14,536 research outputs found
Entanglement entropy in top-down models
We explore holographic entanglement entropy in ten-dimensional supergravity
solutions. It has been proposed that entanglement entropy can be computed in
such top-down models using minimal surfaces which asymptotically wrap the
compact part of the geometry. We show explicitly in a wide range of examples
that the holographic entanglement entropy thus computed agrees with the
entanglement entropy computed using the Ryu-Takayanagi formula from the
lower-dimensional Einstein metric obtained from reduction over the compact
space. Our examples include not only consistent truncations but also cases in
which no consistent truncation exists and Kaluza-Klein holography is used to
identify the lower-dimensional Einstein metric. We then give a general proof,
based on the Lewkowycz-Maldacena approach, of the top-down entanglement entropy
formula.Comment: 40 page
Entanglement entropy and differential entropy for massive flavors
In this paper we compute the holographic entanglement entropy for massive
flavors in the D3-D7 system, for arbitrary mass and various entangling region
geometries. We show that the universal terms in the entanglement entropy
exactly match those computed in the dual theory using conformal perturbation
theory. We derive holographically the universal terms in the entanglement
entropy for a CFT perturbed by a relevant operator, up to second order in the
coupling; our results are valid for any entangling region geometry. We present
a new method for computing the entanglement entropy of any top-down brane probe
system using Kaluza-Klein holography and illustrate our results with massive
flavors at finite density. Finally we discuss the differential entropy for
brane probe systems, emphasising that the differential entropy captures only
the effective lower-dimensional Einstein metric rather than the ten-dimensional
geometry.Comment: 54 pages, 8 figures; v2 references and comments adde
Efficient Hamiltonian programming in qubit arrays with nearest-neighbour couplings
We consider the problem of selectively controlling couplings in a practical
quantum processor with always-on interactions that are diagonal in the
computational basis, using sequences of local NOT gates. This methodology is
well-known in NMR implementations, but previous approaches do not scale
efficiently for the general fully-connected Hamiltonian, where the complexity
of finding time-optimal solutions makes them only practical up to a few tens of
qubits. Given the rapid growth in the number of qubits in cutting-edge quantum
processors, it is of interest to investigate the applicability of this control
scheme to much larger scale systems with realistic restrictions on
connectivity. Here we present an efficient scheme to find near time-optimal
solutions that can be applied to engineered qubit arrays with local
connectivity for any number of qubits, indicating the potential for practical
quantum computing in such systems.Comment: 5 pages, 5 figures. Shortened and clarified from previous versio
Large Deployable Reflector (LDR) system concept and technology definition study. Volume 2: Technology assessment and technology development plan
A study was conducted to define reasonable and representative LDR system concepts for the purpose of defining a technology development program aimed at providing the requisite technological capability necessary to start LDR development by the end of 1991. This volume presents thirteen technology assessments and technology development plans, as well as an overview and summary of the LDR concepts. Twenty-two proposed augmentation projects are described (selected from more than 30 candidates). The five LDR technology areas most in need of supplementary support are: cryogenic cooling; astronaut assembly of the optically precise LDR in space; active segmented primary mirror; dynamic structural control; and primary mirror contamination control. Three broad, time-phased, five-year programs were synthesized from the 22 projects, scheduled, and funding requirements estimated
Exploring the limits of multiplexed photon-pair sources for the preparation of pure single-photon states
Current sources of heralded single photons based on nonlinear optics operate
in a probabilistic manner. In order to build quantum-enhanced devices based
around the use of single photons, compact, turn-key and deterministic sources
are required. A possible solution is to multiplex a number of sources to
increase the single-photon generation probability and in so doing reducing the
waiting time to deliver large numbers of photons simultaneously, from
independent sources. Previously it has been shown that, in the ideal case, 17
multiplexed sources allow deterministic generation of heralded single photons
[Christ and Silberhorn, Phys. Rev. A 85, 023829 (2012)]. Here we extend this
analysis to include undesirable effects of detector inefficiency and photon
loss on a number of multiplexed sources using a variety of different detectors
for heralding. We compare these systems for fixed signal-to-noise ratio to
allow a direct comparison of performance for real- world heralded single photon
sources.Comment: 10 pages, 7 figures. Equation 18 changed to include power of a half
in the binomial facto
Temporal Loop Multiplexing: A resource efficient scheme for multiplexed photon-pair sources
Single photons are a vital resource for photonic quantum information
processing. However, even state-of-the-art single photon sources based on
photon-pair generation and heralding detection have only a low probability of
delivering a single photon when one is requested. We analyse a scheme that uses
a switched fibre delay loop to increase the delivery probability per time bin
of single photons from heralded sources. We show that, for realistic
experimental parameters, combining the output of up to 15 pulses can yield a
performance improvement of a factor of 10. We consider the future performance
of this scheme with likely component improvements.Comment: 5 pages, 4 figure
Achieving a wide field near infrared camera for the Calar Alto 3.5m telescope
The ongoing development of large infrared array detectors has enabled wide
field, deep surveys to be undertaken. There are, however, a number of
challenges in building an infrared instrument which has both excellent optical
quality and high sensitivity over a wide field. We discuss these problems in
the context of building a wide field imaging camera for the 3.5m telescope at
Calar Alto with the new 2K*2K HgCdTe HAWAII-2 focal plane array. Our final
design is a prime focus camera with a 15' field-of-view, called Omega 2000. To
achieve excellent optical quality over the whole field, we have had to dispense
with the reimaging optics and cold Lyot stop. We show that creative baffling
schemes, including the use of undersized baffles, can compensate for the lost K
band sensitivity. A moving baffle will be employed in Omega 2000 to allow full
transmission in the non-thermal J and H bands.Comment: To appear in the SPIE proceedings of ``Optical and IR Telescope
Instrumentation and Detectors'', Munich, March 200
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