42 research outputs found
Dephasing and thermal smearing in an electromechanical which-path device
In an electromechanical which-path device electrons travelling through an
Aharonov-Bohm ring with a quantum dot in one of the arms are dephased by an
interaction with the fundamental flexural mode of a radio-frequency cantilever,
leading to a reduction in the visibilty of the interference fringes. However,
at finite temperatures time-averaged measurement of the current leads to a
fringe visibility which is reduced partly by dephasing of the electrons and
partly by a thermal smearing effect. The balance between thermal smearing and
dephasing predicted by a calculation depends very strongly on the choice of
cantilever basis states used. The interaction between the cantilever and its
environment is expected to select the coherent state basis for the cantilever
and hence lead to a dephasing rate which is substantially lower than that which
would arise if instead the Fock states were selected.Comment: To appear in Phonons 2001 Proceedings (Physica B
Synchronization of micromasers
We investigate synchronization effects in quantum self-sustained oscillators theoretically using the micromaser as a model system. We use the probability distribution for the relative phase as a tool for quantifying the emergence of preferred phases when two micromasers are coupled together. Using perturbation theory, we show that the behavior of the phase distribution is strongly dependent on exactly how the oscillators are coupled. In the quantum regime where photon occupation numbers are low we find that although synchronization effects are rather weak, they are nevertheless significantly stronger than expected from a semiclassical description of the phase dynamics. We also compare the behavior of the phase distribution with the mutual information of the two oscillators and show that they can behave in rather different ways
Mechanical Lamb-shift analogue for the Cooper-pair box
We estimate the correction to the Cooper-pair box energy level splitting due
to the quantum motion of a coupled micromechanical gate electrode. While the
correction due to zero-point motion is very small, it should be possible to
observe thermal motion-induced corrections to the photon-assisted tunneling
current.Comment: To appear in Phonons 2001 Proceedings (Physica B
A cavity-Cooper pair transistor scheme for investigating quantum optomechanics in the ultrastrong coupling regime
We propose a scheme involving a Cooper pair transistor (CPT) embedded in a superconducting microwave cavity, where the CPT serves as a charge tunable quantum inductor to facilitate ultra-strong coupling between photons in the cavity and a nano- to meso-scale mechanical resonator. The mechanical resonator is capacitively coupled to the CPT, such that mechanical displacements of the resonator cause a shift in the CPT inductance and hence the cavity's resonant frequency. The amplification provided by the CPT is sufficient for the zero point motion of the mechanical resonator alone to cause a significant change in the cavity resonance. Conversely, a single photon in the cavity causes a shift in the mechanical resonator position on the order of its zero point motion. As a result, the cavity-Cooper pair transistor coupled to a mechanical resonator will be able to access a regime in which single photons can affect single phonons and vice versa. Realizing this ultra strong coupling regime will facilitate the creation of non-classical states of the mechanical resonator, as well as the means to accurately characterize such states by measuring the cavity photon field
Feedback cooling of a nanomechanical resonator
Cooled, low-loss nanomechanical resonators offer the prospect of directly
observing the quantum dynamics of mesoscopic systems. However, the present
state of the art requires cooling down to the milliKelvin regime in order to
observe quantum effects. Here we present an active feedback strategy based on
continuous observation of the resonator position for the purpose of obtaining
these low temperatures. In addition, we apply this to an experimentally
realizable configuration, where the position monitoring is carried out by a
single-electron transistor. Our estimates indicate that with current technology
this technique is likely to bring the required low temperatures within reach.Comment: 10 pages, RevTex4, 4 color eps figure
Impact of van der Waals forces on the classical shuttle instability
The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage larger than a threshold voltage . This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage . Associated with the process is also
hysteresis in the I-V characteristics.Comment: minor revisions, updated references, Article published in Phys. Rev.
B 69, 035309 (2004
Quantum Measurement of a Coupled Nanomechanical Resonator -- Cooper-Pair Box System
We show two effects as a result of considering the second-order correction to
the spectrum of a nanomechanical resonator electrostatically coupled to a
Cooper-pair box. The spectrum of the Cooper-pair box is modified in a way which
depends on the Fock state of the resonator. Similarly, the frequency of the
resonator becomes dependent on the state of the Cooper-pair box. We consider
whether these frequency shifts could be utilized to prepare the nanomechanical
resonator in a Fock state, to perform a quantum non-demolition measurement of
the resonator Fock state, and to distinguish the phase states of the
Cooper-pair box
Numerical analysis of the radio-frequency single-electron transistor operation
We have analyzed numerically the response and noise-limited charge
sensitivity of a radio-frequency single-electron transistor (RF-SET) in a
non-superconducting state using the orthodox theory. In particular, we have
studied the performance dependence on the quality factor Q of the tank circuit
for Q both below and above the value corresponding to the impedance matching
between the coaxial cable and SET.Comment: 14 page
Entanglement Sharing in the Two-Atom Tavis-Cummings Model
Individual members of an ensemble of identical systems coupled to a common
probe can become entangled with one another, even when they do not interact
directly. We investigate how this type of multipartite entanglement is
generated in the context of a system consisting of two two-level atoms
resonantly coupled to a single mode of the electromagnetic field. The dynamical
evolution is studied in terms of the entanglements in the different bipartite
partitions of the system, as quantified by the I-tangle. We also propose a
generalization of the so-called residual tangle that quantifies the inherent
three-body correlations in our tripartite system. This enables us to completely
characterize the phenomenon of entanglement sharing in the case of the two-atom
Tavis-Cummings model, a system of both theoretical and experimental interest.Comment: 11 pages, 4 figures, submitted to PRA, v3 contains corrections to
small error
Steering of a Bosonic Mode with a Double Quantum Dot
We investigate the transport and coherence properties of a double quantum dot
coupled to a single damped boson mode. Our numerically results reveal how the
properties of the boson distribution can be steered by altering parameters of
the electronic system such as the energy difference between the dots.
Quadrature amplitude variances and the Wigner function are employed to
illustrate how the state of the boson mode can be controlled by a stationary
electron current through the dots.Comment: 10 pages, 6 figures, to appear in Phys. Rev.