530 research outputs found
Compensating for Beamsplitter Asymmetries in Quantum Interference Experiments
The visibility of the quantum interference "dip" seen in the Hong-Ou-Mandel
experiment is optimized when a symmetric 50/50 beamsplitter is used in the
interferometer. Here we show that the reduction in visibility caused by an
asymmetric beamsplitter can be compensated by manipulating the polarization
states of the two input photons. We experimentally demonstrate this by using a
highly asymmetric 10/90 beamsplitter, and converting an initial dip visibility
of 22% to a compensated value of 99%.Comment: 3 pages, 4 figure
Measuring the absolute photo detection efficiency using photon number correlations
We present two methods for determining the absolute detection efficiency of
photon-counting detectors directly from their singles rates under illumination
from a nonclassical light source. One method is based on a continuous variable
analogue to coincidence counting in discrete photon experiments, but does not
actually rely on high detector time resolutions. The second method is based on
difference detection which is a typical detection scheme in continuous variable
quantum optics experiments. Since no coincidence detection is required with
either method, they are useful for detection efficiency measurements of photo
detectors with detector time resolutions far too low to resolve coincidence
events.Comment: 6 pages, 1 figure, journal reference adde
Experimental nonlinear sign shift for linear optics quantum computation
We have realized the nonlinear sign shift (NS) operation for photonic
qubits.This operation shifts the phase of two photons reflected by a beam
splitter using an extra single photon and measurement. We show that the
conditional phase shift is in clear agreement with
theory. Our results show that by using an ancilla photon and conditional
detection, nonlinear optical effects can be implemented using only linear
optical elements. This experiment represents an essential step for linear
optical implementations of scalable quantum computation.Comment: 4 pages, 4 figure
Electronic Structure of Mixed Perovskite Oxides
Based on the tight-binding method, electronic bands of the mixed perovskite oxides are calculated in order to develop the electronic theory of ferroelectric phase transitions in these mixtures which are difficult to describe within the phenomenological theories. Diagonal elements of Hamiltonian matrix of parent materials are assumed to differ by 0.1eV and mixtures are simulated by lattices of supercells containing 2(3) = 8 or 3(3) = 27 unit cells randomly assigned to either material. The width of the conduction and valence bands have maxima and the band gap has a minimum at intermediate mixing ratio. Results are in agreement with those of other analyses on random systems and even 2(3)-cell computation seems to serve as a first approximation for our purpose
Hybrid quantum repeater using bright coherent light
We describe a quantum repeater protocol for long-distance quantum
communication. In this scheme, entanglement is created between qubits at
intermediate stations of the channel by using a weak dispersive light-matter
interaction and distributing the outgoing bright coherent light pulses among
the stations. Noisy entangled pairs of electronic spin are then prepared with
high success probability via homodyne detection and postselection. The local
gates for entanglement purification and swapping are deterministic and
measurement-free, based upon the same coherent-light resources and weak
interactions as for the initial entanglement distribution. Finally, the
entanglement is stored in a nuclear-spin-based quantum memory. With our system,
qubit-communication rates approaching 100 Hz over 1280 km with fidelities near
99% are possible for reasonable local gate errors.Comment: title changed, final published versio
Fault-tolerant quantum computation with cluster states
The one-way quantum computing model introduced by Raussendorf and Briegel
[Phys. Rev. Lett. 86 (22), 5188-5191 (2001)] shows that it is possible to
quantum compute using only a fixed entangled resource known as a cluster state,
and adaptive single-qubit measurements. This model is the basis for several
practical proposals for quantum computation, including a promising proposal for
optical quantum computation based on cluster states [M. A. Nielsen,
arXiv:quant-ph/0402005, accepted to appear in Phys. Rev. Lett.]. A significant
open question is whether such proposals are scalable in the presence of
physically realistic noise. In this paper we prove two threshold theorems which
show that scalable fault-tolerant quantum computation may be achieved in
implementations based on cluster states, provided the noise in the
implementations is below some constant threshold value. Our first threshold
theorem applies to a class of implementations in which entangling gates are
applied deterministically, but with a small amount of noise. We expect this
threshold to be applicable in a wide variety of physical systems. Our second
threshold theorem is specifically adapted to proposals such as the optical
cluster-state proposal, in which non-deterministic entangling gates are used. A
critical technical component of our proofs is two powerful theorems which
relate the properties of noisy unitary operations restricted to act on a
subspace of state space to extensions of those operations acting on the entire
state space.Comment: 31 pages, 54 figure
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