# Publications by Alexander Lvovsky

## Darkness of two-mode squeezed light in ?-type atomic system

NEW JOURNAL OF PHYSICS **22** (2020) ARTN 013014

## Measuring fluorescence into a nanofiber by observing field quadrature noise

Optics Letters Optical Society of America **44** (2019) 1678-1681

We perform balanced homodyne detection of the electromagnetic field in a single-mode tapered optical nanofiber surrounded by rubidium atoms in a magneto-optical trap. Resonant fluorescence of atoms into the nanofiber mode manifests itself as increased quantum noise of the field quadratures. The autocorrelation function of the homodyne detector's output photocurrent exhibits exponential fall-off with a decay time constant of 26.3±0.6 ns, which is consistent with the theoretical expectation under our experimental conditions. To the best of our knowledge, this is the first experiment in which fluorescence into a tapered optical nanofiber has been observed and measured by balanced optical homodyne detection.

## Annealing by simulating the coherent Ising machine.

Optics express **27** (2019) 10288-10295

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.

## Entanglement of macroscopically distinct states of light

Optica **6** (2019) 1425-1430

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Schrödinger’s famous Gedankenexperiment has inspired multiple generations of physicists to think about apparent paradoxes that arise when the logic of quantum physics is applied to macroscopic objects. The development of quantum technologies enabled us to produce physical analogues of Schrödinger’s cats, such as superpositions of macroscopically distinct states as well as entangled states of microscopic and macroscopic entities. Here we take one step further and prepare an optical state which, in Schrödinger’s language, is equivalent to a superposition of two cats, one of which is dead and the other alive, but it is not known in which state each individual cat is. Specifically, the alive and dead states are, respectively, the displaced single photon and displaced vacuum (coherent state), with the magnitude of displacement being on a scale of 108 photons. These two states have significantly different photon statistics and are therefore macroscopically distinguishable.

## Quantum technologies in Russia

QUANTUM SCIENCE AND TECHNOLOGY **4** (2019) ARTN 040501

## Schrödinger cats for quantum internet

2018 Conference on Lasers and Electro-Optics, CLEO 2018 - Proceedings (2018)

© 2018 OSA. We generate and characterize an entangled state between qubits encoded in single-photon polarizations and superpositions of opposite-amplitude coherent states. We demonstrate its application for exchanging quantum information between discrete-and continuous-variable encodings of quantum information.

## Quantum-secured blockchain

QUANTUM SCIENCE AND TECHNOLOGY **3** (2018) UNSP 035004

## Schro¨dinger cats for quantum internet

Optics InfoBase Conference Papers **Part F93-CLEO_QELS 2018** (2018)

© OSA 2018. We generate and characterize an entangled state between qubits encoded in single-photon polarizations and superpositions of opposite-amplitude coherent states. We demonstrate its application for exchanging quantum information between discrete- and continuous-variable encodings of quantum information.

## Optical nanofiber temperature monitoring via double heterodyne detection

AIP ADVANCES **8** (2018) ARTN 055005

## Towards interconversion between discrete- and continuous-variable encodings in quantum optics

FOURTH INTERNATIONAL CONFERENCE ON QUANTUM TECHNOLOGIES (ICQT-2017) **1936** (2018) ARTN 020017

## Generating and breeding optical Schrodinger's cat states

FOURTH INTERNATIONAL CONFERENCE ON QUANTUM TECHNOLOGIES (ICQT-2017) **1936** (2018) ARTN 020018

## Quantum Physics An Introduction Based on Photons

Springer, 2018

The book uses a mathematically simple physical system – photon polarization – as the visualization tool, permitting the student to see the entangled beauty of the quantum world from the very first pages.

## Two-level masers as heat-to-work converters.

Proceedings of the National Academy of Sciences of the United States of America **115** (2018) 9941-9944

Heat engines, which cyclically transform heat into work, are ubiquitous in technology. Lasers and masers may be viewed as heat engines that rely on population inversion or coherence in the active medium. Here we put forward an unconventional paradigm of a remarkably simple and robust electromagnetic heat-powered engine that bears basic differences to any known maser or laser: The proposed device makes use of only one Raman transition and does not rely on population inversion or coherence in its two-level working medium. Nor does it require any coherent driving. The engine can be powered by the ambient temperature difference between the sky and the ground surface. Its autonomous character and "free" power source make this engine conceptually and technologically enticing.

