28 research outputs found
Loss-tolerant quantum enhanced metrology and state engineering via the reverse Hong-Ou-Mandel effect
Preparing highly entangled quantum states between remote parties is a major
challenge for quantum communications [1-8]. Particularly promising in this
context are the N00N states, which are entangled N-photon wavepackets
delocalized between two different locations, providing measurement sensitivity
limited only by the uncertainty principle [1, 10-15]. However, these states are
notoriously vulnerable to losses, making it difficult both to share them
between remote locations, and to recombine them to exploit interference
effects. Here we address this challenge by utilizing the reverse version of the
Hong-Ou-Mandel effect [16] to prepare a high-fidelity two-photon N00N state
shared between two parties connected by a lossy optical channel. Furthermore,
we demonstrate that the enhanced phase sensitivity can be directly exploited in
the two distant locations, and we remotely prepare superpositions of coherent
states, known as Schr\"odinger's cat states" [17, 18]
Synthesis of the Einstein-Podolsky-Rosen entanglement in a sequence of two single-mode squeezers
Synthesis of the Einstein-Podolsky-Rosen entangled state --- the primary
entangled resource in continuous-variable quantum-optical information
processing --- is a technological challenge of great importance. Here we
propose and implement a new scheme of generating this state. Two nonlinear
optical crystals, positioned back-to-back in the waist of a pump beam, function
as single-pass degenerate optical parametric amplifiers and produce single-mode
squeezed vacuum states in orthogonal polarization modes, but in the same
spatiotemporal mode. A subsequent pair of waveplates acts as a beam splitter,
entangling the two polarization modes to generate the Einstein-Podolsky-Rosen
state. This technique takes advantage of the strong nonlinearity associated
with type-I phase-matching configuration while at the same time eliminating the
need for actively stabilizing the optical phase between the two squeezers,
which typically arises if these squeezers are spatially separated. We
demonstrate our method in an experiment, preparing a 1.4 dB two-mode squeezed
state and characterizing it via two-mode homodyne tomography.Comment: 4 pages, 3 figure
Annealing by simulating the coherent Ising machine
The coherent Ising machine (CIM) enables efficient sampling of low-lying
energy states of the Ising Hamiltonian with all-to-all connectivity by encoding
the spins in the amplitudes of pulsed modes in an optical parametric oscillator
(OPO). The interaction between the pulses is realized by means of
measurement-based optoelectronic feedforward which enhances the gain for
lower-energy spin configurations. We present an efficient method of simulating
the CIM on a classical computer that outperforms the CIM itself as well as the
noisy mean-field annealer in terms of both the quality of the samples and the
computational speed. It is furthermore advantageous with respect to the CIM in
that it can handle Ising Hamiltonians with arbitrary real-valued node coupling
strengths. These results illuminate the nature of the faster performance
exhibited by the CIM and may give rise to a new class of quantum-inspired
algorithms of classical annealing that can successfully compete with existing
methods
Passive superresolution imaging of incoherent objects
We investigate Hermite Gaussian Imaging (HGI) -- a novel passive
super-resolution technique -- for complex 2D incoherent objects in the
sub-Rayleigh regime. The method consists of measuring the field's spatial mode
components in the image plane in the overcomplete basis of Hermite-Gaussian
modes and their superpositions and subsequently using a deep neural network to
reconstruct the object from these measurements. We show a three-fold resolution
improvement over direct imaging. Our HGI reconstruction retains its superiority
even if the same neural network is applied to improve the resolution of direct
imaging. This superiority is also preserved in the presence of shot noise. Our
findings are the first step towards passive super-resolution imaging protocols
in fluorescent microscopy and astronomy.Comment: 6 pages, 8 figure
Undoing the effect of loss on quantum entanglement
Entanglement distillation is a process via which the strength and purity of
quantum entanglement can be increased probabilistically. It is a key step in
many quantum communication and computation protocols. In particular,
entanglement distillation is a necessary component of the quantum repeater, a
device which counters the degradation of entanglement that inevitably occurs
due to losses in a communication line. Here we report an experiment on
distilling the Einstein-Podolsky-Rosen (EPR) state of light, the workhorse of
continuous-variable entanglement, using the technique of noiseless
amplification. In contrast to previous implementations, the entanglement
enhancement factor achievable by our technique is not fundamentally limited and
permits recovering an EPR state with a macroscopic level of entanglement no
matter how low the initial entanglement or how high the loss may be. In
particular, we recover the original level of entanglement after one of the EPR
modes has passed through a channel with a loss factor of 20. The level of
entanglement in our distilled state is higher than that achievable by direct
transmission of any state through a similar loss channel. This is a key
bench-marking step towards the realization of a practical continuous-variable
quantum repeater and other CV quantum protocols.Comment: 8 pages, 5 figure