1,627 research outputs found
Density Fluctuations in the Oscillatory Phase of Nonclassical Inflaton in FRW Universe
Using coherent and squeezed state formalisms of quantum optics for a
minimally coupled non-classical inflaton in the FRW mertic is studied, in
semiclassical theory of gravity. The leading order solution for the
semiclassical Einstein equations in the coherent, squeezed and squeezed vacuum
states are obtained perturbatively and are exhibit powerlaw expansion
behaviour. The validity of the semiclassical theory is examined in the squeezed
vacuum state in the oscillatory phase of the inflaton. The semiclassical theory
in the oscillatory phase of the non-classical inflaton holds only if the
associated squeezing parameter is much less compared to unity. Quantum
fluctuations of the inflaton is also examined in coherent and squeezed state
formalisms.Comment: 14 pages, 2 figures, To appear in Int.J.Mod.Phys.
Optical cavities as amplitude filters for squeezed fields
We explore the use of Fabry-P\'erot cavities as high-pass filters for
squeezed light, and show that they can increase the sensitivity of
interferometric gravitational-wave detectors without the need for long
(kilometer scale) filter cavities. We derive the parameters for the filters,
and analyze the performance of several possible cavity configurations in the
context of a future gravitational-wave interferometer with squeezed light
(vacuum) injected into the output port.Comment: 9 pages, 6 figure
Upper Bound on the region of Separable States near the Maximally Mixed State
A lower bound on the amount of noise that must be added to a GHZ-like
entangled state to make it separable (also called the random robustness) is
found using the transposition condition. The bound is applicable to arbitrary
numbers of subsystems, and dimensions of Hilbert space, and is shown to be
exact for qubits. The new bound is compared to previous such bounds on this
quantity, and found to be stronger in all cases. It implies that increasing the
number of subsystems, rather than increasing their Hilbert space dimension is a
more effective way of increasing entanglement. An explicit decomposition into
an ensemble of separable states, when the state is not entangled,is given for
the case of qubits.Comment: 2 figures. accepted J. Opt. B: Quantum Semiclass. Opt. (2000
Quantum Mechanics and Linearized Gravitational Waves
The interaction of classical gravitational waves (GW) with matter is studied
within a quantum mechanical framework. The classical equations of motion in the
long wave-length limit is quantized and a Schroedinger equation for the
interaction of GW with matter is proposed. Due to its quadrapole nature, the GW
interacts with matter by producing squeezed quantum states. The resultant
hamiltonian is quite different from one would expect from general principles,
however. The interaction of GW with the free particle, the harmonic oscillator
and the hydrogen atom is then studied using this hamiltonian.Comment: 24 pages, written in REVTE
Entanglement and bifurcations in Jahn-Teller models
We compare and contrast the entanglement in the ground state of two
Jahn-Teller models. The system models the coupling of a
two-level electronic system, or qubit, to a single oscillator mode, while the
models the qubit coupled to two independent, degenerate
oscillator modes. In the absence of a transverse magnetic field applied to the
qubit, both systems exhibit a degenerate ground state. Whereas there always
exists a completely separable ground state in the system, the
ground states of the model always exhibit entanglement. For
the case we aim to clarify results from previous work, alluding
to a link between the ground state entanglement characteristics and a
bifurcation of a fixed point in the classical analogue. In the
case we make use of an ansatz for the ground state. We
compare this ansatz to exact numerical calculations and use it to investigate
how the entanglement is shared between the three system degrees of freedom.Comment: 11 pages, 9 figures, comments welcome; 2 references adde
Squeezed Light for the Interferometric Detection of High Frequency Gravitational Waves
The quantum noise of the light field is a fundamental noise source in
interferometric gravitational wave detectors. Injected squeezed light is
capable of reducing the quantum noise contribution to the detector noise floor
to values that surpass the so-called Standard-Quantum-Limit (SQL). In
particular, squeezed light is useful for the detection of gravitational waves
at high frequencies where interferometers are typically shot-noise limited,
although the SQL might not be beaten in this case. We theoretically analyze the
quantum noise of the signal-recycled laser interferometric gravitational-wave
detector GEO600 with additional input and output optics, namely
frequency-dependent squeezing of the vacuum state of light entering the dark
port and frequency-dependent homodyne detection. We focus on the frequency
range between 1 kHz and 10 kHz, where, although signal recycled, the detector
is still shot-noise limited. It is found that the GEO600 detector with present
design parameters will benefit from frequency dependent squeezed light.
Assuming a squeezing strength of -6 dB in quantum noise variance, the
interferometer will become thermal noise limited up to 4 kHz without further
reduction of bandwidth. At higher frequencies the linear noise spectral density
of GEO600 will still be dominated by shot-noise and improved by a factor of
10^{6dB/20dB}~2 according to the squeezing strength assumed. The interferometer
might reach a strain sensitivity of 6x10^{-23} above 1 kHz (tunable) with a
bandwidth of around 350 Hz. We propose a scheme to implement the desired
frequency dependent squeezing by introducing an additional optical component to
GEO600s signal-recycling cavity.Comment: Presentation at AMALDI Conference 2003 in Pis
Improving Detectors Using Entangling Quantum Copiers
We present a detection scheme which using imperfect detectors, and imperfect
quantum copying machines (which entangle the copies), allows one to extract
more information from an incoming signal, than with the imperfect detectors
alone.Comment: 4 pages, 2 figures, REVTeX, to be published in Phys. Rev.
Qudit Entanglement
We consider the separability of various joint states of D-dimensional quantum systems, which we call 'qudits'. We derive two main results: (i) the separability condition for a two-qudit state that is a mixture of the maximally mixed state and a maximally entangled state; (ii) lower and upper bounds on the size of the neighorhood of separable states surrounding the maximally mixed state for N qudits
Hilbert--Schmidt volume of the set of mixed quantum states
We compute the volume of the convex N^2-1 dimensional set M_N of density
matrices of size N with respect to the Hilbert-Schmidt measure. The hyper--area
of the boundary of this set is also found and its ratio to the volume provides
an information about the complex structure of M_N. Similar investigations are
also performed for the smaller set of all real density matrices. As an
intermediate step we analyze volumes of the unitary and orthogonal groups and
of the flag manifolds.Comment: 13 revtex pages, ver 3: minor improvement
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
- …