62 research outputs found
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 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 and an
operational dark count rate , mainly due to the
out-of-equilibrium microwave photons in the input line. The corresponding power
sensitivity is , one order of
magnitude lower than the state of the art. The detector operates continuously
over hour timescales with a duty cycle , and offers
frequency tunability of 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 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
.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
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