843 research outputs found
Phase diffusion pattern in quantum nondemolition systems
We quantitatively analyze the dynamics of the quantum phase distribution
associated with the reduced density matrix of a system, as the system evolves
under the influence of its environment with an energy-preserving quantum
nondemolition (QND) type of coupling. We take the system to be either an
oscillator (harmonic or anharmonic) or a two-level atom (or equivalently, a
spin-1/2 system), and model the environment as a bath of harmonic oscillators,
initially in a general squeezed thermal state. The impact of the different
environmental parameters is explicitly brought out as the system starts out in
various initial states. The results are applicable to a variety of physical
systems now studied experimentally with QND measurements.Comment: 18 pages, REVTeX, 8 figure
Dual-Resonator Speed Meter for a Free Test Mass
A description and analysis are given of a ``speed meter'' for monitoring a
classical force that acts on a test mass. This speed meter is based on two
microwave resonators (``dual resonators''), one of which couples evanescently
to the position of the test mass. The sloshing of the resulting signal between
the resonators, and a wise choice of where to place the resonators' output
waveguide, produce a signal in the waveguide that (for sufficiently low
frequencies) is proportional to the test-mass velocity (speed) rather than its
position. This permits the speed meter to achieve force-measurement
sensitivities better than the standard quantum limit (SQL), both when operating
in a narrow-band mode and a wide-band mode. A scrutiny of experimental issues
shows that it is feasible, with current technology, to construct a
demonstration speed meter that beats the wide-band SQL by a factor 2. A concept
is sketched for an adaptation of this speed meter to optical frequencies; this
adaptation forms the basis for a possible LIGO-III interferometer that could
beat the gravitational-wave standard quantum limit h_SQL, but perhaps only by a
factor 1/xi = h_SQL/h ~ 3 (constrained by losses in the optics) and at the
price of a very high circulating optical power --- larger by 1/xi^2 than that
required to reach the SQL.Comment: RevTex: 13 pages with 4 embedded figures (two .eps format and two
drawn in TeX); Submitted to Physical Review
Persistent Rabi oscillations probed via low-frequency noise correlation
The qubit Rabi oscillations are known to be non-decaying (though with a
fluctuating phase) if the qubit is continuously monitored in the weak-coupling
regime. In this paper we propose an experiment to demonstrate these persistent
Rabi oscillations via low-frequency noise correlation. The idea is to measure a
qubit by two detectors, biased stroboscopically at the Rabi frequency. The
low-frequency noise depends on the relative phase between the two combs of
biasing pulses, with a strong increase of telegraph noise in both detectors for
the in-phase or anti-phase combs. This happens because of self-synchronization
between the persistent Rabi oscillations and measurement pulses. Almost perfect
correlation of the noise in the two detectors for the in-phase regime and
almost perfect anticorrelation for the anti-phase regime indicates a presence
of synchronized persistent Rabi oscillations. The experiment can be realized
with semiconductor or superconductor qubits.Comment: 5 page
Schroedinger Cat States of a Nanomechanical Resonator
We present a scheme of generating large-amplitude Schr\"{o}dinger cat states
and entanglement in a coupled system of nanomechanical resonator and single
Cooper pair box (SCPB), without being limited by the magnitude of the coupling.
It is shown that the entanglement between the resonator and the SCPB can be
detected by a spectroscopic method.Comment: 1 figur
Superconducting re-entrant cavity transducer for a resonant bar gravitational radiation antenna
Copyright @ American Institute of PhysicsA 10‐GHz superconducting niobium re‐entrant cavity parametric transducer was developed for use in a cryogenic 1.5‐tonne Nb resonant bar gravitational radiation antenna. The transducer has a very high electrical Q (6×105 at 4.2 K), and was operated at high cavity fields without degrading the Q. A very high electromechanical coupling between the antenna and the transducer was therefore achieved. The highest coupling attained, constrained by the available pump power, was 0.11. If the transducer were to be operated in conjunction with a wideband impedance matching element, an antenna bandwidth comparable to the frequency of the antenna would be attained. The temperature dependence of the Q of the transducer was in good agreement with theory. At temperatures above about 6 K the Q was degraded by the increase in the BCS surface resistance, while at lower temperatures the Q was limited by radiative losses
Counting statistics and detector properties of quantum point contacts
Quantum detector properties of the quantum point contact (QPC) are analyzed
for arbitrary electron transparency and coupling strength to the measured
system and are shown to be determined by the electron counting statistics.
Conditions of the quantum-limited operation of the QPC detector which prevent
information loss through the scattering time and scattering phases are found
for arbitrary coupling. We show that the phase information can be restored and
used for the quantum-limited detection by inclusion of the QPC detector in the
electronic Mach-Zehnder interferometer.Comment: 4 pages, 2 figures, published versio
Minimum Length from Quantum Mechanics and Classical General Relativity
We derive fundamental limits on measurements of position, arising from
quantum mechanics and classical general relativity. First, we show that any
primitive probe or target used in an experiment must be larger than the Planck
length, . This suggests a Planck-size {\it minimum ball} of uncertainty in
any measurement. Next, we study interferometers (such as LIGO) whose precision
is much finer than the size of any individual components and hence are not
obviously limited by the minimum ball. Nevertheless, we deduce a fundamental
limit on their accuracy of order . Our results imply a {\it device
independent} limit on possible position measurements.Comment: 8 pages, latex, to appear in the Physical Review Letter
High-sensitivity force measurement using entangled probes
We show the possibility to improve the measurement sensitivity of a weak
force by using two meters in an entangled state. This latter can be achieved by
exploiting radiation pressure effects.Comment: ReVTeX file, 11 pages, 2 eps figure
Increasing future gravitational-wave detectors sensitivity by means of amplitude filter cavities and quantum entanglement
The future laser interferometric gravitational-wave detectors sensitivity can
be improved using squeezed light. In particular, recently a scheme which uses
the optical field with frequency dependent squeeze factor, prepared by means of
a relatively short (~30 m) amplitude filter cavity, was proposed
\cite{Corbitt2004-3}. Here we consider an improved version of this scheme,
which allows to further reduce the quantum noise by exploiting the quantum
entanglement between the optical fields at the filter cavity two ports.Comment: 10 pages, 7 figure
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
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