3,747 research outputs found
Helstrom Theorem by No-Signaling Condition
We prove a special case of Helstrom theorem by using no-signaling condition
in the special theory of relativity that faster-than-light communication is
impossible.Comment: Minor corrections (A reference added, discussion part deleted, typos
in equations corrected), 2 pages, RevTe
Unified Treatment of Heterodyne Detection: the Shapiro-Wagner and Caves Frameworks
A comparative study is performed on two heterodyne systems of photon
detectors expressed in terms of a signal annihilation operator and an image
band creation operator called Shapiro-Wagner and Caves' frame, respectively.
This approach is based on the introduction of a convenient operator
which allows a unified formulation of both cases. For the Shapiro-Wagner
scheme, where , quantum phase and amplitude
are exactly defined in the context of relative number state (RNS)
representation, while a procedure is devised to handle suitably and in a
consistent way Caves' framework, characterized by , within the approximate simultaneous measurements of
noncommuting variables. In such a case RNS phase and amplitude make sense only
approximately.Comment: 25 pages. Just very minor editorial cosmetic change
Phase-covariant cloning of coherent states
We consider the problem of phase-covariant cloning for coherent states. We
show that an experimental scheme based on ideal phase measurement and
feedforward outperforms the semiclassical procedure of ideal phase measurement
and preparation in terms of fidelity. A realistic scheme where the ideal phase
measurement is replaced with double-homodyne detection is shown to be unable to
overcome the semiclassical cloning strategy. On the other hand, such a
realistic scheme is better than semiclassical cloning based on double-homodyne
phase measurement and preparation.Comment: 6 pages, 2 figures; updated references and minor corrections; in
press on Physical Review
Factors Associated with Depression among University Students in Malaysia: A Cross-sectional Study
Depression is a recurrent mental health problem among younger demographics, and university students are particularly susceptible owing to stress, workload and independent living, amongst other factors. This study explores the prevalence of depression and the factors influencing depression among university students in Malaysia. This cross-sectional study involved 1,023 university students (response rate 90.4%). Depression was assessed using the Centre for Epidemiological Studies Short Depression Scale (CESD -10). Binary logistic regression was used to determine predictors of depression based on sociodemographic, physiological, lifestyle, and health characteristics. Approximately 30% of respondents experienced depression, and 4.4% of this category suffered severe depression. This study demonstrates that instances of depression were 2.52 times higher (95% CI: 1.71-3.71) in second year students compared to first year students, and 1.63 times higher (95% CI: 1.08-2.45) in students staying outside campus compared to students staying inside campus. Students from poor, not well-off, and quite well-off family background had 15.26 (95% CI: 2.77-84.88), 4.85 (95% CI: 1.01-23.34) and 5.62 times (95% CI: 1.16-27.25) higher chance for depression than wealthier students, respectively. Students with mild, moderate, and severe sleeping problems were 2.50 times (95% CI: 1.61-3.88), 3.34 times (95% CI: 2.18-5.11), and 3.66 times (95% CI: 1. 93 -6. 94) more likely to be depressed than those without sleeping problem, respectively. Students with post-traumatic stress disorder (PTSD) were 1.42 times higher (95% CI: 1.07-2.56) to suffer from depression. This study concludes that higher education institutions need to pay special attention to the mental health of those students especially those in their second year, living off campus, from lower economic backgrounds, with sleeping problem, or suffering PTSD
Standard Quantum Limits for broadband position measurement
I utilize the Caves-Milburn model for continuous position measurements to
formulate a broadband version of the Standard Quantum Limit (SQL) for
monitoring the position of a free mass, and illustrate the use of Kalman
filtering to recover the SQL for estimating a weak classical force that acts on
a quantum-mechanical test particle under continuous observation. These
derivations are intended to clarify the interpretation of SQL's in the context
of continuous quantum measurement.Comment: Replaced version: changed title, fixed algebra error at the very end,
conclusions modified accordingly. Four pages, one eps figur
Minimum-error discrimination between mixed quantum states
We derive a general lower bound on the minimum-error probability for {\it
ambiguous discrimination} between arbitrary mixed quantum states with given
prior probabilities. When , this bound is precisely the well-known
Helstrom limit. Also, we give a general lower bound on the minimum-error
probability for discriminating quantum operations. Then we further analyze how
this lower bound is attainable for ambiguous discrimination of mixed quantum
states by presenting necessary and sufficient conditions related to it.
