2,796 research outputs found
Optical Quantum Computation with Perpetually Coupled Spins
The possibility of using strongly and continuously interacting spins for
quantum computation has recently been discussed. Here we present a simple
optical scheme that achieves this goal while avoiding the drawbacks of earlier
proposals. We employ a third state, accessed by a classical laser field, to
create an effective barrier to information transfer. The mechanism proves to be
highly efficient both for continuous and pulsed laser modes; moreover it is
very robust, tolerating high decay rates for the excited states. The approach
is applicable to a broad range of systems, in particular dense structures such
as solid state self-assembled (e.g., molecular) devices. Importantly, there are
existing structures upon which `first step' experiments could be immediately
performed.Comment: 5 pages including 3 figures. Updated to published versio
On the concept of pressure in quantum mechanics
Heat and work are fundamental concepts for thermodynamical systems. When
these are scaled down to the quantum level they require appropriate embeddings.
Here we show that the dependence of the particle spectrum on system size giving
rise to a formal definition of pressure can, indeed, be correlated with an
external mechanical degree of freedom, modelled as a spatial coordinate of a
quantum oscillator. Under specific conditions this correlation is reminiscent
of that occurring in the classical manometer.Comment: 7 pages, 3 figure
Measurement models for time-resolved spectroscopy: a comment
We present an exactly solvable model for photon emission, which allows us to
examine the evolution of the photon wavefunction in space and time. We apply
this model to coherent phenomena in three-level systems with a special emphasis
on the photon detection process.Comment: 14 pages RevTex, 4 figure
Small quantum networks operating as quantum thermodynamic machines
We show that a 3-qubit system as studied for quantum information purposes can
alternatively be used as a thermodynamic machine when driven in finite time and
interfaced between two split baths. The spins are arranged in a chain where the
working spin in the middle exercises Carnot cycles the area of which defines
the exchanged work. The cycle orientation (sign of the exchanged work) flips as
the difference of bath temperatures goes through a critical value.Comment: RevTeX, 4 pages, 7 figures. Replaced by version accepted for
publication in EP
Abrupt and gradual changes of information through the Kane solid state computer
The susceptibility of the transformed information to the filed and system
parameters is investigated for the Kane solid state computer. It has been
shown, that the field polarization and the initial state of the system play the
central roles on the abrupt and gradual quench of the purity and the fidelity.
If the field and the initial state are in different polarizations, then the
purity and the fidelity decrease abruptly, while for the common polarization
the decay is gradual and smooth. For some class of initial states one can send
the information without any loss. Therefore, by controlling on the devices one
can increase the time of safe communication, reduce the amount of exchange
information between the state and its environment and minimize the purity
decrease rate
Passive, broadband and low-frequency suppression of laser amplitude noise to the shot-noise limit using hollow-core fibre
We use hollow-core fibre to preserve the spectrum and temporal profile of
picosecond laser pulses in CBD to suppress 2.6 dB of amplitude noise at MHz
noise frequencies, to within 0.01 dB of the shot-noise limit. We provide an
enhanced version of the CBD scheme that concatenates circuits to suppress over
multiple frequencies and over broad frequency ranges --- we perform a first
demonstration that reduces total excess amplitude noise, between 2 - 6 MHz, by
85%. These demonstrations enable passive, broad-band, all-guided fibre laser
technology operating at the shot-noise limit.Comment: 8 pages, 8 figure
Complex joint probabilities as expressions of determinism in quantum mechanics
The density operator of a quantum state can be represented as a complex joint
probability of any two observables whose eigenstates have non-zero mutual
overlap. Transformations to a new basis set are then expressed in terms of
complex conditional probabilities that describe the fundamental relation
between precise statements about the three different observables. Since such
transformations merely change the representation of the quantum state, these
conditional probabilities provide a state-independent definition of the
deterministic relation between the outcomes of different quantum measurements.
In this paper, it is shown how classical reality emerges as an approximation to
the fundamental laws of quantum determinism expressed by complex conditional
probabilities. The quantum mechanical origin of phase spaces and trajectories
is identified and implications for the interpretation of quantum measurements
are considered. It is argued that the transformation laws of quantum
determinism provide a fundamental description of the measurement dependence of
empirical reality.Comment: 12 pages, including 1 figure, updated introduction includes
references to the historical background of complex joint probabilities and to
related work by Lars M. Johanse
Ignorance based inference of optimality in thermodynamic processes
We derive ignorance based prior distribution to quantify incomplete
information and show its use to estimate the optimal work characteristics of a
heat engine.Comment: Latex, 10 pages, 3 figure
State Measurements with Short Laser Pulses and Lower-Efficiency Photon Detectors
It has been proposed by Cook (Phys. Scr. T 21, 49 (1988)) to use a short
probe laser pulse for state measurements of two-level systems. In previous work
we have investigated to what extent this proposal fulfills the projection
postulate if ideal photon detectors are considered. For detectors with overall
efficiency less than 1 complications arise for single systems, and for this
case we present a simple criterion for a laser pulse to act as a state
measurement and to cause an almost complete state reduction.Comment: 13 pages, LaTeX; submitted to J. mod. Op
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