811 research outputs found
Natural orbits of atomic Cooper pairs in a nonuniform Fermi gas
We examine the basic mode structure of atomic Cooper pairs in an
inhomogeneous Fermi gas. Based on the properties of Bogoliubov quasi-particle
vacuum, the single particle density matrix and the anomalous density matrix
share the same set of eigenfunctions. These eigenfunctions correspond to
natural pairing orbits associated with the BCS ground state. We investigate
these orbits for a Fermi gas in a spherical harmonic trap, and construct the
wave function of a Cooper pair in the form of Schmidt decomposition. The issue
of spatial quantum entanglement between constituent atoms in a pair is
addressed.Comment: 14 pages, 4 figures, submitted to Phys. Rev.
Phase shifts in nonresonant coherent excitation
Far-off-resonant pulsed laser fields produce negligible excitation between
two atomic states but may induce considerable phase shifts. The acquired phases
are usually calculated by using the adiabatic-elimination approximation. We
analyze the accuracy of this approximation and derive the conditions for its
applicability to the calculation of the phases. We account for various sources
of imperfections, ranging from higher terms in the adiabatic-elimination
expansion and irreversible population loss to couplings to additional states.
We find that, as far as the phase shifts are concerned, the adiabatic
elimination is accurate only for a very large detuning. We show that the
adiabatic approximation is a far more accurate method for evaluating the phase
shifts, with a vast domain of validity; the accuracy is further enhanced by
superadiabatic corrections, which reduce the error well below .
Moreover, owing to the effect of adiabatic population return, the adiabatic and
superadiabatic approximations allow one to calculate the phase shifts even for
a moderately large detuning, and even when the peak Rabi frequency is larger
than the detuning; in these regimes the adiabatic elimination is completely
inapplicable. We also derive several exact expressions for the phases using
exactly soluble two-state and three-state analytical models.Comment: 10 pages, 7 figure
Realization of a space reversal operator
In this paper we propose the realization of a bosonic-fermionic interaction
in the context of trapped ions whose effect upon the ion center of mass degrees
of freedom is properly speaking a spatial inversion. The physical system and
its features are accurately described and some applications are briefly
discussed.Comment: 9 pages; to appear in Rep. Math. Phys., in summer 200
Quantum Cryptography with Coherent States
The safety of a quantum key distribution system relies on the fact that any
eavesdropping attempt on the quantum channel creates errors in the
transmission. For a given error rate, the amount of information that may have
leaked to the eavesdropper depends on both the particular system and the
eavesdropping strategy. In this work, we discuss quantum cryptographic
protocols based on the transmission of weak coherent states and present a new
system, based on a symbiosis of two existing ones, and for which the
information available to the eavesdropper is significantly reduced. This system
is therefore safer than the two previous ones. We also suggest a possible
experimental implementation.Comment: 20 pp. Revtex, Figures available from the authors upon request, To be
published in PRA (March 95
Quantum State Disturbance vs. Information Gain: Uncertainty Relations for Quantum Information
When an observer wants to identify a quantum state, which is known to be one
of a given set of non-orthogonal states, the act of observation causes a
disturbance to that state. We investigate the tradeoff between the information
gain and that disturbance. This issue has important applications in quantum
cryptography. The optimal detection method, for a given tolerated disturbance,
is explicitly found in the case of two equiprobable non-orthogonal pure states.Comment: 20 pages, standard LaTeX, four png figures (also available from the
authors: [email protected] and [email protected]
Experimental Detection of Entanglement with Polarized Photons
We report on the first experimental realization of the entanglement witness
for polarization entangled photons. It represents a recently discovered
significant quantum information protocol which is based on few local
measurements. The present demonstration has been applied to the so-called
Werner states, a family of ''mixed'' quantum states that include both entangled
and non entangled states. These states have been generated by a novel high
brilliance source of entanglement which allows to continuously tune the degree
of mixedness
Security of quantum cryptography using balanced homodyne detection
In this paper we investigate the security of a quantum cryptographic scheme
which utilizes balanced homodyne detection and weak coherent pulse (WCP). The
performance of the system is mainly characterized by the intensity of the WCP
and postselected threshold. Two of the simplest intercept/resend eavesdropping
attacks are analyzed. The secure key gain for a given loss is also discussed in
terms of the pulse intensity and threshold.Comment: RevTeX4, 8pages, 7 figure
Direct measurement of non-linear properties of bipartite quantum states
Non-linear properties of quantum states, such as entropy or entanglement,
quantify important physical resources and are frequently used in quantum
information science. They are usually calculated from a full description of a
quantum state, even though they depend only on a small number parameters that
specify the state. Here we extract a non-local and a non-linear quantity,
namely the Renyi entropy, from local measurements on two pairs of polarization
entangled photons. We also introduce a "phase marking" technique which allows
to select uncorrupted outcomes even with non-deterministic sources of entangled
photons. We use our experimental data to demonstrate the violation of entropic
inequalities. They are examples of a non-linear entanglement witnesses and
their power exceeds all linear tests for quantum entanglement based on all
possible Bell-CHSH inequalities.Comment: To appear on PRL with minor change
Using of small-scale quantum computers in cryptography with many-qubit entangled states
We propose a new cryptographic protocol. It is suggested to encode
information in ordinary binary form into many-qubit entangled states with the
help of a quantum computer. A state of qubits (realized, e.g., with photons) is
transmitted through a quantum channel to the addressee, who applies a quantum
computer tuned to realize the inverse unitary transformation decoding of the
message. Different ways of eavesdropping are considered, and an estimate of the
time needed for determining the secret unitary transformation is given. It is
shown that using even small quantum computers can serve as a basis for very
efficient cryptographic protocols. For a suggested cryptographic protocol, the
time scale on which communication can be considered secure is exponential in
the number of qubits in the entangled states and in the number of gates used to
construct the quantum network
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