110 research outputs found
Quantum cryptography via parametric downconversion
The use of quantum bits (qubits) in cryptography holds the promise of secure
cryptographic quantum key distribution schemes. It is based usually on
single-photon polarization states. Unfortunately, the implemented ``qubits'' in
the usual weak pulse experiments are not true two-level systems, and quantum
key distribution based on these imperfect qubits is totally insecure in the
presence of high (realistic) loss rate. In this work, we investigate another
potential implementation: qubits generated using a process of parametric
downconversion. We find that, to first (two-photon) and second (four-photon)
order in the parametric downconversion small parameter, this implementation of
quantum key distribution is equivalent to the theoretical version.
Once realistic measurements are taken into account, quantum key distribution
based on parametric downconversion suffers also from sensitivity to extremely
high (nonrealistic) losses. By choosing the small parameter of the process
according to the loss rates, both implementations of quantum key distribution
can in principle become secure against the attack studied in this paper.
However, adjusting the small parameter to the required levels seems to be
impractical in the weak pulse process. On the other hand, this can easily be
done in the parametric downconversion process, making it a much more promising
implementation.Comment: 6 pages, Latex (a special style file is attached). Presented in
QCM'98 conference. Similar results regarding the insecurity of weak-pulse
schemes were also presented by Norbert Lutkenhaus in the same conferenc
Interval oscillation theorems for asecond-order linear differential equation
AbstractInterval oscillation criteria are given for the forced second-order linear differential equation Ly(t) = (p(t)y′)′ + q(t)y = ƒ(t), tε (0, ∞), where p, q, ƒ are locally integrable functions and p(t) > 0, for t > 0. No restriction is imposed on ƒ(t) to be the second derivative of an oscillatory function as assumed by Kartsatos [1). Our results also allow both q and f to change sign in the neighborhood at infinity. In particular, we show that all solutions of y″ + c(sin t)y = tβ cos t with β ≥ 0 are oscillatory, for c ≥ 1.3448. This improves an estimate given by Nasr [2] for the linear equation
Image and Coherence Transfer in the Stimulated Down-conversion Process
The intensity transverse profile of the light produced in the process of
stimulated down-conversion is derived. A quantum-mechanical treatment is used.
We show that the angular spectrum of the pump laser can be transferred to the
stimulated down-converted beam, so that images can also be transferred from the
pump to the down-converted beam. We also show that the transfer can occur from
the stimulating beam to the down-converted one. Finally, we study the process
of diffraction through an arbitrarily shaped screen. For the special case of a
double-slit, the interference pattern is explicitly obtained. The visibility
for the spontaneous emitted light is in accordance with the van Cittert -
Zernike theorem for incoherent light, while the visibility for the stimulated
emitted light is unity. The overall visibility is in accordance with previous
experimental results
Entanglement dynamics of three-qubit states in noisy channels
We study entanglement dynamics of the three-qubit system which is initially
prepared in pure Greenberger-Horne- Zeilinger (GHZ) or W state and transmitted
through one of the Pauli channels or the
depolarizing channel. With the help of the lower bound for three-qubit
concurrence we show that the W state preserves more entanglement than the GHZ
state in transmission through the Pauli channel . For the Pauli
channels and the depolarizing channel, however, the
entanglement of the GHZ state is more resistant against decoherence than the
W-type entanglement. We also briefly discuss how the accuracy of the lower
bound approximation depends on the rank of the density matrix under
consideration.Comment: 2 figures, 32 reference
Quantum teleportation of light beams
We experimentally demonstrate quantum teleportation for continuous variables
using squeezed-state entanglement. The teleportation fidelity for a real
experimental system is calculated explicitly, including relevant imperfection
factors such as propagation losses, detection inefficiencies and phase
fluctuations. The inferred fidelity for input coherent states is F = 0.61 +-
0.02, which when corrected for the efficiency of detection by the output
observer, gives a fidelity of 0.62. By contrast, the projected result based on
the independently measured entanglement and efficiencies is 0.69. The
teleportation protocol is explained in detail, including a discussion of
discrepancy between experiment and theory, as well as of the limitations of the
current apparatus.Comment: 17 pages, 19 figures, submitted to PR
Constraint on teleportation over multipartite pure states
We first define a quantity exhibiting the usefulness of bipartite quantum
states for teleportation, called the quantum teleportation capability, and then
investigate its restricted shareability in multi-party quantum systems. In this
work, we verify that the quantum teleportation capability has a monogamous
property in its shareability for arbitrary three-qutrit pure states by
employing the monogamy inequality in terms of the negativity.Comment: 4 pages, 1 figur
Quantum spiral bandwidth of entangled two-photon states
We put forward the concept of quantum spiral bandwidth of the spatial mode
function of the two-photon entangled state in spontaneous parametric
downconversion. We obtain the bandwidth using the eigenstates of the orbital
angular momentum of the biphoton states, and reveal its dependence with the
length of the down converting crystal and waist of the pump beam. The
connection between the quantum spiral bandwidth and the entropy of entanglement
of the quantum state is discussed.Comment: 10 pages, 3 figure
Quantum diffraction and interference of spatially correlated photon pairs generated by spontaneous parametric down-conversion
We demonstrate one- and two-photon diffraction and interference experiments
utilizing parametric down-converted photon pairs (biphotons) and a transmission
grating. With two-photon detection, the biphoton exhibits a
diffraction-interference pattern equivalent to that of an effective single
particle that is associated with half the wavelength of the constituent
photons. With one-photon detection, however no diffraction-interference pattern
is observed. We show that these phenomena originate from the spatial quantum
correlation between the down-converted photons.Comment: 4 pages, 5 figure
On quantum teleportation with beam-splitter-generated entanglement
Following the lead of Cochrane, Milburn, and Munro [Phys. Rev. A {\bf 62},
062307 (2000)], we investigate theoretically quantum teleportation by means of
the number-sum and phase-difference variables. We study Fock-state entanglement
generated by a beam splitter and show that two-mode Fock-state inputs can be
entangled by a beam splitter into close approximations of maximally entangled
eigenstates of the phase difference and the photon-number sum
(Einstein-Podolsky-Rosen -- EPR -- states). Such states could be experimentally
feasible with on-demand single-photon sources. We show that the teleportation
fidelity can reach near unity when such ``quasi-EPR'' states are used as the
quantum channel.Comment: 7 pages (two-column), 7 figures, submitted to Phys. Rev. A. Text
unmodified, postscript error correcte
Experimental investigation of continuous variable quantum teleportation
We report the experimental demonstration of quantum teleportation of the
quadrature amplitudes of a light field. Our experiment was stably locked for
long periods, and was analyzed in terms of fidelity, F; and with signal
transfer, T_{q}=T^{+}+T^{-}, and noise correlation, V_{q}=V_{in|out}^{+}
V_{in|out}^{-}. We observed an optimum fidelity of 0.64 +/- 0.02, T_{q}= 1.06
+/- 0.02 and V_{q} =0.96 +/- 0.10. We discuss the significance of both T_{q}>1
and V_{q}<1 and their relation to the teleportation no-cloning limit.Comment: 4 pages, 4 figure
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