859 research outputs found
Random quantum correlations and density operator distributions
Consider the question: what statistical ensemble corresponds to minimal prior
knowledge about a quantum system ? For the case where the system is in fact
known to be in a pure state there is an obvious answer, corresponding to the
unique unitarily-invariant measure on the Hilbert sphere. However, the problem
is open for the general case where states are described by density operators.
Here two approaches to the problem are investigated.
The first approach assumes that the system is randomly correlated with a
second system, where the ensemble of composite systems is described by a random
pure state. Results for qubits randomly correlated with other systems are
presented, including average entanglement entropies. It is shown that maximum
correlation is guaranteed in the limit as one system becomes
infinite-dimensional.
The second approach relies on choosing a metric on the space of density
operators, and generating a corresponding ensemble from the induced volume
element. Comparisons between the approaches are made for qubits, for which the
second approach (based on the Bures metric) yields the most symmetric, and
hence the least informative, ensemble of density operators.Comment: 13 pages, no figures; a new page of additional notes at end draws
attention to 3 new references and their relevanc
Estimated workload intensity during volunteer aquarium dives
Background: This study aimed to characterize the physiological demands of working dives on volunteer divers at a public aquarium in the USA. Aims: To estimate the workloads associated with volunteer dives in a US aquarium. Methods: Participants completed a medical and diving history questionnaire. Measurements included blood pressure before and after diving and continuous ECG (Holter) monitoring during diving. Dive profiles were recorded using loggers. Mean workload was estimated from total air consumption. Results Twenty-seven divers recorded 49 air dives over 5 days. Two-thirds were male and ages ranged from 40 to 78 years. Typically, each diver made two dives with a 30-60 min surface interval. Mean heart rate while diving was 100 beats per minute (bpm). Mean estimated workload during the dives recorded during this study was 5.8 metabolic equivalents (METS), with a range from 4.1 to 10.5. The highest mean recorded heart rate was 120 bpm over 40 min, vacuuming the floor in the shark exhibit. Conclusions: Given the mean age of this sample and the prevalence of cardiovascular risk factors (body mass index, high cholesterol and hypertension), it may be prudent for aquariums to regularly monitor SAC/kg and heart rate in volunteer divers, to identify which tasks require the highest workload intensity. Divers with existing cardiovascular risk factors might then be employed in dives with lighter workloads. In conclusion, volunteer dives at this aquarium required a mean workload intensity that was described by recreational divers as moderate. The highest workload, at 10 METS for 23 min, would be considered by many recreational divers as exhausting
\Lambda-buildings and base change functors
We prove an analog of the base change functor of \Lambda-trees in the setting
of generalized affine buildings. The proof is mainly based on local and global
combinatorics of the associated spherical buildings. As an application we
obtain that the class of generalized affine building is closed under ultracones
and asymptotic cones. Other applications involve a complex of groups
decompositions and fixed point theorems for certain classes of generalized
affine buildings.Comment: revised version, 29 pages, to appear in Geom. Dedicat
Application of precise 142Nd/144Nd analysis of small samples to inclusions in diamonds (Finsch, South Africa) and Hadean Zircons (Jack Hills, Western Australia)
146Sm-142Nd and 147Sm-143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The stud
A Two-Step Quantum Direct Communication Protocol Using Einstein-Podolsky-Rosen Pair Block
A protocol for quantum secure direct communication using blocks of EPR pairs
is proposed. A set of ordered EPR pairs is used as a data block for sending
secret message directly. The ordered EPR set is divided into two particle
sequences, a checking sequence and a message-coding sequence. After
transmitting the checking sequence, the two parties of communication check
eavesdropping by measuring a fraction of particles randomly chosen, with random
choice of two sets of measuring bases. After insuring the security of the
quantum channel, the sender, Alice encodes the secret message directly on the
message-coding sequence and send them to Bob. By combining the checking and
message-coding sequences together, Bob is able to read out the encoded messages
directly. The scheme is secure because an eavesdropper cannot get both
sequences simultaneously. We also discuss issues in a noisy channel.Comment: 8 pages and 2 figures. To appear in Phys Rev
Charge Conjugation Invariance of the Vacuum and the Cosmological Constant Problem
We propose a method of field quantization which uses an indefinite metric in
a Hilbert space of state vectors. The action for gravity and the standard model
includes, as well as the positive energy fermion and boson fields, negative
energy fields. The Hamiltonian for the action leads through charge conjugation
invariance symmetry of the vacuum to a cancellation of the zero-point vacuum
energy and a vanishing cosmological constant in the presence of a gravitational
field. To guarantee the stability of the vacuum, we introduce a Dirac sea
`hole' theory of quantization for gravity as well as the standard model. The
vacuum is defined to be fully occupied by negative energy particles with a hole
in the Dirac sea, corresponding to an anti-particle. We postulate that the
negative energy bosons in the vacuum satisfy a para-statistics that leads to a
para-Pauli exclusion principle for the negative energy bosons in the vacuum,
while the positive energy bosons in the Hilbert space obey the usual
Bose-Einstein statistics. This assures that the vacuum is stable for both
fermions and bosons. Restrictions on the para-operator Hamiltonian density lead
to selection rules that prohibit positive energy para-bosons from being
observable. The problem of deriving a positive energy spectrum and a consistent
unitary field theory from a pseudo-Hermitian Hamiltonian is investigated.Comment: 15 pages, Latex file, no figures. Typos corrected. To be published in
Physics Letters
A Model for the Stray Light Contamination of the UVCS Instrument on SOHO
We present a detailed model of stray-light suppression in the spectrometer
channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO
spacecraft. The control of diffracted and scattered stray light from the bright
solar disk is one of the most important tasks of a coronagraph. We compute the
fractions of light that diffract past the UVCS external occulter and
non-specularly pass into the spectrometer slit. The diffracted component of the
stray light depends on the finite aperture of the primary mirror and on its
figure. The amount of non-specular scattering depends mainly on the
micro-roughness of the mirror. For reasonable choices of these quantities, the
modeled stray-light fraction agrees well with measurements of stray light made
both in the laboratory and during the UVCS mission. The models were constructed
for the bright H I Lyman alpha emission line, but they are applicable to other
spectral lines as well.Comment: 19 pages, 5 figures, Solar Physics, in pres
Experiment towards continuous-variable entanglement swapping: Highly correlated four-partite quantum state
We present a protocol for performing entanglement swapping with intense
pulsed beams. In a first step, the generation of amplitude correlations between
two systems that have never interacted directly is demonstrated. This is
verified in direct detection with electronic modulation of the detected
photocurrents. The measured correlations are better than expected from a
classical reconstruction scheme. In the entanglement swapping process, a
four--partite entangled state is generated. We prove experimentally that the
amplitudes of the four optical modes are quantum correlated 3 dB below shot
noise, which is due to the potential four--party entanglement.Comment: 9 pages, 10 figures, update of references 9 and 10; minor
inconsistency in notation removed; format for units in the figures change
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