594 research outputs found
Adaptive Measurements in the Optical Quantum Information Laboratory
Adaptive techniques make practical many quantum measurements that would
otherwise be beyond current laboratory capabilities. For example: they allow
discrimination of nonorthogonal states with a probability of error equal to the
Helstrom bound; they allow measurement of the phase of a quantum oscillator
with accuracy approaching (or in some cases attaining) the Heisenberg limit;
and they allow estimation of phase in interferometry with a variance scaling at
the Heisenberg limit, using only single qubit measurement and control. Each of
these examples has close links with quantum information, in particular
experimental optical quantum information: the first is a basic quantum
communication protocol; the second has potential application in linear optical
quantum computing; the third uses an adaptive protocol inspired by the quantum
phase estimation algorithm. We discuss each of these examples, and their
implementation in the laboratory, but concentrate upon the last, which was
published most recently [Higgins {\em et al.}, Nature vol. 450, p. 393, 2007].Comment: 12 pages, invited paper to be published in IEEE Journal of Selected
Topics in Quantum Electronics: Quantum Communications and Information Scienc
Solid-Liquid Phase Diagrams for Binary Metallic Alloys: Adjustable Interatomic Potentials
We develop a new approach to determining LJ-EAM potentials for alloys and use
these to determine the solid-liquid phase diagrams for binary metallic alloys
using Kofke's Gibbs-Duhem integration technique combined with semigrand
canonical Monte Carlo simulations. We demonstrate that it is possible to
produce a wide-range of experimentally observed binary phase diagrams (with no
intermetallic phases) by reference to the atomic sizes and cohesive energies of
the two elemental materials. In some cases, it is useful to employ a single
adjustable parameter to adjust the phase diagram (we provided a good choice for
this free parameter). Next, we perform a systematic investigation of the effect
of relative atomic sizes and cohesive energies of the elements on the binary
phase diagrams. We then show that this approach leads to good agreement with
several experimental binary phase diagrams. The main benefit of this approach
is not the accurately reproduction of experimental phase diagrams, but rather
to provide a method by which material properties can be continuously changed in
simulations studies. This is one of the keys to the use of atomistic
simulations to understand mechanisms and properties in a manner not available
to experiment
Using weak values to experimentally determine "negative probabilities" in a two-photon state with Bell correlations
Bipartite quantum entangled systems can exhibit measurement correlations that
violate Bell inequalities, revealing the profoundly counter-intuitive nature of
the physical universe. These correlations reflect the impossibility of
constructing a joint probability distribution for all values of all the
different properties observed in Bell inequality tests. Physically, the
impossibility of measuring such a distribution experimentally, as a set of
relative frequencies, is due to the quantum back-action of projective
measurements. Weakly coupling to a quantum probe, however, produces minimal
back-action, and so enables a weak measurement of the projector of one
observable, followed by a projective measurement of a non-commuting observable.
By this technique it is possible to empirically measure weak-valued
probabilities for all of the values of the observables relevant to a Bell test.
The marginals of this joint distribution, which we experimentally determine,
reproduces all of the observable quantum statistics including a violation of
the Bell inequality, which we independently measure. This is possible because
our distribution, like the weak values for projectors on which it is built, is
not constrained to the interval [0, 1]. It was first pointed out by Feynman
that, for explaining singlet-state correlations within "a [local] hidden
variable view of nature ... everything works fine if we permit negative
probabilities". However, there are infinitely many such theories. Our method,
involving "weak-valued probabilities", singles out a unique set of
probabilities, and moreover does so empirically.Comment: 9 pages, 3 figure
Quantum Non-demolition Measurements on Qubits
We discuss the characterization and properties of quantum non-demolition
(QND) measurements on qubit systems. We introduce figures of merit which can be
applied to systems of any Hilbert space dimension thus providing universal
criteria for characterizing QND measurements. We discuss the controlled-NOT
gate and an optical implementation as examples of QND devices for qubits. We
also discuss the QND measurement of weak values
Measuring a photonic qubit without destroying it
Measuring the polarisation of a single photon typically results in its
destruction. We propose, demonstrate, and completely characterise a
\emph{quantum non-demolition} (QND) scheme for realising such a measurement
non-destructively. This scheme uses only linear optics and photo-detection of
ancillary modes to induce a strong non-linearity at the single photon level,
non-deterministically. We vary this QND measurement continuously into the weak
regime, and use it to perform a non-destructive test of complementarity in
quantum mechanics. Our scheme realises the most advanced general measurement of
a qubit: it is non-destructive, can be made in any basis, and with arbitrary
strength.Comment: 4 pages, 3 figure
Heralded quantum steering over a high-loss channel
Entanglement is the key resource for many long-range quantum information
tasks, including secure communication and fundamental tests of quantum physics.
These tasks require robust verification of shared entanglement, but performing
it over long distances is presently technologically intractable because the
loss through an optical fiber or free-space channel opens up a detection
loophole. We design and experimentally demonstrate a scheme that verifies
entanglement in the presence of at least dB of added loss,
equivalent to approximately km of telecommunication fiber. Our protocol
relies on entanglement swapping to herald the presence of a photon after the
lossy channel, enabling event-ready implementation of quantum steering. This
result overcomes the key barrier in device-independent communication under
realistic high-loss scenarios and in the realization of a quantum repeater.Comment: 8 pages, 5 figure
The simplest demonstrations of quantum nonlocality
We investigate the complexity cost of demonstrating the key types of nonclassical correlations-Bell inequality violation, Einstein, Podolsky, Rosen (EPR)-steering, and entanglement-with independent agents, theoretically and in a photonic experiment. We show that the complexity cost exhibits a hierarchy among these three tasks, mirroring the recently discovered hierarchy for how robust they are to noise. For Bell inequality violations, the simplest test is the well-known Clauser-Horne-Shimony-Holt test, but for EPR-steering and entanglement the tests that involve the fewest number of detection patterns require nonprojective measurements. The simplest EPR-steering test requires a choice of projective measurement for one agent and a single nonprojective measurement for the other, while the simplest entanglement test uses just a single nonprojective measurement for each agent. In both of these cases, we derive our inequalities using the concept of circular two-designs. This leads to the interesting feature that in our photonic demonstrations, the correlation of interest is independent of the angle between the linear polarizers used by the two parties, which thus require no alignment
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