1,395 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.
Characterising exo-ringsystems around fast-rotating stars using the Rossiter-McLaughlin effect
Planetary rings produce a distinct shape distortion in transit lightcurves.
However, to accurately model such lightcurves the observations need to cover
the entire transit, especially ingress and egress, as well as an out-of-transit
baseline. Such observations can be challenging for long period planets, where
the transits may last for over a day. Planetary rings will also impact the
shape of absorption lines in the stellar spectrum, as the planet and rings
cover different parts of the rotating star (the Rossiter-McLaughlin effect).
These line-profile distortions depend on the size, structure, opacity,
obliquity and sky projected angle of the ring system. For slow rotating stars,
this mainly impacts the amplitude of the induced velocity shift, however, for
fast rotating stars the large velocity gradient across the star allows the line
distortion to be resolved, enabling direct determination of the ring
parameters. We demonstrate that by modeling these distortions we can recover
ring system parameters (sky-projected angle, obliquity and size) using only a
small part of the transit. Substructure in the rings, e.g. gaps, can be
recovered if the width of the features () relative to the size of the
star is similar to the intrinsic velocity resolution (set by the width of the
local stellar profile, ) relative to the stellar rotation velocity (
sin, i.e. sin/). This opens up a new
way to study the ring systems around planets with long orbital periods, where
observations of the full transit, covering the ingress and egress, are not
always feasible.Comment: Accepted for publication in MNRA
Coherent Quantum Dynamics of a Superconducting Flux Qubit
We have observed coherent time evolution between two quantum states of a
superconducting flux qubit comprising three Josephson junctions in a loop. The
superposition of the two states carrying opposite macroscopic persistent
currents is manipulated by resonant microwave pulses. Readout by means of
switching-event measurement with an attached superconducting quantum
interference device revealed quantum-state oscillations with high fidelity.
Under strong microwave driving it was possible to induce hundreds of coherent
oscillations. Pulsed operations on this first sample yielded a relaxation time
of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These
results are promising for future solid-state quantum computing.Comment: submitted 2 December 2002; accepted 4 February 200
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
Parametric coupling for superconducting qubits
We propose a scheme to couple two superconducting charge or flux qubits
biased at their symmetry points with unequal energy splittings. Modulating the
coupling constant between two qubits at the sum or difference of their two
frequencies allows to bring them into resonance in the rotating frame.
Switching on and off the modulation amounts to switching on and off the
coupling which can be realized at nanosecond speed. We discuss various physical
implementations of this idea, and find that our scheme can lead to rapid
operation of a two-qubit gate.Comment: 6 page
QND measurement of a superconducting qubit in the weakly projective regime
Quantum state detectors based on switching of hysteretic Josephson junctions
biased close to their critical current are simple to use but have strong
back-action. We show that the back-action of a DC-switching detector can be
considerably reduced by limiting the switching voltage and using a fast
cryogenic amplifier, such that a single readout can be completed within 25 ns
at a repetition rate of 1 MHz without loss of contrast. Based on a sequence of
two successive readouts we show that the measurement has a clear quantum
non-demolition character, with a QND fidelity of 75 %.Comment: submitted to PR
Detection of a persistent-current qubit by resonant activation
We present the implementation of a new scheme to detect the quantum state of
a persistent-current qubit. It relies on the dependency of the measuring
Superconducting Quantum Interference Device (SQUID) plasma frequency on the
qubit state, which we detect by resonant activation. With a measurement pulse
of only 5ns, we observed Rabi oscillations with high visibility (65%).Comment: 4 pages, 4 figures, submitted to PRB Rapid Co
Decoherence of Flux Qubits Coupled to Electronic Circuits
On the way to solid-state quantum computing, overcoming decoherence is the
central issue. In this contribution, we discuss the modeling of decoherence of
a superonducting flux qubit coupled to dissipative electronic circuitry. We
discuss its impact on single qubit decoherence rates and on the performance of
two-qubit gates. These results can be used for designing decoherence-optimal
setups.Comment: 16 pages, 5 figures, to appear in Advances in Solid State Physics,
Vol. 43 (2003
Phase-slip flux qubits
In thin superconducting wires, phase-slip by thermal activation near the
critical temperature is a well-known effect. It has recently become clear that
phase-slip by quantum tunnelling through the energy barrier can also have a
significant rate at low temperatures. In this paper it is suggested that
quantum phase-slip can be used to realize a superconducting quantum bit without
Josephson junctions. A loop containing a nanofabricated very thin wire is
biased with an externally applied magnetic flux of half a flux quantum,
resulting in two states with opposite circulating current and equal energy.
Quantum phase-slip should provide coherent coupling between these two
macroscopic states. Numbers are given for a wire of amorphous niobium-silicon
that can be fabricated with advanced electron beam lithography.Comment: Submitted to New Journal of Physics, special issue solid state
quantum informatio
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