2,738 research outputs found
Deterministic Controlled-NOT gate for single-photon two-qubit quantum logic
We demonstrate a robust implementation of a deterministic linear-optical
Controlled-NOT (CNOT) gate for single-photon two-qubit quantum logic. A
polarization Sagnac interferometer with an embedded 45-oriented dove
prism is used to enable the polarization control qubit to act on the momentum
(spatial) target qubit of the same photon. The CNOT gate requires no active
stabilization because the two spatial modes share a common path, and it is used
to entangle the polarization and momentum qubits.Comment: 10 pages, 4 figures. Typos corrected, referee comments and
correction
Attacking quantum key distribution with single-photon two-qubit quantum logic
The Fuchs-Peres-Brandt (FPB) probe realizes the most powerful individual
attack on Bennett-Brassard 1984 quantum key distribution (BB84 QKD) by means of
a single controlled-NOT (CNOT) gate. This paper describes a complete physical
simulation of the FPB-probe attack on polarization-based BB84 QKD using a
deterministic CNOT constructed from single-photon two-qubit quantum logic.
Adding polarization-preserving quantum nondemolition measurements of photon
number to this configuration converts the physical simulation into a true
deterministic realization of the FPB attack.Comment: 8 pages, 9 figures; references added, 1 new figure, appendix
expanded; accepted for publication in Phys. Rev.
A pulsed Sagnac source of narrowband polarization-entangled photons
We demonstrate pulsed operation of a bidirectionally pumped polarization
Sagnac interferometric down-conversion source and its generation of narrowband,
high-visibility polarization-entangled photons. Driven by a narrowband,
mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II
phase-matched periodically poled KTiOPO crystal is capable of producing
0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with
1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a
mean photon-pair generation rate of as high as 0.7 pair per pulse, at which
multi-pair events dominate and significantly reduce the two-photon
quantum-interference visibility. For low generation probability , the
reduced visibility is independent of the throughput efficiency and
of the polarization analysis basis, which can be utilized to yield an accurate
estimate of the generation rate . At low we have characterized
the source entanglement quality in three different ways: average
quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt
parameter of , and quantum state tomography with 98.85%
singlet-state fidelity. The narrowband pulsed Sagnac source of entangled
photons is suitable for use in quantum information processing applications such
as free-space quantum key distribution.Comment: 10 pages, 6 figures, accepted for publication in Phys. Rev.
Entanglement-Enhanced Sensing in a Lossy and Noisy Environment
Nonclassical states are essential for optics-based quantum information processing, but their fragility limits their utility for practical scenarios in which loss and noise inevitably degrade, if not destroy, nonclassicality. Exploiting nonclassical states in quantum metrology yields sensitivity advantages over all classical schemes delivering the same energy per measurement interval to the sample being probed. These enhancements, almost without exception, are severely diminished by quantum decoherence. Here, we experimentally demonstrate an entanglement-enhanced sensing system that is resilient to quantum decoherence. We employ entanglement to realize a 20% signal-to-noise ratio improvement over the optimum classical scheme in an entanglement-breaking environment plagued by 14Â dB of loss and a noise background 75Â dB stronger than the returned probe light. Our result suggests that advantageous quantum-sensing technology could be developed for practical situations.United States. Army Research Office (Grant W911NF-10-1-0430)United States. Office of Naval Research (Grant N00014-13-1-0774
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