236 research outputs found
Direct measurement of the Wigner function by photon counting
We report a direct measurement of the Wigner function characterizing the
quantum state of a light mode. The experimental scheme is based on the
representation of the Wigner function as an expectation value of a displaced
photon number parity operator. This allowed us to scan the phase space
point-by-point, and obtain the complete Wigner function without using any
numerical reconstruction algorithms.Comment: 4 pages, REVTe
Information gain versus state disturbance for a single qubit
The trade-off between the information gain and the state disturbance is
derived for quantum operations on a single qubit prepared in a uniformly
distributed pure state. The derivation is valid for a class of measures
quantifying the state disturbance and the information gain which satisfy
certain invariance conditions. This class includes in particular the Shannon
entropy versus the operation fidelity. The central role in the derivation is
played by efficient quantum operations, which leave the system in a pure output
state for any measurement outcome. It is pointed out that the optimality of
efficient quantum operations among those inducing a given operator-valued
measure is related to Davies' characterization of convex invariant functions on
hermitian operators.Comment: 17 pages, LaTeX, osid.sty. Substantially expanded and generalize
Immunity of information encoded in decoherence-free subspaces to particle loss
We demonstrate that for an ensemble of qudits, subjected to collective
decoherence in the form of perfectly correlated random SU(d) unitaries, quantum
superpositions stored in the decoherence free subspace are fully immune against
the removal of one particle. This provides a feasible scheme to protect quantum
information encoded in the polarization state of a sequence of photons against
both collective depolarization and one photon loss, and can be demonstrated
with photon quadruplets using currently available technology.Comment: to appear in Phys. Rev. A; 5 pages, 2 figures; content changed a bit
(the property demonstrated explicitly on a 4 qubit state
Iterative maximum-likelihood reconstruction in quantum homodyne tomography
I propose an iterative expectation maximization algorithm for reconstructing
a quantum optical ensemble from a set of balanced homodyne measurements
performed on an optical state. The algorithm applies directly to the acquired
data, bypassing the intermediate step of calculating marginal distributions.
The advantages of the new method are made manifest by comparing it with the
traditional inverse Radon transformation technique
Hybrid noiseless subsystems for quantum communication over optical fibers
We derive the general structure of noiseless subsystems for optical radiation
contained in a sequence of pulses undergoing collective depolarization in an
optical fiber. This result is used to identify optimal ways to implement
quantum communication over a collectively depolarizing channel, which in
general combine various degrees of freedom, such as polarization and phase,
into joint hybrid schemes for protecting quantum coherence.Comment: 5 pages, 1 figur
Direct measurement of optical quasidistribution functions: multimode theory and homodyne tests of Bell's inequalities
We develop a multimode theory of direct homodyne measurements of quantum
optical quasidistribution functions. We demonstrate that unbalanced homodyning
with appropriately shaped auxiliary coherent fields allows one to sample
point-by-point different phase space representations of the electromagnetic
field. Our analysis includes practical factors that are likely to affect the
outcome of a realistic experiment, such as non-unit detection efficiency,
imperfect mode matching, and dark counts. We apply the developed theory to
discuss feasibility of observing a loophole-free violation of Bell's
inequalities by measuring joint two-mode quasidistribution functions under
locality conditions by photon counting. We determine the range of parameters of
the experimental setup that enable violation of Bell's inequalities for two
states exhibiting entanglement in the Fock basis: a one-photon Fock state
divided by a 50:50 beam splitter, and a two-mode squeezed vacuum state produced
in the process of non-degenerate parametric down-conversion.Comment: 18 pages, 7 figure
Exploiting entanglement in communication channels with correlated noise
We develop a model for a noisy communication channel in which the noise
affecting consecutive transmissions is correlated. This model is motivated by
fluctuating birefringence of fiber optic links. We analyze the role of
entanglement of the input states in optimizing the classical capacity of such a
channel. Assuming a general form of an ensemble for two consecutive
transmissions, we derive tight bounds on the classical channel capacity
depending on whether the input states used for communication are separable or
entangled across different temporal slots. This result demonstrates that by an
appropriate choice, the channel capacity may be notably enhanced by exploiting
entanglement.Comment: 9 pages, 5 figure
A Comparison of Quantum Oracles
A standard quantum oracle for a general function is
defined to act on two input states and return two outputs, with inputs
and () returning outputs and
. However, if is known to be a one-to-one function, a
simpler oracle, , which returns given , can also be
defined. We consider the relative strengths of these oracles. We define a
simple promise problem which minimal quantum oracles can solve exponentially
faster than classical oracles, via an algorithm which cannot be naively adapted
to standard quantum oracles. We show that can be constructed by invoking
and once each, while invocations of
and/or are required to construct .Comment: 4 pages, 1 figure; Final version, with an extended discussion of
oracle inverses. To appear in Phys Rev
Quantum homodyne tomography with a priori constraints
I present a novel algorithm for reconstructing the Wigner function from
homodyne statistics. The proposed method, based on maximum-likelihood
estimation, is capable of compensating for detection losses in a numerically
stable way.Comment: 4 pages, REVTeX, 2 figure
Multi-Component Bell Inequality and its Violation for Continuous Variable Systems
Multi-component correlation functions are developed by utilizing d-outcome
measurements. Based on the multi-component correlation functions, we propose a
Bell inequality for bipartite d-dimensional systems. Violation of the Bell
inequality for continuous variable (CV) systems is investigated. The violation
of the original Einstein-Podolsky-Rosen state can exceed the Cirel'son bound,
the maximal violation is 2.96981. For finite value of squeezing parameter,
violation strength of CV states increases with dimension d. Numerical results
show that the violation strength of CV states with finite squeezing parameter
is stronger than that of original EPR state.Comment: 5 pages and 1 figure, rewritten version, accepted by Phys. Rev.
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