64 research outputs found
Photons with sub-Planckian Energy Cannot Efficiently Probe Space-Time Foam
Extra-galactic sources of photons have been used to constrain space-time
quantum fluctuations in the Universe. In these proposals, the fundamental
"fuzziness" of distance caused by space-time quantum fluctuations has been
directly identified with fluctuations in optical paths. Phase-front
corrugations deduced from these optical-path fluctuations are then applied to
light from extra-galactic point sources, and used to constrain various models
of quantum gravity. However, when a photon propagates in three spatial
dimensions, it does not follow a specific ray, but rather samples a finite,
three-dimensional region around that ray --- thereby averaging over space-time
quantum fluctuations all through that region. We use a simple, random-walk type
model to demonstrate that, once the appropriate wave optics is applied, the
averaging of neighboring space-time fluctuations will cause much less
distortion to the phase front. In our model, the extra suppression factor due
to diffraction is the wave length in units of the Planck length, which is at
least for astronomical observations.Comment: This is a revised version of arXiv:gr-qc/060509
Enhancing the bandwidth of gravitational-wave detectors with unstable optomechanical filters
For gravitational-wave interferometric detectors, there is a tradeoff between
the detector bandwidth and peak sensitivity when focusing on the shot noise
level. This has to do with the frequency-dependent propagation phase lag
(positive dispersion) of the signal. We consider embedding an active unstable
filter---a cavity-assisted optomechanical device operating in the instability
regime---inside the interferometer to compensate the phase, and using feedback
control to stabilize the entire system. We show that this scheme in principle
can enhance the bandwidth without sacrificing the peak sensitivity. However,
there is one practical difficulty for implementing it due to the thermal
fluctuation of the mechanical oscillator in the optomechanical filter, which
puts a very stringent requirement on the environmental temperature and the
mechanical quality factor.Comment: 5 pages and 6 figures. Comments are welcom
Quantum noise of white light cavity using double-pumped gain medium
Laser interferometric gravitational-wave detectors implement Fabry-Perot
cavities to increase their peak sensitivity. However, this is at cost of
reducing their detection bandwidth, which origins from the propagation phase
delay of the light. The "white-light-cavity" idea, first proposed by Wicht et
al. [Optics Communications 134, 431 (1997)], is to circumvent this limitation
by introducing anomalous dispersion, using double-pumped gain medium, to
compensate for such phase delay. In this article, starting from the Hamiltonian
of atom-light interaction, we apply the input-output formalism to evaluate the
quantum noise of the system. We find that apart from the additional noise
associated with the parametric amplification process noticed by others, the
stability condition for the entire system poses an additional constraint.
Through surveying the parameter regimes where the gain medium remains stable
(not lasing) and stationary, we find that there is no net enhancement of the
shot-noise limited sensitivity. Therefore, other gain mediums or different
parameter regimes shall be explored for realizing the white light cavity.Comment: 12 pages, 7 figure
Extraction of energy from gravitational waves by laser interferometer detectors
In this paper we discuss the energy interaction between gravitational waves
and laser interferom- eter gravitational wave detectors. We show that the
widely held view that the laser interferometer gravitational wave detector
absorbs no energy from gravitational waves is only valid under the
approximation of a frequency-independent optomechanical coupling strength and a
pump laser without detuning with respect to the resonance of the
interferometer. For a strongly detuned interferometer, the optical-damping
dynamics dissipates gravitational wave energy through the interaction between
the test masses and the optical field. For a non-detuned interferometer, the
frequency-dependence of the optomechanical coupling strength causes a tiny
energy dissipation, which is proved to be equivalent to the Doppler friction
raised by Braginsky et.al.Comment: 20 pages, 7 figure
Towards the Fundamental Quantum Limit of Linear Measurements of Classical Signals
The quantum Cram\'er-Rao bound (QCRB) sets a fundamental limit for the
measurement of classical signals with detectors operating in the quantum
regime. Using linear-response theory and the Heisenberg uncertainty relation,
we derive a general condition for achieving such a fundamental limit. When
applied to classical displacement measurements with a test mass, this condition
leads to an explicit connection between the QCRB and the Standard Quantum Limit
which arises from a tradeoff between the measurement imprecision and quantum
backaction; the QCRB can be viewed as an outcome of a quantum non-demolition
measurement with the backaction evaded. Additionally, we show that the test
mass is more a resource for improving measurement sensitivity than a victim of
the quantum backaction, which suggests a new approach to enhancing the
sensitivity of a broad class of sensors. We illustrate these points with laser
interferometric gravitational wave detectors.Comment: revised version with supplemental materials adde
A Hybrid Rydberg Quantum Gate for Quantum Network
The high fidelity storage, distribution and processing of quantum information
prefers qubits with different physical properties. Thus, hybrid quantum gates
interfacing different types of qubits are essential for the realization of
complex quantum network structures. A Rydberg-atom based physical quantum CZ
gate is proposed to hybridly process the polarisation-encoded single-photon
optical qubit and the "Schroedinger cat" microwave qubit. The degradation of
the fidelity under the influence of various noise channels, such as microwave
cavity loss, sponetanous emission of atom states, and non-adiabaticity effect,
etc, has been analyised through detailed theoretical analysis by deriving
input-output relation of qubit fields. The feasibility and the challenges of
the protocol within current technology are also discussed by analysing the
possible experimental parameter settings
Accretion-modified stellar-mass black hole distribution and milli-Hz gravitational wave backgrounds from galaxy centre
Gas accretion of embedded stellar-mass black holes\,(sBHs) or stars in the
accretion disk of active galactic nuclei\,(AGNs) will modify the mass
distribution of these sBHs and stars, which will also affect the migration of
the sBHs/stars. With the introduction of the mass accretion effect, we simulate
the evolution of the sBH/star distribution function in a consistent way by
extending the Fokker-Planck equation of sBH/star distributions to the
mass-varying scenario, and explore the mass distribution of sBHs in the nuclear
region of the galaxy centre. We find that the sBHs can grow up to several tens
solar mass and form heavier sBH binaries, which will be helpful for us to
understand the black-hole mass distribution as observed by the current and
future ground-based gravitational wave detectors\,(e.g., LIGO/VIRGO, ET and
Cosmic Explorer). We further estimate the event rate of extreme mass-ratio
inspirals\,(EMRI) for sBH surrounding the massive black hole and calculate the
stochastic gravitational wave\,(GW) background of the EMRIs. We find that the
background can be detected in future space-borne GW detectors after considering
the sBHs embedded in the AGN disk, while the mass accretion has a slight effect
on the GW background.Comment: 15 pages, 8 figures, Accepted by MNRA
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