492 research outputs found
Serial-parallel conversion for single photons with heralding signals
We present serial-parallel conversion for a heralded single photon source
(heralded SPS). We theoretically show that with the heralding signal, the
serial-parallel converter can route a stream of n photons to n different
spatial modes more efficiently than is the case without using a heralding
signal. We also experimentally demonstrate serial-parallel conversion for two
photons generated from a heralded SPS. We achieve a conversion efficiency of
0.533 \pm 0.003, which exceeds the maximum achievable efficiency of 0.5 for
serial-parallel conversion using unheralded photons, and is double the
efficiency (0.25) for that using beamsplitters. When the losses in the optical
converter are corrected for, the efficiency of the current setup can be
increased up to 0.996 \pm 0.006.Comment: 8 pages, 5 figure
Nonlocal Position Changes of a Photon Revealed by Quantum Routers
Since its publication, Aharonov and Vaidman's three-box paradox has undergone
three major advances: i). A non-counterfactual scheme by the same authors in
2003 with strong rather than weak measurements for verifying the particle's
subtle presence in two boxes. ii) A realization of the latter by Okamoto and
Takeuchi in 2016. iii) A dynamic version by Aharonov et al. in 2017, with
disappearance and reappearance of the particle. We now combine these advances
together. Using photonic quantum routers the particle acts like a quantum
"shutter." It is initially split between Boxes A, B and C, the latter located
far away from the former two. The shutter particle's whereabouts can then be
followed by a probe photon, split in both space and time and reflected by the
shutter in its varying locations. Measuring the former is expected to reveal
the following time-evolution: The shutter particle was, with certainty, in
boxes A+C at t1, then only in C at t2, and finally in B+C at t3. Another branch
of the split probe photon can show that boxes A+B were empty at t2. A Bell-like
theorem applied to this experiment challenges any alternative interpretation
that avoids disappearance-reappearance in favor of local hidden variables.Comment: Revised versio
Demonstration of an optical quantum controlled-NOT gate without path interference
We report the first experimental demonstration of an optical quantum
controlled-NOT gate without any path interference, where the two interacting
path interferometers of the original proposals (Phys. Rev. A {\bf 66}, 024308
(2001), Phys. Rev. A {\bf 65}, 012314 (2002)) have been replaced by three
partially polarizing beam splitters with suitable polarization dependent
transmittances and reflectances. The performance of the device is evaluated
using a recently proposed method (Phys. Rev. Lett. {\bf 94}, 160504 (2005)), by
which the quantum process fidelity and the entanglement capability can be
estimated from the 32 measurement results of two classical truth tables,
significantly less than the 256 measurement results required for full quantum
tomography.Comment: 4 pages, 3 figure
Beating the Standard Quantum Limit with Four Entangled Photons
Precision measurements are important across all fields of science. In
particular, optical phase measurements can be used to measure distance,
position, displacement, acceleration and optical path length. Quantum
entanglement enables higher precision than would otherwise be possible. We
demonstrate an optical phase measurement with an entangled four photon
interference visibility greater than the threshold to beat the standard quantum
limit--the limit attainable without entanglement. These results open the way
for new high-precision measurement applications.Comment: 5 pages, 4 figures Author name was slightly modifie
Implementation of a quantum controlled-SWAP gate with photonic circuits
Quantum information science addresses how the processing and transmission of
information are affected by uniquely quantum mechanical phenomena. Combination
of two-qubit gates has been used to realize quantum circuits, however,
scalability is becoming a critical problem. The use of three-qubit gates may
simplify the structure of quantum circuits dramatically. Among them, the
controlled-SWAP (Fredkin) gates are essential since they can be directly
applied to important protocols, e.g., error correction, fingerprinting, and
optimal cloning. Here we report a realization of the Fredkin gate for photonic
qubits. We achieve a fidelity of 0.85 in the computational basis and an output
state fidelity of 0.81 for a 3-photon Greenberger-Horne-Zeilinger state. The
estimated process fidelity of 0.77 indicates that our Fredkin gate can be
applied to various quantum tasks.Comment: 9 pages, 4 figures, Sci. Rep. 7, 45353 (2017
Dispersion cancellation in high resolution two-photon interference
The dispersion cancellation observed in Hong-Ou-Mandel (HOM) interference
between frequency-entangled photon pairs has been the basis of quantum optical
coherence tomography and quantum clock synchronization. Here we explore the
effect of phase dispersion on ultranarrow HOM dips. We show that the
higher-order dispersion, the line width of the pump laser, and the spectral
shape of the parametric fluorescence have a strong effect on the dispersion
cancellation in the high-resolution regime with several experimental
verifications. Perfect dispersion cancellation with a linewidth of 3\mu m is
also demonstrated through 25 mm of water.Comment: 6 pages, 6 figure
Analysis of an experimental quantum logic gate by complementary classical operations
Quantum logic gates can perform calculations much more efficiently than their
classical counterparts. However, the level of control needed to obtain a
reliable quantum operation is correspondingly higher. In order to evaluate the
performance of experimental quantum gates, it is therefore necessary to
identify the essential features that indicate quantum coherent operation. In
this paper, we show that an efficient characterization of an experimental
device can be obtained by investigating the classical logic operations on a
pair of complementary basis sets. It is then possible to obtain reliable
predictions about the quantum coherent operations of the gate such as
entanglement generation and Bell state discrimination even without performing
these operations directly.Comment: 14 pages, 1 figure, 3 tables, Brief Review for Modern Physics Letters
A, includes a more detailed analysis of the experimental data in Phys. Rev.
Lett. 95, 210506 (2005) (quant-ph/0506263). v2 has minor corrections in
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