274 research outputs found
Geometric quantum gates robust against stochastic control errors
We analyze a scheme for quantum computation where quantum gates can be
continuously changed from standard dynamic gates to purely geometric ones.
These gates are enacted by controlling a set of parameters that are subject to
unwanted stochastic fluctuations. This kind of noise results in a departure
from the ideal case that can be quantified by a gate fidelity. We find that the
maximum of this fidelity corresponds to quantum gates with a vanishing
dynamical phase.Comment: 4 pager
Quantum state reconstruction with imperfect rotations on an inhomogeneously broadened ensemble of qubits
We present a method for performing quantum state reconstruction on qubits and
qubit registers in the presence of decoherence and inhomogeneous broadening.
The method assumes only rudimentary single qubit rotations as well as knowledge
of decoherence and loss mechanisms. We show that full state reconstruction is
possible even in the case where single qubit rotations may only be performed
imperfectly. Furthermore we show that for ensemble quantum computing proposals,
quantum state reconstruction is possible even if the ensemble experiences
inhomogeneous broadening and if only imperfect qubit manipulations are
available during state preparation and reconstruction.Comment: 6 pages, 5 figure
Protected Rabi oscillation induced by natural interactions among physical qubits
For a system composed of nine qubits, we show that natural interactions among
the qubits induce the time evolution that can be regarded, at discrete times,
as the Rabi oscillation of a logical qubit. Neither fine tuning of the
parameters nor switching of the interactions is necessary. Although
straightforward application of quantum error correction fails, we propose a
protocol by which the logical Rabi oscillation is protected against all
single-qubit errors. The present method thus opens a simple and realistic way
of protecting the unitary time evolution against noise.Comment: In this revised manuscript, new sections V, VI, VII and new
appendices A, B, C have been added to give detailed discussions. 13 pages, 4
figure
Cooperative effects in Josephson junctions in a cavity in the strong coupling regime
We analyze the behavior of systems of two and three qubits made by Josephson
junctions, treated in the two level approximation, driven by a radiation mode
in a cavity. The regime we consider is a strong coupling one recently
experimentally reached for a single junction. Rabi oscillations are obtained
with the frequency proportional to integer order Bessel functions in the limit
of a large photon number, similarly to the case of the single qubit. A
selection rule is derived for the appearance of Rabi oscillations. A quantum
amplifier built with a large number of Josephson junctions in a cavity in the
strong coupling regime is also described.Comment: 9 pages, no figures. Version accepted for publication in Physical
Review
Zeeman energy and spin relaxation in a one-electron quantum dot
We have measured the relaxation time, T1, of the spin of a single electron
confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic
field, applied parallel to the two-dimensional electron gas in which the
quantum dot is defined, Zeeman splitting of the orbital states is directly
observed by measurements of electron transport through the dot. By applying
short voltage pulses, we can populate the excited spin state with one electron
and monitor relaxation of the spin. We find a lower bound on T1 of 50
microseconds at 7.5 T, only limited by our signal-to-noise ratio. A continuous
measurement of the charge on the dot has no observable effect on the spin
relaxation.Comment: Replaced with the version published in Phys. Rev. Let
Semiconductor few-electron quantum dot operated as a bipolar spin filter
We study the spin states of a few-electron quantum dot defined in a
two-dimensional electron gas, by applying a large in-plane magnetic field. We
observe the Zeeman splitting of the two-electron spin triplet states. Also, the
one-electron Zeeman splitting is clearly resolved at both the zero-to-one and
the one-to-two electron transition. Since the spin of the electrons transmitted
through the dot is opposite at these two transitions, this device can be
employed as an electrically tunable, bipolar spin filter. Calculations and
measurements show that higher-order tunnel processes and spin-orbit interaction
have a negligible effect on the polarization.Comment: 4 pages, 3 figure
Experimental implementation of high-fidelity unconventional geometric quantum gates using NMR interferometer
Following a key idea of unconventional geometric quantum computation
developed earlier [Phys. Rev. Lett. 91, 197902 (2003)], here we propose a more
general scheme in such an intriguing way: , where and are respectively the dynamic and
geometric phases accumulated in the quantum gate operation, with as a
constant and being dependent only on the geometric feature of the
operation. More arrestingly, we demonstrate the first experiment to implement a
universal set of such kind of generalized unconventional geometric quantum
gates with high fidelity in an NMR system.Comment: 4 pages, 3 figure
Simultaneous Spin-Charge Relaxation in Double Quantum Dots
We investigate phonon-induced spin and charge relaxation mediated by
spin-orbit and hyperfine interactions for a single electron confined within a
double quantum dot. A simple toy model incorporating both direct decay to the
ground state of the double dot and indirect decay via an intermediate excited
state yields an electron spin relaxation rate that varies non-monotonically
with the detuning between the dots. We confirm this model with experiments
performed on a GaAs double dot, demonstrating that the relaxation rate exhibits
the expected detuning dependence and can be electrically tuned over several
orders of magnitude. Our analysis suggests that spin-orbit mediated relaxation
via phonons serves as the dominant mechanism through which the double-dot
electron spin-flip rate varies with detuning.Comment: 5 pages, 3 figures, Supplemental Material (2 pages, 2 figures
Decoherence of a two-state atom driven by coherent light
Recent studies of the decoherence induced by the quantum nature of the laser
field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308
(2002); S. J. van Enk and H. J. Kimble, Quantum Inf. and Comp. 2, 1 (2002)]
have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and
the proposal made that all decoherence is due to spontaneous emission. We
analyze the problem within the formalism of cascaded open quantum systems. Our
conclusions agree with the Itano proposal. We show that the decoherence,
nevertheless, may be divided into two parts--that due to forwards scattering
and to scattering out of the laser mode. Previous authors attribute the former
to the quantum nature of the laser field.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
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