte quantique universelle en utilisant l interaction entre les deux qubits. Ce système possède donc toutes les caractéristiques d un processeur quantique universel à deux qubits. On se sert de ce processeur pour implémenter la porte quantique universelle sqrt(iSWAP), qu on caractérise en utilisant une methode de tomographie du processus quantique. On obtient une fidélité de porte de 90 %. On utilise cette porte universelle pour réaliser des états intriqués à deux qubits afin d effectuer un test de l inégalité de Bell. On observe une violation de la limite classique de l équation par 22 écart-types après avoir corrigé les erreurs de lecture. En utilisant la porte iSWAP, on implémente l algorithme de Grover pour deux qubits. Cet algorithme de recherche trouve parmi quatre états x {00,01,10,11}, l état y qui satisfait f(y)=1, où f est une fonction de recherche telle que f(x y)=0. L'implémentation de cet algorithme permet de trouver la réponse au problème de recherche avec une probabilité comprise entre 52 % et 67 %, dépassant en performance les algorithmes classiques, dont les probabilités de succès sont limitées à 25 %. En conséquence, cette expérience est une preuve du concept d'accélération quantique. Enfin, nous proposons une architecture scalable qui pourrait résoudre certains problèmes des architectures courantes de qubits supraconducteursPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF