369 research outputs found
Dissipative stabilization of entangled cat states using a driven Bose-Hubbard dimer
We analyze a modified Bose-Hubbard model, where two cavities having on-site
Kerr interactions are subject to two-photon driving and correlated dissipation.
We derive an exact solution for the steady state of this interacting
driven-dissipative system, and use it show that the system permits the
preparation and stabilization of pure entangled non-Gaussian states, so-called
entangled cat states. Unlike previous proposals for dissipative stabilization
of such states, our approach requires only a linear coupling to a single
engineered reservoir (as opposed to nonlinear couplings to two or more
reservoirs). Our scheme is within the reach of state-of-the-art experiments in
circuit QED.Comment: 5 pages main text, 5 pages appendices, 6 figure
Sequential Strong Measurements and Heat Vision
We study scenarios where a finite set of non-demolition von-Neumann
measurements are available. We note that, in some situations, repeated
application of such measurements allows estimating an infinite number of
parameters of the initial quantum state, and illustrate the point with a
physical example. We then move on to study how the system under observation is
perturbed after several rounds of projective measurements. While in the finite
dimensional case the effect of this perturbation always saturates, there are
some instances of infinite dimensional systems where such a perturbation is
accumulative, and the act of retrieving information about the system increases
its energy indefinitely (i.e., we have `Heat Vision'). We analyze this effect
and discuss a specific physical system with two dichotomic von-Neumann
measurements where Heat Vision is expected to show.Comment: See the Appendix for weird examples of heat visio
Quantum nano-electromechanics with electrons, quasiparticles and Cooper pairs: effective bath descriptions and strong feedback effects
Using a quantum noise approach, we discuss the physics of both normal metal
and superconducting single electron transistors (SET) coupled to mechanical
resonators. Particular attention is paid to the regime where transport occurs
via incoherent Cooper-pair tunneling (either via the Josephson quasiparticle
(JQP) or double Josephson quasiparticle (DJQP) process). We show that,
surprisingly, the back-action of tunneling Cooper pairs (or superconducting
quasiparticles) can be used to significantly cool the oscillator. We also
discuss the physical origin of negative damping effects in this system, and how
they can lead to a regime of strong electro-mechanical feedback, where despite
a weak SET - oscillator coupling, the motion of the oscillator strongly effects
the tunneling of the Cooper pairs. We show that in this regime, the oscillator
is characterized by an energy-dependent effective temperature. Finally, we
discuss the strong analogy between back-action effects of incoherent
Cooper-pair tunneling and ponderomotive effects in an optical cavity with a
moveable mirror; in our case, tunneling Cooper pairs play the role of the
cavity photons.Comment: 27 pages, 7 figures; submitted to the New Journal of Physics focus
issue on Nano-electromechanical Systems; error in references correcte
The information about the state of a qubit gained by a weakly coupled detector
We analyze the information that one can learn about the state of a quantum
two-level system, i.e. a qubit, when probed weakly by a nearby detector. In
particular, we focus on the case when the qubit Hamiltonian and the qubit's
operator being probed by the detector do not commute. Because the qubit's state
keeps evolving while being probed and because the measurement data is mixed
with a detector-related background noise, one might expect the detector to fail
in this case. We show, however, that under suitable conditions and by proper
analysis of the measurement data useful information about the state of the
qubit can be extracted. It turns out that the measurement basis is
stochastically determined every time the experiment is repeated. We analyze in
detail the probability distributions that govern the choice of measurement
bases. We also analyze the information acquisition rate and show that it is
largely unaffected by the apparent conflict between the measurement and
intrinsic qubit dynamics. We discuss the relation between our analysis and the
stochastic master equation that describes the evolution of the qubit's state
under the influence of measurement and decoherence. In particular, we write
down a stochastic equation that encompasses the usual stochastic master
equation for the evolution of the qubit's density matrix and additionally
contains the measurement information that can be extracted from the observed
signal.Comment: 21 pages (two column), 8 figure
Noise and Measurement Efficiency of a Partially Coherent Mesoscopic Detector
We study the noise properties and efficiency of a mesoscopic resonant-level
conductor which is used as a quantum detector, in the regime where transport
through the level is only partially phase coherent. We contrast models in which
detector incoherence arises from escape to a voltage probe, versus those in
which it arises from a random time-dependent potential. Particular attention is
paid to the back-action charge noise of the system. While the average detector
current is similar in all models, we find that its noise properties and
measurement efficiency are sensitive both to the degree of coherence and to the
nature of the dephasing source. Detector incoherence prevents quantum limited
detection, except in the non-generic case where the source of dephasing is not
associated with extra unobserved information. This latter case can be realized
in a version of the voltage probe model.Comment: 15 pages, 5 figures; revised dicussion of voltage probe model
Quantum master equation descriptions of a nanomechanical resonator coupled to a single-electron transistor
We analyse the quantum dynamics of a nanomechanical resonator coupled to a
normal-state single-electron transistor (SET). Starting from a microscopic
description of the system, we derive a master equation for the SET island
charge and resonator which is valid in the limit of weak electro-mechanical
coupling. Using this master equation we show that, apart from brief transients,
the resonator always behaves like a damped harmonic oscillator with a shifted
frequency and relaxes into a thermal-like steady state. Although the behaviour
remains qualitatively the same, we find that the magnitude of the resonator
damping rate and frequency shift depend very sensitively on the relative
magnitudes of the resonator period and the electron tunnelling time. Maximum
damping occurs when the electrical and mechanical time-scales are the same, but
the frequency shift is greatest when the resonator moves much more slowly than
the island charge. We then derive reduced master equations which describe just
the resonator dynamics. By making slightly different approximations, we obtain
two different reduced master equations for the resonator. Apart from minor
differences, the two reduced master equations give rise to a consistent picture
of the resonator dynamics which matches that obtained from the master equation
including the SET island charge.Comment: 22 pages, 4 figure
Application of Resonance Perturbation Theory to Dynamics of Magnetization in Spin Systems Interacting with Local and Collective Bosonic Reservoirs
We apply our recently developed resonance perturbation theory to describe the
dynamics of magnetization in paramagnetic spin systems interacting
simultaneously with local and collective bosonic environments. We derive
explicit expressions for the evolution of the reduced density matrix elements.
This allows us to calculate explicitly the dynamics of the macroscopic
magnetization, including characteristic relaxation and dephasing time-scales.
We demonstrate that collective effects (i) do not influence the character of
the relaxation processes but merely renormalize the relaxation times, and (ii)
significantly modify the dephasing times, leading in some cases to a
complicated (time inhomogeneous) dynamics of the transverse magnetization,
governed by an effective time-dependent magnetic field
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