209 research outputs found
Effects of photon losses on phase estimation near the Heisenberg limit using coherent light and squeezed vacuum
Two path interferometry with coherent states and squeezed vacuum can achieve
phase sensitivities close to the Heisenberg limit when the average photon
number of the squeezed vacuum is close to the average photon number of the
coherent light. Here, we investigate the phase sensitivity of such states in
the presence of photon losses. It is shown that the Cramer-Rao bound of phase
sensitivity can be achieved experimentally by using a weak local oscillator and
photon counting in the output. The phase sensitivity is then given by the
Fisher information F of the state. In the limit of high squeezing, the ratio
(F-N)/N^2 of Fisher information above shot noise to the square of the average
photon number N depends only on the average number of photons lost, n_loss, and
the fraction of squeezed vacuum photons mu. For mu=1/2, the effect of losses is
given by (F-N)/N^2=1/(1+2 n_loss). The possibility of increasing the robustness
against losses by lowering the squeezing fraction mu is considered and an
optimized result is derived. However, the improvements are rather small, with a
maximal improvement by a factor of two at high losses.Comment: 7 pages, including 6 figure
High photon number path entanglement in the interference of spontaneously downconverted photon pairs with coherent laser light
We show that the quantum interference between downconverted photon pairs and
photons from coherent laser light can produce a maximally path entangled
N-photon output component with a fidelity greater than 90% for arbitrarily high
photon numbers. A simple beam splitter operation can thus transform the
2-photon coherence of down-converted light into an almost optimal N-photon
coherence.Comment: 5 pages, including 2 figures and 1 table, final version for
publication as rapid communication in Phys. Rev.
Implementation of a quantum controlled-SWAP gate with photonic circuits
Quantum information science addresses how the processing and transmission of
information are affected by uniquely quantum mechanical phenomena. Combination
of two-qubit gates has been used to realize quantum circuits, however,
scalability is becoming a critical problem. The use of three-qubit gates may
simplify the structure of quantum circuits dramatically. Among them, the
controlled-SWAP (Fredkin) gates are essential since they can be directly
applied to important protocols, e.g., error correction, fingerprinting, and
optimal cloning. Here we report a realization of the Fredkin gate for photonic
qubits. We achieve a fidelity of 0.85 in the computational basis and an output
state fidelity of 0.81 for a 3-photon Greenberger-Horne-Zeilinger state. The
estimated process fidelity of 0.77 indicates that our Fredkin gate can be
applied to various quantum tasks.Comment: 9 pages, 4 figures, Sci. Rep. 7, 45353 (2017
Quantum-enhanced phase estimation using optical spin squeezing
Quantum metrology enables estimation of optical phase shifts with precision
beyond the shot-noise limit. One way to exceed this limit is to use squeezed
states, where the quantum noise of one observable is reduced at the expense of
increased quantum noise for its complementary partner. Because shot-noise
limits the phase sensitivity of all classical states, reduced noise in the
average value for the observable being measured allows for improved phase
sensitivity. However, additional phase sensitivity can be achieved using phase
estimation strategies that account for the full distribution of measurement
outcomes. Here we experimentally investigate the phase sensitivity of a
five-particle optical spin-squeezed state generated by photon subtraction from
a parametric downconversion photon source. The Fisher information for all
photon-number outcomes shows it is possible to obtain a quantum advantage of
1.58 compared to the shot-noise limit, even though due to experimental
imperfection, the average noise for the relevant spin-observable does not
achieve sub-shot-noise precision. Our demonstration implies improved
performance of spin squeezing for applications to quantum metrology.Comment: 8 pages, 5 figure
Controlling and measuring a superposition of position and momentum
The dynamics of a particle propagating in free space is described by its
position and momentum, where quantum mechanics prohibits the simultaneous
identification of two non-commutative physical quantities. Recently, a lower
bound on the probability of finding a particle after propagating for a given
time has been derived for well-defined initial constraints on position and
momentum under the assumption that particles travel in straight lines. Here, we
investigate this lower limit experimentally with photons. We prepared a
superposition of position and momentum states by using slits, lenses and an
interferometer, and observed a quantum interference between position and
momentum. The lower bound was then evaluated using the initial state and the
result was 5.9\% below this classical bound.Comment: 5 pages, 4 figure
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