239 research outputs found
Quantum homodyne tomography of a two-photon Fock state
We present a continuous-variable experimental analysis of a two-photon Fock
state of free-propagating light. This state is obtained from a pulsed
non-degenerate parametric amplifier, which produces two intensity-correlated
twin beams. Counting two photons in one beam projects the other beam in the
desired two-photon Fock state, which is analyzed by using a pulsed homodyne
detection. The Wigner function of the measured state is clearly negative. We
developed a detailed analytic model which allows a fast and efficient analysis
of the experimental results.Comment: 4 pages, 6 figures Revised version : corrected typo and reference
Loss-tolerant parity measurement for distant quantum bits
We propose a scheme to measure the parity of two distant qubits, while
ensuring that losses on the quantum channel between them does not destroy
coherences within the parity subspaces. This capability enables deterministic
preparation of highly entangled qubit states whose fidelity is not limited by
the transmission loss. The key observation is that for a probe electromagnetic
field in a particular quantum state, namely a superposition of two coherent
states of opposite phases, the transmission loss stochastically applies a
near-unitary back-action on the probe state. This leads to a parity measurement
protocol where the main effect of the transmission losses is a decrease in the
measurement strength. By repeating the non-destructive (weak) parity
measurement, one achieves a high-fidelity entanglement in spite of a
significant transmission loss
Increasing entanglement between Gaussian states by coherent photon subtraction
We experimentally demonstrate that the entanglement between Gaussian
entangled states can be increased by non-Gaussian operations. Coherent
subtraction of single photons from Gaussian quadrature-entangled light pulses,
created by a non-degenerate parametric amplifier, produces delocalized states
with negative Wigner functions and complex structures, more entangled than the
initial states in terms of negativity. The experimental results are in very
good agreement with the theoretical predictions
Dispersive optical nonlinearities in an EIT-Rydberg medium
We investigate dispersive optical nonlinearities that arise from Rydberg
excitation blockade in cold Rydberg gases. We consider a two-photon transition
scheme and study the non-linear response to a weak optical probe in presence of
a strong control beam. For very low probe fields, the dominant nonlinearities
are of the third order and they can be exactly evaluated in a steady state
regime. In a more general case, the change in average atomic populations and
coherences due to Rydberg interactions can be characterized by properly defined
scaling parameters, which are generally complex numbers but in certain
situations take the usual meaning of the number of atoms in a blockade sphere.
They can be used in a simple "universal scaling" formula to determine the
dispersive optical nonlinearity of the medium. We also develop a novel
technique to account for the Rydberg interaction effects, by simplifying the
treatment of nonlocal interaction terms, the so-called collisional integrals.
We find algebraic relations that only involve two-body correlations, which can
be solved numerically. All average populations and coherences are then obtained
straightforwardly.Comment: 9 pages, 4 figure
All-optical generation of states for "Encoding a qubit in an oscillator"
Both discrete and continuous systems can be used to encode quantum
information. Most quantum computation schemes propose encoding qubits in
two-level systems, such as a two-level atom or an electron spin. Others exploit
the use of an infinite-dimensional system, such as a harmonic oscillator. In
"Encoding a qubit in an oscillator" [Phys. Rev. A 64 012310 (2001)], Gottesman,
Kitaev, and Preskill (GKP) combined these approaches when they proposed a
fault-tolerant quantum computation scheme in which a qubit is encoded in the
continuous position and momentum degrees of freedom of an oscillator. One
advantage of this scheme is that it can be performed by use of relatively
simple linear optical devices, squeezing, and homodyne detection. However, we
lack a practical method to prepare the initial GKP states. Here we propose the
generation of an approximate GKP state by using superpositions of optical
coherent states (sometimes called "Schr\"odinger cat states"), squeezing,
linear optical devices, and homodyne detection.Comment: 4 pages, 3 figures. Submitted to Optics Letter
Generating non-Gaussian states using collisions between Rydberg polaritons
We investigate theoretically the deterministic generation of quantum states
with negative Wigner functions, by using giant non-linearities due to
collisional interactions between Rydberg polaritons. The state resulting from
the polariton interactions may be transferred with high fidelity into a
photonic state, which can be analyzed using homodyne detection followed by
quantum tomography. Besides generating highly non-classical states of the
light, this method can also provide a very sensitive probe for the physics of
the collisions involving Rydberg states.Comment: 5 pages, 3 figure
Rydberg-induced optical nonlinearities from a cold atomic ensemble trapped inside a cavity
We experimentally characterize the optical nonlinear response of a cold
atomic medium placed inside an optical cavity, and excited to Rydberg states.
The excitation to S and D Rydberg levels is carried out via a two-photon
transition in an EIT (electromagnetically induced transparency) configuration,
with a weak (red) probe beam on the lower transition, and a strong (blue)
coupling beam on the upper transition. The observed optical nonlinearities
induced by S states for the probe beam can be explained using a semi-classical
model with van der Waals' interactions. For the D states, it appears necessary
to take into account a dynamical decay of Rydberg excitations into a long-lived
dark state. We show that the measured nonlinearities can be explained by using
a Rydberg bubble model with a dynamical decay.Comment: 8 pages, 6 figure
Quantum optical non-linearities induced by Rydberg-Rydberg interactions: a perturbative approach
In this article, we theoretically study the quantum statistical properties of
the light transmitted through or reflected from an optical cavity, filled by an
atomic medium with strong optical non-linearity induced by Rydberg-Rydberg van
der Waals interactions. Atoms are driven on a two-photon transition from their
ground state to a Rydberg level via an intermediate state by the combination of
a weak signal field and a strong control beam. By using a perturbative
approach, we get analytic results which remain valid in the regime of weak
feeding fields, even when the intermediate state becomes resonant. Therefore
they allow us to investigate quantitatively new features associated with the
resonant behaviour of the system. We also propose an effective non-linear
three-boson model of the system which, in addition to leading to the same
analytic results as the original problem, sheds light on the physical processes
at work in the system
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