525 research outputs found
Entanglement Dynamics in a Dispersively Coupled Qubit-Oscillator System
We study entanglement dynamics in a system consisting of a qubit dispersively
coupled to a finite-temperature, dissipative, driven oscillator. We show that
there are two generic ways to generate entanglement: one can entangle the qubit
either with the phase or the amplitude of the oscillator's motion. Using an
exact solution of the relevant quantum master equation, we study the robustness
of both these kinds of entanglement against the effects of dissipation and
temperature; in the limit of zero temperature (but finite damping), a simple
analytic expression is derived for the logarithmic negativity. We also discuss
how the generated entanglement may be detected via dephasing revivals, being
mindful that revivals can occur even in the absence of any useful entanglement.
Our results have relevance to quantum electromechanics, as well as to circuit
QED systems.Comment: 5 pages, 5 figure
Full counting statistics and conditional evolution in a nanoelectromechanical system
We study theoretically the full distribution of transferred charge in a
tunnel junction (or quantum point contact) coupled to a nanomechanical
oscillator, as well as the conditional evolution of the oscillator. Even if the
oscillator is very weakly coupled to the tunnel junction, it can strongly
affect the tunneling statistics and lead to a highly non-Gaussian distribution.
Conversely, given a particular measurement history of the current, the
oscillator energy distribution may be localized and highly non-thermal. We also
discuss non-Gaussian correlations between the oscillator motion and tunneling
electrons; these show that the tunneling back-action cannot be fully described
as an effective thermal bath coupled to the oscillator.Comment: 7 pages; figure added; typos correcte
Dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor
We present an analysis of the dynamics of a nanomechanical resonator coupled
to a superconducting single electron transistor (SSET) in the vicinity of the
Josephson quasiparticle (JQP) and double Josephson quasiparticle (DJQP)
resonances. For weak coupling and wide separation of dynamical timescales, we
find that for either superconducting resonance the dynamics of the resonator is
given by a Fokker-Planck equation, i.e., the SSET behaves effectively as an
equilibrium heat bath, characterised by an effective temperature, which also
damps the resonator and renormalizes its frequency. Depending on the gate and
drain-source voltage bias points with respect to the superconducting resonance,
the SSET can also give rise to an instability in the mechanical resonator
marked by negative damping and temperature within the appropriate Fokker-Planck
equation. Furthermore, sufficiently close to a resonance, we find that the
Fokker-Planck description breaks down. We also point out that there is a close
analogy between coupling a nanomechanical resonator to a SSET in the vicinity
of the JQP resonance and Doppler cooling of atoms by means of lasers
Laser-like Instabilities in Quantum Nano-electromechanical Systems
We discuss negative damping regimes in quantum nano-electromechanical systems
formed by coupling a mechanical oscillator to a single-electron transistor
(normal or superconducting). Using an analogy to a laser with a tunable
atom-field coupling, we demonstrate how these effects scale with system
parameters. We also discuss the fluctuation physics of both the oscillator and
the single-electron transistor in this regime, and the degree to which the
oscillator motion is coherent.Comment: 4+ pages, 1 figure; reference to the work of Dykman and Krivoglaz
adde
Damping of a nanomechanical oscillator strongly coupled to a quantum dot
We present theoretical and experimental results on the mechanical damping of
an atomic force microscope cantilever strongly coupled to a self-assembled InAs
quantum dot. When the cantilever oscillation amplitude is large, its motion
dominates the charge dynamics of the dot which in turn leads to nonlinear,
amplitude-dependent damping of the cantilever. We observe highly asymmetric
lineshapes of Coulomb blockade peaks in the damping that reflect the degeneracy
of energy levels on the dot, in excellent agreement with our strong coupling
theory. Furthermore, we predict that excited state spectroscopy is possible by
studying the damping versus oscillation amplitude, in analogy to varying the
amplitude of an ac gate voltage.Comment: 4+ pages, 4 figure
Heisenberg-limited qubit readout with two-mode squeezed light
We show how to use two-mode squeezed light to exponentially enhance
cavity-based dispersive qubit measurement. Our scheme enables true
Heisenberg-limited scaling of the measurement, and crucially, is not restricted
to small dispersive couplings or unrealistically long measurement times. It
involves coupling a qubit dispersively to two cavities, and making use of a
symmetry in the dynamics of joint cavity quadratures (a so-called
quantum-mechanics-free subsystem). We discuss the basic scaling of the scheme
and its robustness against imperfections, as well as a realistic implementation
in circuit quantum electrodynamics.Comment: 5 pages, 4 figures, Supplemental Materia
Mechanically probing coherent tunnelling in a double quantum dot
We study theoretically the interaction between the charge dynamics of a
few-electron double quantum dot and a capacitively-coupled AFM cantilever, a
setup realized in several recent experiments. We demonstrate that the
dot-induced frequency shift and damping of the cantilever can be used as a
sensitive probe of coherent inter-dot tunnelling, and that these effects can be
used to quantitatively extract both the magnitude of the coherent interdot
tunneling and (in some cases) the value of the double-dot T_1 time. We also
show how the adiabatic modulation of the double-dot eigenstates by the
cantilever motion leads to new effects compared to the single-dot case.Comment: 6 pages, 2 figure
Cooling a nanomechanical resonator with quantum back-action
Quantum mechanics demands that the act of measurement must affect the
measured object. When a linear amplifier is used to continuously monitor the
position of an object, the Heisenberg uncertainty relationship requires that
the object be driven by force impulses, called back-action. Here we measure the
back-action of a superconducting single-electron transistor (SSET) on a
radiofrequency nanomechanical resonator. The conductance of the SSET, which is
capacitively coupled to the resonator, provides a sensitive probe of the
latter's position;back-action effects manifest themselves as an effective
thermal bath, the properties of which depend sensitively on SSET bias
conditions. Surprisingly, when the SSET is biased near a transport resonance,
we observe cooling of the nanomechanical mode from 550mK to 300mK-- an effect
that is analogous to laser cooling in atomic physics. Our measurements have
implications for nanomechanical readout of quantum information devices and the
limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic
resonance force microscopy). Furthermore, we anticipate the use of these
backaction effects to prepare ultracold and quantum states of mechanical
structures, which would not be accessible with existing technology.Comment: 28 pages, 7 figures; accepted for publication in Natur
Using a qubit to measure photon number statistics of a driven, thermal oscillator
We demonstrate theoretically how photon number statistics of a driven, damped
oscillator at finite temperature can be extracted by measuring the dephasing
spectrum of a two-level system dispersively coupled to the oscillator; we thus
extend the work of Dykman (1987) and Gambetta et al. (2006). We carefully
consider the fidelity of this scheme-- to what extent does the measurement
reflect the initial number statistics of the mode? We also derive analytic
results for the dephasing of a qubit by a driven, thermal mode, and compare
results obtained at different levels of approximation. Our results have
relevance both to experiments in circuit cavity QED, as well as to
nano-electromechanical systems.Comment: 11 pages; 2 figures adde
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