40 research outputs found
Quantum memory for microwave photons in an inhomogeneously broadened spin ensemble
We propose a multi-mode quantum memory protocol able to store the quantum
state of the field in a microwave resonator into an ensemble of electronic
spins. The stored information is protected against inhomogeneous broadening of
the spin ensemble by spin-echo techniques resulting in memory times orders of
magnitude longer than previously achieved. By calculating the evolution of the
first and second moments of the spin-cavity system variables for realistic
experimental parameters, we show that a memory based on NV center spins in
diamond can store a qubit encoded on the |0> and |1> Fock states of the field
with 80% fidelity.Comment: 5 pages, 4 figures, 11 pages supplementary materia
Coupling a single Nitrogen-Vacancy center to a superconducting flux qubit in the far off resonance regime
We present a theoretical proposal to couple a single Nitrogen-Vacancy (NV)
center to a superconducting flux qubit (FQ) in the regime where both systems
are off resonance. The coupling between both quantum devices is achieved
through the strong driving of the flux qubit by a classical microwave field
that creates dressed states with an experimentally controlled characteristic
frequency. We discuss several applications such as controlling the NV center's
state by manipulation of the flux qubit, performing the NV center full
tomography and using the NV center as a quantum memory. The effect of
decoherence and its consequences to the proposed applications are also
analyzed. Our results provide a theoretical framework describing a promising
hybrid system for quantum information processing, which combines the advantages
of fast manipulation and long coherence times.Comment: 8 pages, 9 figure
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
Quantum technologies with hybrid systems
International audienceAn extensively pursued current direction of research in physics aims at the development of practical technologies that exploit the effects of quantum mechanics. As part of this ongoing effort, devices for quantum information processing, secure communication, and high-precision sensing are being implemented with diverse systems, ranging from photons, atoms, and spins to mesoscopic superconducting and nanomechanical structures. Their physical properties make some of these systems better suited than others for specific tasks; thus, photons are well suited for transmitting quantum information, weakly interacting spins can serve as long-lived quantum memories, and superconducting elements can rapidly process information encoded in their quantum states. A central goal of the envisaged quantum technologies is to develop devices that can simultaneously perform several of these tasks, namely, reliably store, process, and transmit quantum information. Hybrid quantum systems composed of different physical components with complementary functionalities may provide precisely such multitasking capabilities. This article reviews some of the driving theoretical ideas and first experimental realizations of hybrid quantum systems and the opportunities and challenges they present and offers a glance at the near-and long-term perspectives of this fascinating and rapidly expanding field. hybrid quantum systems | quantum technologies | quantum information During the last several decades, quantum physics has evolved from being primarily the conceptual framework for the description of microscopic phenomena to providing inspiration for new technological applications. A range of ideas for quantum information processing (1) and secure communication (2, 3), quantum enhanced sensing (4–8), and the simulation of complex dynamics (9–14) has given rise to expectations that society may before long benefit from such quantum technologies. These developments are driven by our rapidly evolving abilities to experimentally manipulate and control quantum dynamics in diverse systems, ranging from single photons (2, 13), atoms and ions (11, 12), and individual electron and nuclear spins (15–17), to mesoscopic super-conducting (14, 18) and nanomechanical devices (19, 20). As a rule, each of these systems can execute one or a few specific tasks, but no single system can be universally suitable for all envisioned applications. Thus, photons are best suited for transmitting quantum information, weakly interacting spins may serve as long-lived quantum memories , and the dynamics of electronic states of atoms or electric charges in semiconductors and superconducting elements may realize rapid processing of information encoded in their quantum states. The implementation of devices that can simultaneously perform several or all of these tasks, e.g., reliably store, process, and transmit quantum states, calls for a new paradigm: that of hybrid quantum systems (HQSs) (15, 21–24). HQSs attain their multitasking capabilities by combining different physical components with complementary functionalities. Many of the early ideas for HQSs emerged from the field of quantum information processing and communication (QIPC) and were, to a large extent, inspired by the development of QIPC architectures in which superconducting qubits are coupled to high-quality microwave resonators (18, 25). Super-conducting qubits are very-well-controlled quantum systems (26, 27), but in contrast to atoms, they suffer from comparatively short coherence times and do not couple coherently to optical photons. A microwave resonator, such as, for example, a lumped-element LC-circuit or coplanar waveguide (CPW) res-onator, can serve as an interface between superconducting qubits and also between superconducting qubits and other quantum systems with longer coherence times and optical transitions (18, 22, 23, 28). It has thus been proposed to couple superconducting qubits, via a " microwave quantum bus, " to ions (29), atoms (30–32), polar molecules (33), electrons confined above a liquid helium surface (34), and spin-doped crystals (15, 35–37). With the recent advances in the control of micro-and nanomechanical systems (19, 20), the use of a mechanical quantum bus ha
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
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
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
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