406 research outputs found
Secure Key Distribution by Swapping Quantum Entanglement
We report two key distribution schemes achieved by swapping quantum
entanglement. Using two Bell states, two bits of secret key can be shared
between two distant parties that play symmetric and equal roles. We also
address eavesdropping attacks against the schemes.Comment: 4 pages, 2 figures, 3 tables. The revised version will appear in
Phys. Rev.
Multi-photon entanglement from distant single photon sources on demand
We describe a scheme that allows for the generation of any desired N-photon
state on demand. Under ideal conditions, this requires only N single photon
sources, laser pulses and linear optics elements. First, the sources should be
initialised with the help of single-qubit rotations and repeat-until-success
two-qubit quantum gates [Lim et al., Phys. Rev. Lett. 95, 030305 (2005)].
Afterwards, the state of the sources can be mapped onto the state of N newly
generated photons whenever needed.Comment: 9 pages, 3 figure
Secure Deterministic Communication Without Entanglement
We propose a protocol for deterministic communication that does not make use
of entanglement. It exploits nonorthogonal states in a two-way quantum channel
attaining significant improvement of security and efficiency over already known
cryptographic protocols. The presented scheme, being deterministic, can be
devoted to direct communication as well as to key distribution, and its
experimental realization is feasible with present day technology.Comment: 4 pages, 2 figures. Corrected typos in the field "Authors"; added one
referenc
Double jumps and transition rates for two dipole-interacting atoms
Cooperative effects in the fluorescence of two dipole-interacting atoms, with
macroscopic quantum jumps (light and dark periods), are investigated. The
transition rates between different intensity periods are calculated in closed
form and are used to determine the rates of double jumps between periods of
double intensity and dark periods, the mean duration of the three intensity
periods and the mean rate of their occurrence. We predict, to our knowledge for
the first time, cooperative effects for double jumps, for atomic distances from
one and to ten wave lengths of the strong transition. The double jump rate, as
a function of the atomic distance, can show oscillations of up to 30% at
distances of about a wave length, and oscillations are still noticeable at a
distance of ten wave lengths. The cooperative effects of the quantities and
their characteristic behavior turn out to be strongly dependent on the laser
detuning.Comment: Substantially revised versio
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
A rate equation approach to cavity mediated laser cooling
The cooling rate for cavity mediated laser cooling scales as the Lamb-Dicke
parameter eta squared. A proper analysis of the cooling process hence needs to
take terms up to eta^2 in the system dynamics into account. In this paper, we
present such an analysis for a standard scenario of cavity mediated laser
cooling with eta << 1. Our results confirm that there are many similarities
between ordinary and cavity mediated laser cooling. However, for a weakly
confined particle inside a strongly coupled cavity, which is the most
interesting case for the cooling of molecules, numerical results indicate that
even more detailed calculations are needed to model the cooling process
accurately.Comment: 15 pages, 10 figures, minor corrections, PRA (in press
Randomized Dynamical Decoupling Techniques for Coherent Quantum Control
The need for strategies able to accurately manipulate quantum dynamics is
ubiquitous in quantum control and quantum information processing. We
investigate two scenarios where randomized dynamical decoupling techniques
become more advantageous with respect to standard deterministic methods in
switching off unwanted dynamical evolution in a closed quantum system: when
dealing with decoupling cycles which involve a large number of control actions
and/or when seeking long-time quantum information storage. Highly effective
hybrid decoupling schemes, which combine deterministic and stochastic features
are discussed, as well as the benefits of sequentially implementing a
concatenated method, applied at short times, followed by a hybrid protocol,
employed at longer times. A quantum register consisting of a chain of spin-1/2
particles interacting via the Heisenberg interaction is used as a model for the
analysis throughout.Comment: 7 pages, 2 figures. Replaced with final version. Invited talk
delivered at the XXXVI Winter Colloquium on the Physics of Quantum
Electronics, Snowbird, Jan 2006. To be published in J. Mod. Optic
Enhanced Convergence and Robust Performance of Randomized Dynamical Decoupling
We demonstrate the advantages of randomization in coherent quantum dynamical
control. For systems which are either time-varying or require decoupling cycles
involving a large number of operations, we find that simple randomized
protocols offer superior convergence and stability as compared to deterministic
counterparts. In addition, we show how randomization always allows to
outperform purely deterministic schemes at long times, including combinatorial
and concatenated methods. General criteria for optimally interpolating between
deterministic and stochastic design are proposed and illustrated in explicit
decoupling scenarios relevant to quantum information storage.Comment: 4 pages, 3 figures, replaced with final versio
Quantum Direct Communication with Authentication
We propose two Quantum Direct Communication (QDC) protocols with user
authentication. Users can identify each other by checking the correlation of
Greenberger-Horne-Zeilinger (GHZ) states. Alice can directly send a secret
message to Bob using the remaining GHZ states after authentication. Our second
QDC protocol can be used even though there is no quantum link between Alice and
Bob. The security of the transmitted message is guaranteed by properties of
entanglement of GHZ states.Comment: 9 pages, 3 figures and 2 table
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