1,774 research outputs found
Entanglement-free Heisenberg-limited phase estimation
Measurement underpins all quantitative science. A key example is the
measurement of optical phase, used in length metrology and many other
applications. Advances in precision measurement have consistently led to
important scientific discoveries. At the fundamental level, measurement
precision is limited by the number N of quantum resources (such as photons)
that are used. Standard measurement schemes, using each resource independently,
lead to a phase uncertainty that scales as 1/sqrt(N) - known as the standard
quantum limit. However, it has long been conjectured that it should be possible
to achieve a precision limited only by the Heisenberg uncertainty principle,
dramatically improving the scaling to 1/N. It is commonly thought that
achieving this improvement requires the use of exotic quantum entangled states,
such as the NOON state. These states are extremely difficult to generate.
Measurement schemes with counted photons or ions have been performed with N <=
6, but few have surpassed the standard quantum limit and none have shown
Heisenberg-limited scaling. Here we demonstrate experimentally a
Heisenberg-limited phase estimation procedure. We replace entangled input
states with multiple applications of the phase shift on unentangled
single-photon states. We generalize Kitaev's phase estimation algorithm using
adaptive measurement theory to achieve a standard deviation scaling at the
Heisenberg limit. For the largest number of resources used (N = 378), we
estimate an unknown phase with a variance more than 10 dB below the standard
quantum limit; achieving this variance would require more than 4,000 resources
using standard interferometry. Our results represent a drastic reduction in the
complexity of achieving quantum-enhanced measurement precision.Comment: Published in Nature. This is the final versio
General framework for estimating the ultimate precision limit in noisy quantum-enhanced metrology
The estimation of parameters characterizing dynamical processes is central to
science and technology. The estimation error changes with the number N of
resources employed in the experiment (which could quantify, for instance, the
number of probes or the probing energy). Typically, it scales as 1/N^(1/2).
Quantum strategies may improve the precision, for noiseless processes, by an
extra factor 1/N^(1/2). For noisy processes, it is not known in general if and
when this improvement can be achieved. Here we propose a general framework for
obtaining attainable and useful lower bounds for the ultimate limit of
precision in noisy systems. We apply this bound to lossy optical interferometry
and atomic spectroscopy in the presence of dephasing, showing that it captures
the main features of the transition from the 1/N to the 1/N^(1/2) behaviour as
N increases, independently of the initial state of the probes, and even with
use of adaptive feedback.Comment: Published in Nature Physics. This is the revised submitted version.
The supplementary material can be found at
http://www.nature.com/nphys/journal/v7/n5/extref/nphys1958-s1.pd
Modelling the electronic structure and magnetic properties of LiFeAs and FeSe using hybrid-exchange density functional theory
The electronic structure and magnetic properties of LiFeAs and FeSe have been
studied using hybrid exchange density functional theory. The total energies for
a unit cell in LiFeAs and FeSe with different spin states including
non-magnetic and spin-2 are calculated. The spin-2 configuration has the lower
energy for both LiFeAs and FeSe. The computed anti-ferromagnetic exchange
interactions between spins on the nearest (next nearest) neighbouring Fe atoms
in LiFeAs and FeSe are approximately 14 (17) meV and 6 (13) meV respectively.
The total energies of the checkerboard and stripe-type anti-ferromagnetic
ordering for LiFeAs and FeSe are compared, yielding that for LiFeAs the
checkerboard is lower whereas for FeSe the stripe-type is lower. However, owing
to the fact that the exchange interaction of the next nearest neighbour is
larger than that of the nearest one, which means that the collinear ordering
might be the ground state. These results are in agreement with previous
theoretical calculations and experiments. Especially the calculations for
LiFeAs indicate a co-existence of conducting d-bands at the Fermi surface and
d-orbital magnetism far below the Fermi surface. The theoretical results
presented here might be useful for the experimentalists working on the
electronic structure and magnetism of iron-based superconductors.Comment: 7 pages, 4 figures, 1 table, accepted by Solid State Communication
Effect of phase fluctuation and dephasing on the dynamics of entanglement generation in a correlated emission laser
A detailed study of the effects of phase fluctuation and dephasing on the
dynamics of the entanglement generated from a coherently pumped correlated
emission laser is presented. It is found that the time evolution of the
entanglement is significantly reliant on the phase fluctuation and dephasing,
particularly, at early stages of the lasing process. In the absence of external
driving radiation, the degree of entanglement and intensity turns out to attain
a maximum value just before starting to exhibit oscillation which dies at
longer time scale. However, in case the driving mechanism is on, the
oscillatory nature disappears due to the additional induced coherent
superposition and the degree of entanglement would be larger at steady state.
