328 research outputs found
Quantum state detection of a superconducting flux qubit using a DC-SQUID in the inductive mode
We present a readout method for superconducting flux qubits. The qubit
quantum flux state can be measured by determining the Josephson inductance of
an inductively coupled DC superconducting quantum interference device
(DC-SQUID). We determine the response function of the DC-SQUID and its
back-action on the qubit during measurement. Due to driving, the qubit energy
relaxation rate depends on the spectral density of the measurement circuit
noise at sum and difference frequencies of the qubit Larmor frequency and SQUID
driving frequency. The qubit dephasing rate is proportional to the spectral
density of circuit noise at the SQUID driving frequency. These features of the
backaction are qualitatively different from the case when the SQUID is used in
the usual switching mode. For a particular type of readout circuit with
feasible parameters we find that single shot readout of a superconducting flux
qubit is possible.Comment: 11 pages, 3 figures; submitted to Phys. Rev.
Dynamics of parametric fluctuations induced by quasiparticle tunneling in superconducting flux qubits
We present experiments on the dynamics of a two-state parametric fluctuator
in a superconducting flux qubit. In spectroscopic measurements, the fluctuator
manifests itself as a doublet line. When the qubit is excited in resonance with
one of the two doublet lines, the correlation of readout results exhibits an
exponential time decay which provides a measure of the fluctuator transition
rate. The rate increases with temperature in the interval 40 to 158 mK. Based
on the magnitude of the transition rate and the doublet line splitting we
conclude that the fluctuation is induced by quasiparticle tunneling. These
results demonstrate the importance of considering quasiparticles as a source of
decoherence in flux qubits.Comment: 12 pages, including supplementary informatio
Nondestructive readout for a superconducting flux qubit
We present a new readout method for a superconducting flux qubit, based on
the measurement of the Josephson inductance of a superconducting quantum
interference device that is inductively coupled to the qubit. The intrinsic
flux detection efficiency and back-action are suitable for a fast and
nondestructive determination of the quantum state of the qubit, as needed for
readout of multiple qubits in a quantum computer. We performed spectroscopy of
a flux qubit and we measured relaxation times of the order of 80 .Comment: 4 pages, 4 figures; modified content, figures and references;
accepted for publication in Phys. Rev. Let
High-resolution spatial mapping of a superconducting NbN wire using single-electron detection
Superconducting NbN wires have recently received attention as detectors for
visible and infrared photons. We present experiments in which we use a NbN wire
for high-efficiency (40 %) detection of single electrons with keV energy. We
use the beam of a scanning electron microscope as a focussed, stable, and
calibrated electron source. Scanning the beam over the surface of the wire
provides a map of the detection efficiency. This map shows features as small as
150 nm, revealing wire inhomogeneities. The intrinsic resolution of this
mapping method, superior to optical methods, provides the basis of a
characterization tool relevant for photon detectors.Comment: 2009 IEEE Toronto International Conference, Science and Technology
for Humanity (TIC-STH
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
Low-crosstalk bifurcation detectors for coupled flux qubits
We present experimental results on the crosstalk between two AC-operated
dispersive bifurcation detectors, implemented in a circuit for high-fidelity
readout of two strongly coupled flux qubits. Both phase-dependent and
phase-independent contributions to the crosstalk are analyzed. For proper
tuning of the phase the measured crosstalk is 0.1 % and the correlation between
the measurement outcomes is less than 0.05 %. These results show that
bifurcative readout provides a reliable and generic approach for multi-partite
correlation experiments.Comment: Copyright 2010 American Institute of Physics. This article may be
downloaded for personal use only. Any other use requires prior permission of
the author and the American Institute of Physics. The following article
appeared in Applied Physics Letters and may be found at
http://link.aip.org/link/?apl/96/12350
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