455 research outputs found
Photovoltaic and Rectification Currents in Quantum Dots
We investigate theoretically and experimentally the statistical properties of
dc current through an open quantum dot subject to ac excitation of a
shape-defining gate. The symmetries of rectification current and photovoltaic
current with respect to applied magnetic field are examined. Theory and
experiment are found to be in good agreement throughout a broad range of
frequency and ac power, ranging from adiabatic to nonadiabatic regimes.Comment: 4 pages, 3 figures; related articles at http://marcuslab.harvard.ed
Probing dynamics of an electron-spin ensemble via a superconducting resonator
We study spin relaxation and diffusion in an electron-spin ensemble of
nitrogen impurities in diamond at low temperature (0.25-1.2 K) and polarizing
magnetic field (80-300 mT). Measurements exploit mode- and
temperature-dependent coupling of hyperfine-split sub-ensembles to the
resonator. Temperature-independent spin linewidth and relaxation time suggest
that spin diffusion limits spin relaxation. Depolarization of one sub-ensemble
by resonant pumping of another indicates fast cross-relaxation compared to spin
diffusion, with implications on use of sub-ensembles as independent quantum
memories.Comment: 5 pages, 5 figures, and Supplementary Information (2 figures
Protecting quantum entanglement from leakage and qubit errors via repetitive parity measurements
Protecting quantum information from errors is essential for large-scale
quantum computation. Quantum error correction (QEC) encodes information in
entangled states of many qubits, and performs parity measurements to identify
errors without destroying the encoded information. However, traditional QEC
cannot handle leakage from the qubit computational space. Leakage affects
leading experimental platforms, based on trapped ions and superconducting
circuits, which use effective qubits within many-level physical systems. We
investigate how two-transmon entangled states evolve under repeated parity
measurements, and demonstrate the use of hidden Markov models to detect leakage
using only the record of parity measurement outcomes required for QEC. We show
the stabilization of Bell states over up to 26 parity measurements by
mitigating leakage using postselection, and correcting qubit errors using
Pauli-frame transformations. Our leakage identification method is
computationally efficient and thus compatible with real-time leakage tracking
and correction in larger quantum processors.Comment: 22 pages, 15 figure
Detecting bit-flip errors in a logical qubit using stabilizer measurements
Quantum data is susceptible to decoherence induced by the environment and to
errors in the hardware processing it. A future fault-tolerant quantum computer
will use quantum error correction (QEC) to actively protect against both. In
the smallest QEC codes, the information in one logical qubit is encoded in a
two-dimensional subspace of a larger Hilbert space of multiple physical qubits.
For each code, a set of non-demolition multi-qubit measurements, termed
stabilizers, can discretize and signal physical qubit errors without collapsing
the encoded information. Experimental demonstrations of QEC to date, using
nuclear magnetic resonance, trapped ions, photons, superconducting qubits, and
NV centers in diamond, have circumvented stabilizers at the cost of decoding at
the end of a QEC cycle. This decoding leaves the quantum information vulnerable
to physical qubit errors until re-encoding, violating a basic requirement for
fault tolerance. Using a five-qubit superconducting processor, we realize the
two parity measurements comprising the stabilizers of the three-qubit
repetition code protecting one logical qubit from physical bit-flip errors. We
construct these stabilizers as parallelized indirect measurements using
ancillary qubits, and evidence their non-demolition character by generating
three-qubit entanglement from superposition states. We demonstrate
stabilizer-based quantum error detection (QED) by subjecting a logical qubit to
coherent and incoherent bit-flip errors on its constituent physical qubits.
While increased physical qubit coherence times and shorter QED blocks are
required to actively safeguard quantum information, this demonstration is a
critical step toward larger codes based on multiple parity measurements.Comment: 6 pages, 4 figures, 10 supplementary figure
Charge fluctuations in open chaotic cavities
We present a discussion of the charge response and the charge fluctuations of
mesoscopic chaotic cavities in terms of a generalized Wigner-Smith matrix. The
Wigner-Smith matrix is well known in investigations of time-delay of quantum
scattering. It is expressed in terms of the scattering matrix and its
derivatives with energy. We consider a similar matrix but instead of an energy
derivative we investigate the derivative with regard to the electric potential.
The resulting matrix is then the operator of charge. If this charge operator is
combined with a self-consistent treatment of Coulomb interaction, the charge
operator determines the capacitance of the system, the non-dissipative
ac-linear response, the RC-time with a novel charge relaxation resistance, and
in the presence of transport a resistance that governs the displacement
currents induced into a nearby conductor. In particular these capacitances and
resistances determine the relaxation rate and dephasing rate of a nearby qubit
(a double quantum dot). We discuss the role of screening of mesoscopic chaotic
detectors. Coulomb interaction effects in quantum pumping and in photon
assisted electron-hole shot noise are treated similarly. For the latter we
present novel results for chaotic cavities with non-ideal leads.Comment: 29 pages, 13 figures;v.2--minor changes; contribution for the special
issue of J. Phys. A on "Trends in Quantum Chaotic Scattering
Short-term changes on MRI predict long-term changes on radiography in rheumatoid arthritis: an analysis by an OMERACT Task Force of pooled data from four randomised controlled trials
Objective: In rheumatoid arthritis (RA), MRI provides earlier detection of structural damage than radiography (X-ray) and more sensitive detection of intra-articular inflammation than clinical examination. This analysis was designed to evaluate the ability of early MRI findings to predict subsequent structural damage by X-ray. Methods: Pooled data from four randomised controlled trials (RCTs) involving 1022 RA hands and wrists in early and established RA were analysed. X-rays were scored using van der Heijde-modified or Genant-modified Sharp methods. MRIs were scored using Outcome Measures in Rheumatology (OMERACT) RA MRI Score (RAMRIS). Data were analysed at the patient level using multivariable logistic regression and receiver operating characteristic curve analyses. Results: Progression of MRI erosion scores at Weeks 12 and 24 predicted progression of X-ray erosions at Weeks 24 and 52, with areas under the curve (AUCs) of 0.64 and 0.74, respectively. 12-week and 24-week changes in MRI osteitis scores were similarly predictive of 24-week and 52-week X-ray erosion progressions; pooled AUCs were 0.78 and 0.77, respectively. MRI changes in synovitis at Weeks 12 and 24 also predicted progression of X-ray joint damage (erosion and joint-space narrowing) at Weeks 24 and 52 (AUCs=0.72 and 0.65, respectively). Conclusions: Early changes in joint damage and inflammation detected with MRI predict changes in joint damage evident on subsequent X-rays. These findings support the use of MRI as a valid method for monitoring structural damage in short-duration RCTs
Electron pumping in graphene mechanical resonators
The combination of high frequency vibrations and metallic transport in
graphene makes it a unique material for nano-electromechanical devices. In this
letter, we show that graphene-based nano-electromechanical devices are
extremely well suited for charge pumping, due to the sensitivity of its
transport coefficients to perturbations in electrostatic potential and
mechanical deformations, with the potential for novel small scale devices with
useful applications
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