# Publications by Alexander Lvovsky

## Synthesis of the Einstein-Podolsky-Rosen entanglement in a sequence of two single-mode squeezers.

Optics letters **42** (2017) 132-134

We propose and implement a new scheme of generating the optical Einstein-Podolsky-Rosen entangled state. Parametric down-conversion in two nonlinear crystals, positioned back-to-back in the waist of a pump beam, produces single-mode squeezed vacuum states in orthogonal polarization modes; a subsequent beam splitting entangles them and generates the Einstein-Podolsky-Rosen state. The technique takes advantage of the strong nonlinearity associated with type-0 phase-matching configuration while, at the same time, eliminating the need for actively stabilizing the optical phase between the two single-mode squeezers. We demonstrate our method, preparing a 1.4 dB two-mode squeezed state and characterizing it via two-mode homodyne tomography.

## Far-field linear optical superresolution via heterodyne detection in a higher-order local oscillator mode

OPTICA **3** (2016) 1148-1152

## Loss-tolerant state engineering for quantum-enhanced metrology via the reverse Hong-Ou-Mandel effect.

Nature communications **7** (2016) 11925-

Highly entangled quantum states, shared by remote parties, are vital for quantum communications and metrology. Particularly promising are the N00N states-entangled N-photon wavepackets delocalized between two different locations-which outperform coherent states in measurement sensitivity. However, these states are notoriously vulnerable to losses, making them difficult to both share them between remote locations and recombine in order to exploit interference effects. Here we address this challenge by utilizing the reverse Hong-Ou-Mandel effect to prepare a high-fidelity two-photon N00N state shared between two parties connected by a lossy optical medium. We measure the prepared state by two-mode homodyne tomography, thereby demonstrating that the enhanced phase sensitivity can be exploited without recombining the two parts of the N00N state. Finally, we demonstrate the application of our method to remotely prepare superpositions of coherent states, known as Schrödinger's cat states.

## Squeezed Light

in *Photonics: Scientific Foundations, Technology and Applications*, **1** (2015) 121-163

© 2015 John Wiley & Sons, Inc. The basic idea of squeezing can be understood by considering the quantum harmonic oscillator, familiar from undergraduate quantum mechanics. Squeezing is best visualized by means of the Wigner function-the quantum analog of the phase-space probability density. This chapter discusses the photon number decomposition of the two-mode squeezed state. Most frequently, squeezing is obtained by nonlinear optical wave-mixing processes, in which pairs of photons are emitted into degenerate (single-mode squeezing) or non-degenerate (two-mode squeezing) modes. In order to mathematically describe nonlinear-optical squeezing, the chapter describes the equations for the propagation of classical electromagnetic fields through a nonlinear medium. Amplitude squeezing is then readily observed by measuring the intensity with a single high-efficiency detector and evaluating the variance of the photocurrent noise.

## Tomography of a multimode quantum black box

NEW JOURNAL OF PHYSICS **17** (2015) ARTN 043063

## Undoing the effect of loss on quantum entanglement

NATURE PHOTONICS **9** (2015) 764-768

## Quantum vampire: collapse-free action at a distance by the photon annihilation operator

OPTICA **2** (2015) 112-115

## Complete temporal characterization of a single photon

LIGHT-SCIENCE & APPLICATIONS **4** (2015) ARTN e298

## Efficiencies of quantum optical detectors

PHYSICAL REVIEW A **90** (2014) ARTN 053846

## Generation and tomography of arbitrary optical qubits using transient collective atomic excitations.

Optics letters **39** (2014) 5447-5450

We demonstrate the preparation of heralded Fock-basis qubits (a|0〉+b|1〉) from transient collective spin excitations in a hot atomic vapor. The preparation event is heralded by Raman-scattered photons in a four-wave mixing process seeded by a weak coherent optical excitation. The amplitude and phase of the seed field allow arbitrary control over the qubit coefficients. The qubit state is characterized using balanced homodyne tomography.

## Optomechanical Micro-Macro Entanglement

PHYSICAL REVIEW LETTERS **112** (2014) ARTN 080503

## Distillation of the two-mode squeezed state.

Physical review letters **112** (2014) 070402-

We experimentally demonstrate entanglement distillation of the two-mode squeezed state obtained by parametric down-conversion. Applying the photon annihilation operator to both modes, we raise the fraction of the photon-pair component in the state, resulting in the increase of both squeezing and entanglement by about 50%. Because of the low amount of initial squeezing, the distilled state does not experience significant loss of Gaussian character.

## Observation of micro-macro entanglement of light

NATURE PHYSICS **9** (2013) 541-544

## Creating and detecting micro-macro photon-number entanglement by amplifying and deamplifying a single-photon entangled state.

Physical review letters **110** (2013) 170406-

We propose a scheme for the observation of micro-macro entanglement in photon number based on amplifying and deamplifying a single-photon entangled state in combination with homodyne quantum state tomography. The created micro-macro entangled state, which exists between the amplification and deamplification steps, is a superposition of two components with mean photon numbers that differ by approximately a factor of three. We show that for reasonable values of photon loss it should be possible to detect micro-macro photon-number entanglement where the macrosystem has a mean number of one hundred photons or more.

## Observation of electromagnetically induced transparency in evanescent fields.

Optics express **21** (2013) 6880-6888

We observe and investigate, both experimentally and theoretically, electromagnetically-induced transparency experienced by evanescent fields arising due to total internal reflection from an interface of glass and hot rubidium vapor. This phenomenon manifests itself as a non-Lorentzian peak in the reflectivity spectrum, which features a sharp cusp with a sub-natural width of about 1 MHz. The width of the peak is independent of the thickness of the interaction region, which indicates that the main source of decoherence is likely due to collisions with the cell walls rather than diffusion of atoms. With the inclusion of a coherence-preserving wall coating, this system could be used as an ultra-compact frequency reference.

## Experimental characterization of bosonic creation and annihilation operators.

Physical review letters **110** (2013) 130403-

The photon creation and annihilation operators are cornerstones of the quantum description of the electromagnetic field. They signify the isomorphism of the optical Hilbert space to that of the harmonic oscillator and the bosonic nature of photons. We perform complete experimental characterization (quantum process tomography) of these operators. By measuring their effect on coherent states by means of homodyne tomography, we obtain their process tensor in the Fock basis, which explicitly shows the "raising" and "lowering" properties of these operators with respect to photon number states. This is the first experimental demonstration of complete tomography of nondeterministic quantum processes.

## Narrowband photons from an atomic source

Frontiers in Optics, FIO 2012 (2012)

We demonstrate the efficient generation of narrow-bandwidth photons and photon superposition states. Since the heralded states stem from a transient collective spin excitation in the atomic ensemble, this work allows engineering of arbitrary collective atomic excitation states. © OSA 2012.

## Maximum-likelihood coherent-state quantum process tomography

NEW JOURNAL OF PHYSICS **14** (2012) ARTN 105021

## Versatile wideband balanced detector for quantum optical homodyne tomography

OPTICS COMMUNICATIONS **285** (2012) 5259-5267