4,515 research outputs found

### Quantum state engineering by a coherent superposition of photon subtraction and addition

We study a coherent superposition of field annihilation and creation operator
acting on continuous variable systems and propose its application for quantum
state engineering. Specifically, it is investigated how the superposed
operation transforms a classical state to a nonclassical one, together with
emerging nonclassical effects. We also propose an experimental scheme to
implement this elementary coherent operation and discuss its usefulness to
produce an arbitrary superposition of number states involving up to two
photons.Comment: published version, 7 pages, 8 figure

### Quantum phase estimation using path-symmetric entangled states

We study the sensitivity of phase estimation using a generic class of
path-symmetric entangled states
$|\varphi\rangle|0\rangle+|0\rangle|\varphi\rangle$, where an arbitrary state
$|\varphi\rangle$ occupies one of two modes in quantum superposition. This
class of states includes the previously considered states, i.e. $NOON$ states
and entangled coherent states, as special cases. With its generalization, we
identify the practical limit of phase estimation under energy constraint that
is characterized by the photon statistics of the component state
$|\varphi\rangle$. We first show that quantum Cramer-Rao bound (QCRB) can be
lowered with super-Poissonianity of the state $|\varphi\rangle$. By introducing
a component state of the form
$|\varphi\rangle=\sqrt{q}|1\rangle+\sqrt{1-q}|N\rangle$, we particularly show
that an arbitrarily small QCRB can be achieved even with a finite energy in an
ideal situation. For practical measurement schemes, we consider a parity
measurement and a full photon-counting method to obtain phase-sensitivity.
Without photon loss, the latter scheme employing any path-symmetric states
$|\varphi\rangle|0\rangle+|0\rangle|\varphi\rangle$ achieves the QCRB over the
entire range $[0,2\pi]$ of unknown phase shift $\phi$ whereas the former does
so in a certain confined range of $\phi$. We find that the case of
$|\varphi\rangle=\sqrt{q}|1\rangle+\sqrt{1-q}|N\rangle$ provides the most
robust resource against loss among the considered entangled states over the
whole range of input energy. Finally we also propose experimental schemes to
generate these path-symmetric entangled states.Comment: 10 pages, 5 figures, published versio

### Increasing and decreasing entanglement characteristics for continuous variables by a local photon subtraction

We investigate how the entanglement characteristics of a non-Gaussian
entangled state are increased or decreased by a local photon subtraction
operation. The non-Gaussian entangled state is generated by injecting a
single-mode non-Gaussian state and a vacuum state into a 50:50 beam splitter.
We consider a photon-added coherent state and an odd coherent state as a
single-mode non-Gaussian state. In the regime of small amplitude, we show that
the performance of quantum teleportation and the second-order
Einstein-Podolsky- Rosen-type correlation can both be enhanced, whereas the
degree of entanglement decreases, for the output state when a local photon
subtraction operation is applied to the non-Gaussian entangled state. The
counterintuitive effect is more prominent in the limit of nearly zero
amplitude.Comment: Published version, 7 pages, 3 figure

### Generating arbitrary photon-number entangled states for continuous-variable quantum informatics

We propose two experimental schemes that can produce an arbitrary
photon-number entangled state (PNES) in a finite dimension. This class of
entangled states naturally includes non-Gaussian continuous-variable (CV)
states that may provide some practical advantages over the Gaussian
counterparts (two-mode squeezed states). We particularly compare the
entanglement characteristics of the Gaussian and the non-Gaussian states in
view of the degree of entanglement and the Einstein-Podolsky-Rosen correlation,
and further discuss their applications to the CV teleportation and the
nonlocality test. The experimental imperfection due to the on-off
photodetectors with nonideal efficiency is also considered in our analysis to
show the feasibility of our schemes within existing technologies.Comment: published version, 13 pages, 7 figure

### Single-photon quantum nonlocality: Violation of the Clauser-Horne-Shimony-Holt inequality using feasible measurement setups

We investigate quantum nonlocality of a single-photon entangled state under
feasible measurement techniques consisting of on-off and homodyne detections
along with unitary operations of displacement and squeezing. We test for a
potential violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality, in
which each of the bipartite party has a freedom to choose between 2 measurement
settings, each measurement yielding a binary outcome. We find that
single-photon quantum nonlocality can be detected when two or less of the 4
total measurements are carried out by homodyne detection. The largest violation
of the CHSH inequality is obtained when all four measurements are
squeezed-and-displaced on-off detections. We test robustness of violations
against imperfections in on-off detectors and single-photon sources, finding
that the squeezed-and-displaced measurement schemes perform better than the
displacement-only measurement schemes.Comment: 7+ pages, 7 figures, 1 table, close to published versio

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