754 research outputs found
Phase-dependent decoherence of optical transitions in Pr3+:LaF3 in the presence of a driving field
The decoherence times of orthogonally phased components of the optical
transition dipole moment in a two-level system have been observed to differ by
an order of magnitude. This phase anisotropy is observed in coherent transient
experiments where an optical driving field is present during extended periods
of decoherence. The decoherence time of the component of the dipole moment in
phase with the driving field is extended compared to T_2, obtained from
two-pulse photon echoes, in analogy with the spin locking technique of NMR.Comment: 5 pages, 2 figures; replaced with published versio
Violation of the Leggett-Garg inequality with weak measurements of photons
By weakly measuring the polarization of a photon between two strong
polarization measurements, we experimentally investigate the correlation
between the appearance of anomalous values in quantum weak measurements, and
the violation of realism and non-intrusiveness of measurements. A quantitative
formulation of the latter concept is expressed in terms of a Leggett-Garg
inequality for the outcomes of subsequent measurements of an individual quantum
system. We experimentally violate the Leggett-Garg inequality for several
measurement strengths. Furthermore, we experimentally demonstrate that there is
a one-to-one correlation between achieving strange weak values and violating
the Leggett-Garg inequality.Comment: 5 pages, 4 figure
Experimental optical phase measurement approaching the exact Heisenberg limit
The use of quantum resources can provide measurement precision beyond the
shot-noise limit (SNL). The task of ab initio optical phase measurement---the
estimation of a completely unknown phase---has been experimentally demonstrated
with precision beyond the SNL, and even scaling like the ultimate bound, the
Heisenberg limit (HL), but with an overhead factor. However, existing
approaches have not been able---even in principle---to achieve the best
possible precision, saturating the HL exactly. Here we demonstrate a scheme to
achieve true HL phase measurement, using a combination of three techniques:
entanglement, multiple samplings of the phase shift, and adaptive measurement.
Our experimental demonstration of the scheme uses two photonic qubits, one
double passed, so that, for a successful coincidence detection, the number of
photon-passes is . We achieve a precision that is within of the HL,
surpassing the best precision theoretically achievable with simpler techniques
with . This work represents a fundamental achievement of the ultimate
limits of metrology, and the scheme can be extended to higher and other
physical systems.Comment: (12 pages, 6 figures), typos correcte
Entanglement-enhanced measurement of a completely unknown phase
The high-precision interferometric measurement of an unknown phase is the
basis for metrology in many areas of science and technology. Quantum
entanglement provides an increase in sensitivity, but present techniques have
only surpassed the limits of classical interferometry for the measurement of
small variations about a known phase. Here we introduce a technique that
combines entangled states with an adaptive algorithm to precisely estimate a
completely unspecified phase, obtaining more information per photon that is
possible classically. We use the technique to make the first ab initio
entanglement-enhanced optical phase measurement. This approach will enable
rapid, precise determination of unknown phase shifts using interferometry.Comment: 6 pages, 4 figure
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
Insulin-like growth factor binding protein-5 as a biomarker for detection of early liver disease
Study identifying an Insulin-like growth factor binding protein-5 as a biomarker for detection of early liver disease presented at the annual congress of the british toxicology societ
Quantum gate characterization in an extended Hilbert space
We describe an approach for characterizing the process of quantum gates using
quantum process tomography, by first modeling them in an extended Hilbert
space, which includes non-qubit degrees of freedom. To prevent unphysical
processes from being predicted, present quantum process tomography procedures
incorporate mathematical constraints, which make no assumptions as to the
actual physical nature of the system being described. By contrast, the
procedure presented here ensures physicality by placing physical constraints on
the nature of quantum processes. This allows quantum process tomography to be
performed using a smaller experimental data set, and produces parameters with a
direct physical interpretation. The approach is demonstrated by example of
mode-matching in an all-optical controlled-NOT gate. The techniques described
are non-specific and could be applied to other optical circuits or quantum
computing architectures.Comment: 4 pages, 2 figures, REVTeX (published version
High-Fidelity Z-Measurement Error Correction of Optical Qubits
We demonstrate a quantum error correction scheme that protects against
accidental measurement, using an encoding where the logical state of a single
qubit is encoded into two physical qubits using a non-deterministic photonic
CNOT gate. For the single qubit input states |0>, |1>, |0>+|1>, |0>-|1>,
|0>+i|1>, and |0>-i|1> our encoder produces the appropriate 2-qubit encoded
state with an average fidelity of 0.88(3) and the single qubit decoded states
have an average fidelity of 0.93(5) with the original state. We are able to
decode the 2-qubit state (up to a bit flip) by performing a measurement on one
of the qubits in the logical basis; we find that the 64 1-qubit decoded states
arising from 16 real and imaginary single qubit superposition inputs have an
average fidelity of 0.96(3).Comment: 4 pages, 4 figures, comments welcom
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