Moreover, the degree of entanglement as predicted by the logarithmic negativity
and the Duan-Giedke-Cirac-Zoller criteria exhibits a similar nature when there
is no driving radiation, although such a trend is eroded with increasing
strength of the pumping radiation at longer time scale. The other important
aspect of the phase fluctuation and dephasing is the possibility of relaxing
the time at which the maximum entanglement is detected.Comment: 10 pages, 10 figure
Quantum states made to measure
Recent progress in manipulating quantum states of light and matter brings
quantum-enhanced measurements closer to prospective applications. The current
challenge is to make quantum metrologic strategies robust against
imperfections.Comment: 4 pages, 3 figures, Commentary for Nature Photonic
A high-fidelity noiseless amplifier for quantum light states
Noise is the price to pay when trying to clone or amplify arbitrary quantum
states. The quantum noise associated to linear phase-insensitive amplifiers can
only be avoided by relaxing the requirement of a deterministic operation. Here
we present the experimental realization of a probabilistic noiseless linear
amplifier that is able to amplify coherent states at the highest level of
effective gain and final state fidelity ever reached. Based on a sequence of
photon addition and subtraction, and characterized by a significant
amplification and low distortions, this high-fidelity amplification scheme may
become an essential tool for quantum communications and metrology, by enhancing
the discrimination between partially overlapping quantum states or by
recovering the information transmitted over lossy channels.Comment: 5 pages, 4 figure
Ab-initio Quantum Enhanced Optical Phase Estimation Using Real-time Feedback Control
Optical phase estimation is a vital measurement primitive that is used to
perform accurate measurements of various physical quantities like length,
velocity and displacements. The precision of such measurements can be largely
enhanced by the use of entangled or squeezed states of light as demonstrated in
a variety of different optical systems. Most of these accounts however deal
with the measurement of a very small shift of an already known phase, which is
in stark contrast to ab-initio phase estimation where the initial phase is
unknown. Here we report on the realization of a quantum enhanced and fully
deterministic phase estimation protocol based on real-time feedback control.
Using robust squeezed states of light combined with a real-time Bayesian
estimation feedback algorithm, we demonstrate deterministic phase estimation
with a precision beyond the quantum shot noise limit. The demonstrated protocol
opens up new opportunities for quantum microscopy, quantum metrology and
quantum information processing.Comment: 5 figure
Measurement-Induced Entanglement for Excitation Stored in Remote Atomic Ensembles
A critical requirement for diverse applications in Quantum Information
Science is the capability to disseminate quantum resources over complex quantum
networks. For example, the coherent distribution of entangled quantum states
together with quantum memory to store these states can enable scalable
architectures for quantum computation, communication, and metrology. As a
significant step toward such possibilities, here we report observations of
entanglement between two atomic ensembles located in distinct apparatuses on
different tables. Quantum interference in the detection of a photon emitted by
one of the samples projects the otherwise independent ensembles into an
entangled state with one joint excitation stored remotely in 10^5 atoms at each
site. After a programmable delay, we confirm entanglement by mapping the state
of the atoms to optical fields and by measuring mutual coherences and photon
statistics for these fields. We thereby determine a quantitative lower bound
for the entanglement of the joint state of the ensembles. Our observations
provide a new capability for the distribution and storage of entangled quantum
states, including for scalable quantum communication networks .Comment: 13 pages, 4 figures Submitted for publication on August 31 200
Biological measurement beyond the quantum limit
Quantum noise places a fundamental limit on the per photon sensitivity
attainable in optical measurements. This limit is of particular importance in
biological measurements, where the optical power must be constrained to avoid
damage to the specimen. By using non-classically correlated light, we
demonstrated that the quantum limit can be surpassed in biological
measurements. Quantum enhanced microrheology was performed within yeast cells
by tracking naturally occurring lipid granules with sensitivity 2.4 dB beyond
the quantum noise limit. The viscoelastic properties of the cytoplasm could
thereby be determined with a 64% improved measurement rate. This demonstration
paves the way to apply quantum resources broadly in a biological context
Heralded quantum entanglement between two crystals
Quantum networks require the crucial ability to entangle quantum nodes. A
prominent example is the quantum repeater which allows overcoming the distance
barrier of direct transmission of single photons, provided remote quantum
memories can be entangled in a heralded fashion. Here we report the observation
of heralded entanglement between two ensembles of rare-earth-ions doped into
separate crystals. A heralded single photon is sent through a 50/50
beamsplitter, creating a single-photon entangled state delocalized between two
spatial modes. The quantum state of each mode is subsequently mapped onto a
crystal, leading to an entangled state consisting of a single collective
excitation delocalized between two crystals. This entanglement is revealed by
mapping it back to optical modes and by estimating the concurrence of the
retrieved light state. Our results highlight the potential of rare-earth-ions
doped crystals for entangled quantum nodes and bring quantum networks based on
solid-state resources one step closer.Comment: 10 pages, 5 figure
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