21,982 research outputs found
The Lorentzian distance formula in noncommutative geometry
For almost twenty years, a search for a Lorentzian version of the well-known
Connes' distance formula has been undertaken. Several authors have contributed
to this search, providing important milestones, and the time has now come to
put those elements together in order to get a valid and functional formula.
This paper presents a historical review of the construction and the proof of a
Lorentzian distance formula suitable for noncommutative geometry.Comment: 16 pages, final form, few references adde
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.
Inverse Landau-Zener-Stuckelberg problem for qubit-resonator systems
We consider theoretically a superconducting qubit - nanomechanical resonator
(NR) system, which was realized by LaHaye et al. [Nature 459, 960 (2009)].
First, we study the problem where the state of the strongly driven qubit is
probed through the frequency shift of the low-frequency NR. In the case where
the coupling is capacitive, the measured quantity can be related to the
so-called quantum capacitance. Our theoretical results agree with the
experimentally observed result that, under resonant driving, the frequency
shift repeatedly changes sign. We then formulate and solve the inverse
Landau-Zener-Stuckelberg problem, where we assume the driven qubit's state to
be known (i.e. measured by some other device) and aim to find the parameters of
the qubit's Hamiltonian. In particular, for our system the qubit's bias is
defined by the NR's displacement. This may provide a tool for monitoring of the
NR's position.Comment: 10 pages, 7 figure
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.
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