Furthermore, with a restricted condition, we work out a upper bound on the
minimum-error probability for ambiguous discrimination of mixed quantum states.
Therefore, some sufficient conditions are obtained for the minimum-error
probability attaining this bound. Finally, under the condition of the
minimum-error probability attaining this bound, we compare the minimum-error
probability for {\it ambiguously} discriminating arbitrary mixed quantum
states with the optimal failure probability for {\it unambiguously}
discriminating the same states.Comment: A further revised version, and some results have been adde
Does nonlinear metrology offer improved resolution? Answers from quantum information theory
A number of authors have suggested that nonlinear interactions can enhance
resolution of phase shifts beyond the usual Heisenberg scaling of 1/n, where n
is a measure of resources such as the number of subsystems of the probe state
or the mean photon number of the probe state. These suggestions are based on
calculations of `local precision' for particular nonlinear schemes. However, we
show that there is no simple connection between the local precision and the
average estimation error for these schemes, leading to a scaling puzzle. This
puzzle is partially resolved by a careful analysis of iterative implementations
of the suggested nonlinear schemes. However, it is shown that the suggested
nonlinear schemes are still limited to an exponential scaling in \sqrt{n}.
(This scaling may be compared to the exponential scaling in n which is
achievable if multiple passes are allowed, even for linear schemes.) The
question of whether nonlinear schemes may have a scaling advantage in the
presence of loss is left open.
Our results are based on a new bound for average estimation error that
depends on (i) an entropic measure of the degree to which the probe state can
encode a reference phase value, called the G-asymmetry, and (ii) any prior
information about the phase shift. This bound is asymptotically stronger than
bounds based on the variance of the phase shift generator. The G-asymmetry is
also shown to directly bound the average information gained per estimate. Our
results hold for any prior distribution of the shift parameter, and generalise
to estimates of any shift generated by an operator with discrete eigenvalues.Comment: 8 page
Propagation of squeezed radiation through amplifying or absorbing random media
We analyse how nonclassical features of squeezed radiation (in particular the
sub-Poissonian noise) are degraded when it is transmitted through an amplifying
or absorbing medium with randomly located scattering centra. Both the cases of
direct photodetection and of homodyne detection are considered. Explicit
results are obtained for the dependence of the Fano factor (the ratio of the
noise power and the mean current) on the degree of squeezing of the incident
state, on the length and the mean free path of the medium, the temperature, and
on the absorption or amplification rate.Comment: 8 pages, 4 figure
Quantum-limited force measurement with an optomechanical device
We study the detection of weak coherent forces by means of an optomechanical
device formed by a highly reflecting isolated mirror shined by an intense and
highly monochromatic laser field. Radiation pressure excites a vibrational mode
of the mirror, inducing sidebands of the incident field, which are then
measured by heterodyne detection. We determine the sensitivity of such a scheme
and show that the use of an entangled input state of the two sideband modes
improves the detection, even in the presence of damping and noise acting on the
mechanical mode.Comment: 8 pages, 4 figure
Thermodynamics of Extended Bodies in Special Relativity
Relativistic thermodynamics is generalized to accommodate four dimensional
rotation in a flat spacetime. An extended body can be in equilibrium when its
each element moves along a Killing flow. There are three types of basic Killing
flows in a flat spacetime, each of which corresponds to translational motion,
spatial rotation, and constant linear acceleration; spatial rotation and
constant linear acceleration are regarded as four dimensional rotation.
Translational motion has been mainly investigated in the past literature of
relativistic thermodynamics. Thermodynamics of the other two is derived in the
present paper.Comment: 8 pages, no figur
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