21,090 research outputs found
Rabi Oscillations in Systems with Small Anharmonicity
When a two-level quantum system is irradiated with a microwave signal, in
resonance with the energy difference between the levels, it starts Rabi
oscillation between those states. If there are other states close, in energy,
to the first two, the Rabi signal will also induce transition to those. Here,
we study the probability of transition to the third state, in a three-level
system, while a Rabi oscillation between the first two states is performed. We
investigate the effect of pulse shaping on the probability and suggest methods
for optimizing pulse shapes to reduce transition probability.Comment: 7 pages, 7 figure
Visualizing the Quantum Interaction Picture in Phase Space
We illustrate the correspondence between the quantum Interaction
Picture-evolution of the state of a quantum system in Hilbert space and a
combination of local and global transformations of its Wigner function in phase
space. To this aim, we consider the time-evolution of a quantized harmonic
oscillator driven by both a linear and a quadratic (in terms of bosonic
creation and annihilation operators) potentials and employ the Magnus series to
derive the exact form of the time-evolution operator. In this case, the
Interaction Picture corresponds to a local transformation of phase
space-reference frame into the one that is co-moving with the Wigner function.Comment: Submitted to New Journal of Physic
Difficulty of distinguishing product states locally
Non-locality without entanglement is a rather counter-intuitive phenomenon in
which information may be encoded entirely in product (unentangled) states of
composite quantum systems in such a way that local measurement of the
subsystems is not enough for optimal decoding. For simple examples of pure
product states, the gap in performance is known to be rather small when
arbitrary local strategies are allowed. Here we restrict to local strategies
readily achievable with current technology; those requiring neither a quantum
memory nor joint operations. We show that, even for measurements on pure
product states there can be a large gap between such strategies and
theoretically optimal performance. Thus even in the absence of entanglement
physically realizable local strategies can be far from optimal for extracting
quantum information.Comment: 5 pages, 1 figur
Dependence of the evolution of the cavity radiation of a coherently pumped correlated emission laser on dephasing and phase fluctuation
Analysis of the dynamics of the cavity radiation of a coherently pumped
correlated emission laser is presented. The phase fluctuation and dephasing are
found to affect the time evolution of the two-mode squeezing and intensity of
the cavity radiation significantly. The intensity and degree of the two-mode
squeezing increase at early stages of the process with time, but this trend
changes rapidly afterwards. It is also shown that they increase with phase
fluctuation and dephasing in the strong driving limit, however the situation
appears to be opposite in the weak driving limit. This essentially suggests
that the phase fluctuation and dephasing weaken the coherence induced by a
strong driving mechanism so that the spontaneous emission gets a chance. The
other important aspect of the phase fluctuation, in this regard, is the
relaxation of the time at which the maximum squeezing is manifested as well as
the time in which the radiation remains in a squeezed state.Comment: 10 pages, 12 figure
Steady-state entanglement in a double-well Bose-Einstein condensate through coupling to a superconducting resonator
We consider a two-component Bose-Einstein condensate in a double-well
potential, where the atoms are magnetically coupled to a single-mode of the
microwave field inside a superconducting resonator. We find that the system has
the different dark-state subspaces in the strong- and weak-tunneling regimes,
respectively. In the limit of weak tunnel coupling, steady-state entanglement
between the two spatially separated condensates can be generated by evolving to
a mixture of dark states via the dissipation of the photon field. We show that
the entanglement can be faithfully indicated by an entanglement witness.
Long-lived entangled states are useful for quantum information processing with
atom-chip devices.Comment: 9 pages, 7 figures, minor revisio
Frictional quantum decoherence
The dynamics associated with a measurement-based master equation for quantum
Brownian motion are investigated. A scheme for obtaining time evolution from
general initial conditions is derived. This is applied to analyze dissipation
and decoherence in the evolution of both a Gaussian and a Schr\"{o}dinger cat
initial state. Dependence on the diffusive terms present in the master equation
is discussed with reference to both the coordinate and momentum
representations.Comment: 18 pages, 7 figure
Stable, inflatable life raft for high seas rescue operations
Raft is easily deployed and highly maneuverable in water. It has false bottom of water ballast containers attached to underside, making it exceptionally stable platform from which swimmers can operate. Raft is attachable to external moorings
GaAsP on GaP top solar cells
GaAsP on GaP top solar cells as an attachment to silicon bottom solar cells are being developed. The GaAsP on GaP system offers several advantages for this top solar cell. The most important is that the gallium phosphide substrate provides a rugged, transparent mechanical substrate which does not have to be removed or thinned during processing. Additional advantages are that: (1) gallium phosphide is more oxidation resistant than the III-V aluminum compounds, (2) a range of energy band gaps higher than 1.75 eV is readily available for system efficiency optimization, (3) reliable ohmic contact technology is available from the light-emitting diode industry, and (4) the system readily lends itself to graded band gap structures for additional increases in efficiency
Minimum-error discrimination between three mirror-symmetric states
We present the optimal measurement strategy for distinguishing between three
quantum states exhibiting a mirror symmetry. The three states live in a
two-dimensional Hilbert space, and are thus overcomplete. By mirror symmetry we
understand that the transformation {|+> -> |+>, |-> -> -|->} leaves the set of
states invariant. The obtained measurement strategy minimizes the error
probability. An experimental realization for polarized photons, realizable with
current technology, is suggested.Comment: 4 pages, 2 figure
Nematic Order by Disorder in Spin-2 BECs
The effect of quantum and thermal fluctuations on the phase diagram of spin-2
BECs is examined. They are found to play an important role in the nematic part
of the phase diagram, where a mean-field treatment of two-body interactions is
unable to lift the accidental degeneracy between nematic states. Quantum and
thermal fluctuations resolve this degeneracy, selecting the uniaxial nematic
state, for scattering lengths , and the square biaxial nematic state
for . Paradoxically, the fluctuation induced order is stronger at
higher temperatures, for a range of temperatures below . For the
experimentally relevant cases of spin-2 Rb and Na, we argue that
such fluctuations could successfully compete against other effects like the
quadratic Zeeman field, and stabilize the uniaxial phase for experimentally
realistic conditions. A continuous transition of the Ising type from uniaxial
to square biaxial order is predicted on raising the magnetic field. These
systems present a promising experimental opportunity to realize the `order by
disorder' phenomenon.Comment: 5 pages, 4 figures; 1 reference and 1 minor correctio
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