21 research outputs found
Afterpulsing model based on the quasi-continuous distribution of deep levels in single-photon avalanche diodes
We have performed a statistical characterization of the effect of
afterpulsing in a free-running silicon single-photon detector by measuring the
distribution of afterpulse waiting times in response to pulsed illumination and
fitting it by a sum of exponentials. We show that a high degree of goodness of
fit can be obtained for 5 exponentials, but the physical meaning of estimated
characteristic times is dubious. We show that a continuous limit of the sum of
exponentials with a uniform density between the limiting times gives excellent
fitting results in the full range of the detector response function. This means
that in certain detectors the afterpulsing is caused by a continuous band of
deep levels in the active area of the photodetector.Comment: 10 pages, 4 figure
Asymmetric universal entangling machine
We give a definition of asymmetric universal entangling machine which
entangles a system in an unknown state to a specially prepared ancilla. The
machine produces a fixed state-independent amount of entanglement in exchange
to a fixed degradation of the system state fidelity. We describe explicitly
such a machine for any quantum system having levels and prove its
optimality. We show that a -dimensional ancilla is sufficient for reaching
optimality. The introduced machine is a generalization to a number of widely
investigated universal quantum devices such as the symmetric and asymmetric
quantum cloners, the symmetric quantum entangler, the quantum information
distributor and the universal-NOT gate.Comment: 28 pages, 3 figure
On Preparing Entangled Pairs of Polarization Qubits in the Frequency Non-Degenerate Regime
The problems associated with practical implementation of the scheme proposed
for preparation of arbitrary states of polarization ququarts based on biphotons
are discussed. The influence of frequency dispersion effects are considered,
and the necessity of group velocities dispersion compensation in the frequency
non-degenerate case even for continuous pumping is demonstrated. A method for
this compensation is proposed and implemented experimentally. Physical
restrictions on the quality of prepared two-photon states are revealed.Comment: 9 pages, 6 figure
Autofeedback scheme for preservation of macroscopic coherence in microwave cavities
We present a scheme for controlling the decoherence of a linear superposition
of two coherent states with opposite phases in a high-Q microwave cavity, based
on the injection of appropriately prepared ``probe'' and ``feedback'' Rydberg
atoms, improving the one presented in [D. Vitali et al., Phys. Rev. Lett. 79,
2442 (1997)]. In the present scheme, the information transmission from the
probe to the feedback atom is directly mediated by a second auxiliary cavity.
The detection efficiency for the probe atom is no longer a critical parameter,
and the decoherence time of the superposition state can be significantly
increased using presently available technology.Comment: revtex, 15 pages, 4 eps figure
Information, disturbance and Hamiltonian quantum feedback control
We consider separating the problem of designing Hamiltonian quantum feedback
control algorithms into a measurement (estimation) strategy and a feedback
(control) strategy, and consider optimizing desirable properties of each under
the minimal constraint that the available strength of both is limited. This
motivates concepts of information extraction and disturbance which are distinct
from those usually considered in quantum information theory. Using these
concepts we identify an information trade-off in quantum feedback control.Comment: 13 pages, multicol Revtex, 2 eps figure
Quantum Instabilities and Decoherence Problem
A decoherence problem is discussed by means of quantum continuous measurement theory. It is shown that the conditional state of quantum system interacting with a bath preserves its initial purity. In this presentation decoherence arises as a result of averaging over the stochastic times of reduction moments ("clicks"). A method based on external phase feedback is proposed to slow down the decoherence of field superposition state in an open optical cavity. It is also shown that an atom placed inside the optical cavity plays a role of internal self-organized positive feedback between field and atom, which leads to an exponential increase in the mean dipole moment of the atom for the field initially prepared in a superposition of coherent states, i.e. to quantum instability