1,984 research outputs found
Quantum Computing, Metrology, and Imaging
Information science is entering into a new era in which certain subtleties of
quantum mechanics enables large enhancements in computational efficiency and
communication security. Naturally, precise control of quantum systems required
for the implementation of quantum information processing protocols implies
potential breakthoughs in other sciences and technologies. We discuss recent
developments in quantum control in optical systems and their applications in
metrology and imaging.Comment: 11 pages, 6 figures; Proceedings of SPIE: Fluctuations and Noise in
Photonics and Quantum Optics III (2005
Notes on the Shanghai Expo
1. One of my Chinese classmates mentioned that what really mattered for Chinese visitors were the big exhibitions: Japan, China, Europe, and the US. While still interested in the big exhibits, most Westerners were also astounded by the fact that North Korea and Iran were among the countries with pavilions at the Expo. The pavilions, although far from spectacular, showed a side of the ârogueâ nations that is impossible to see in Western media, which often focuses on the proliferation of nuclear weapons and anti-US sentiment. The North Korean Pavilion showed video clips of the Mass Games and random shots of people working in factories and walking through a college campus. You could even buy North Korean stamps and memorabilia. On a light note, one of my friends bought an Expo passport (a small booklet where you get stamps verifying that you did, indeed, go to certain exhibits). She was denied a stamp at the South Korean Pavilion because of her previous visit to the North Korean exhibit
Alternate Scheme for Optical Cluster-State Generation without Number-Resolving Photon Detectors
We design a controlled-phase gate for linear optical quantum computing by
using photodetectors that cannot resolve photon number. An intrinsic
error-correction circuit corrects errors introduced by the detectors. Our
controlled-phase gate has a 1/4 success probability. Recent development in
cluster-state quantum computing has shown that a two-qubit gate with non-zero
success probability can build an arbitrarily large cluster state with only
polynomial overhead. Hence, it is possible to generate optical cluster states
without number-resolving detectors and with polynomial overhead.Comment: 10 pages, 4 figures, 4 tables; made significant revisions and changed
forma
Super-Resolving Quantum Radar: Coherent-State Sources with Homodyne Detection Suffice to Beat the Diffraction Limit
There has been much recent interest in quantum metrology for applications to
sub-Raleigh ranging and remote sensing such as in quantum radar. For quantum
radar, atmospheric absorption and diffraction rapidly degrades any actively
transmitted quantum states of light, such as N00N states, so that for this
high-loss regime the optimal strategy is to transmit coherent states of light,
which suffer no worse loss than the linear Beer's law for classical radar
attenuation, and which provide sensitivity at the shot-noise limit in the
returned power. We show that coherent radar radiation sources, coupled with a
quantum homodyne detection scheme, provide both longitudinal and angular
super-resolution much below the Rayleigh diffraction limit, with sensitivity at
shot-noise in terms of the detected photon power. Our approach provides a
template for the development of a complete super-resolving quantum radar system
with currently available technology.Comment: 23 pages, content is identical to published versio
Inefficiency of classically simulating linear optical quantum computing with Fock-state inputs
Aaronson and Arkhipov recently used computational complexity theory to argue
that classical computers very likely cannot efficiently simulate linear,
multimode, quantum-optical interferometers with arbitrary Fock-state inputs
[Aaronson and Arkhipov, Theory Comput. 9, 143 (2013)]. Here we present an
elementary argument that utilizes only techniques from quantum optics. We
explicitly construct the Hilbert space for such an interferometer and show that
its dimension scales exponentially with all the physical resources. We also
show in a simple example just how the Schr\"odinger and Heisenberg pictures of
quantum theory, while mathematically equivalent, are not in general
computationally equivalent. Finally, we conclude our argument by comparing the
symmetry requirements of multiparticle bosonic to fermionic interferometers
and, using simple physical reasoning, connect the nonsimulatability of the
bosonic device to the complexity of computing the permanent of a large matrix.Comment: 7 pages, 1 figure Published in PRA Phys. Rev. A 89, 022328 (2014
An All Linear Optical Quantum Memory Based on Quantum Error Correction
When photons are sent through a fiber as part of a quantum communication
protocol, the error that is most difficult to correct is photon loss. Here, we
propose and analyze a two-to-four qubit encoding scheme, which can recover the
loss of one qubit in the transmission. This device acts as a repeater when it
is placed in series to cover a distance larger than the attenuation length of
the fiber, and it acts as an optical quantum memory when it is inserted in a
fiber loop. We call this dual-purpose device a ``quantum transponder.''Comment: 4 pages, 5 figure
Temperature-Dependent Electron-Electron Interaction in Graphene on SrTiO3
The electron band structure of graphene on SrTiO3 substrate has been
investigated as a function of temperature. The high-resolution angle-resolved
photoemission study reveals that the spectral width at Fermi energy and the
Fermi velocity of graphene on SrTiO3 are comparable to those of graphene on a
BN substrate. Near the charge neutrality, the energy-momentum dispersion of
graphene exhibits a strong deviation from the well-known linearity, which is
magnified as temperature decreases. Such modification resembles the
characteristics of enhanced electron-electron interaction. Our results not only
suggest that SrTiO3 can be a plausible candidate as a substrate material for
applications in graphene-based electronics, but also provide a possible route
towards the realization of a new type of strongly correlated electron phases in
the prototypical two-dimensional system via the manipulation of temperature and
a proper choice of dielectric substrates.Comment: 16 pages, 3 figure
High-fidelity linear optical quantum computing with polarization encoding
We show that the KLM scheme [Knill, Laflamme and Milburn, Nature {\bf 409},
46] can be implemented using polarization encoding, thus reducing the number of
path modes required by half. One of the main advantages of this new
implementation is that it naturally incorporates a loss detection mechanism
that makes the probability of a gate introducing a non-detected error, when
non-ideal detectors are considered, dependent only on the detector dark-count
rate and independent of its efficiency. Since very low dark-count rate
detectors are currently available, a high-fidelity gate (probability of error
of order conditional on the gate being successful) can be implemented
using polarization encoding. The detector efficiency determines the overall
success probability of the gate but does not affect its fidelity. This can be
applied to the efficient construction of optical cluster states with very high
fidelity for quantum computing.Comment: 12 pages, 7 figures. Improved construction of high-fidelity entangled
ancilla; references adde
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