## Quantum computers put blockchain security at risk

Nature Springer Nature **563** (2018) 465-467

## Entanglement and teleportation between polarization and wave-like encodings of an optical qubit.

Nature communications **9** (2018) 3672-

Light is an irreplaceable means of communication among various quantum information processing and storage devices. Due to their different physical nature, some of these devices couple more strongly to discrete, and some to continuous degrees of freedom of a quantum optical wave. It is therefore desirable to develop a technological capability to interconvert quantum information encoded in these degrees of freedom. Here we generate and characterize an entangled state between a dual-rail (polarization-encoded) single-photon qubit and a qubit encoded as a superposition of opposite-amplitude coherent states. We furthermore demonstrate the application of this state as a resource for the interfacing of quantum information between these encodings. In particular, we show teleportation of a polarization qubit onto a freely propagating continuous-variable qubit.

## Fisher information for far-field linear optical superresolution via homodyne or heterodyne detection in a higher-order local oscillator mode

Physical Review A **96** (2017)

© 2017 American Physical Society. The distance between two point light sources is difficult to estimate if that distance is below the diffraction (Rayleigh's) resolution limit of the imaging device. A recently proposed technique enhances the precision of this estimation by exploiting the source-separation-dependent coupling of light into higher-order transverse-electromagnetic (TEM) modes, particularly the TEM01 mode of the image. We theoretically analyze the estimation of the source separation by means of homodyne or heterodyne detection with a local oscillator in the TEM01 mode, which is maximally sensitive to the separation in the sub-Rayleigh regime. We calculate the Fisher information associated with this estimation and compare it with direct imaging. For thermal sources, the Fisher information in any mode of the image plane depends nonlinearly on the average received photon number. We show that the per-photon Fisher information surpasses that of direct imaging (in the interesting sub-Rayleigh regime) when the average received photon number per source exceeds two for homodyne detection and four for heterodyne detection.

## Noise spectra in balanced optical detectors based on transimpedance amplifiers.

The Review of scientific instruments **88** (2017) 113109-

We present a thorough theoretical analysis and experimental study of the shot and electronic noise spectra of a balanced optical detector based on an operational amplifier connected in a transimpedance scheme. We identify and quantify the primary parameters responsible for the limitations of the circuit, in particular, the bandwidth and shot-to-electronic noise clearance. We find that the shot noise spectrum can be made consistent with the second-order Butterworth filter, while the electronic noise grows linearly with the second power of the frequency. Good agreement between the theory and experiment is observed; however, the capacitances of the operational amplifier input and the photodiodes appear significantly higher than those specified in manufacturers' datasheets. This observation is confirmed by independent tests.

## Quantum Teleportation Between Discrete and Continuous Encodings of an Optical Qubit.

Physical review letters **118** (2017) 160501-

The transfer of quantum information between physical systems of a different nature is a central matter in quantum technologies. Particularly challenging is the transfer between discrete and continuous degrees of freedom of various harmonic oscillator systems. Here we implement a protocol for teleporting a continuous-variable optical qubit, encoded by means of low-amplitude coherent states, onto a discrete-variable, single-rail qubit-a superposition of the vacuum and single-photon optical states-via a hybrid entangled resource. We test our protocol on a one-dimensional manifold of the input qubit space and demonstrate the mapping onto the equator of the teleported qubit's Bloch sphere with an average fidelity of 0.83±0.04. Our work opens up the way to the wide application of quantum information processing techniques where discrete- and continuous-variable encodings are combined within the same optical circuit.

## Enlargement of optical Schrödinger's cat states

Nature Photonics **11** (2017) 379-382

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Superpositions of macroscopically distinct quantum states, introduced in Schrödinger's famous Gedankenexperiment, are an epitome of quantum â € strangeness' and a natural tool for determining the validity limits of quantum physics. The optical incarnation of Schrödinger's cat (SC) - the superposition of two opposite-amplitude coherent states - is also the backbone of continuous-variable quantum information processing. However, the existing preparation methods limit the amplitudes of the component coherent states, which curtails the state's usefulness for fundamental and practical applications. Here, we convert a pair of negative squeezed SC states of amplitude 1.15 to a single positive SC state of amplitude 1.85 with a success probability of â 1/40.2. The protocol consists in bringing the initial states into interference on a beamsplitter and a subsequent heralding quadrature measurement in one of the output channels. Our technique can be realized iteratively, so arbitrarily high amplitudes can, in principle, be reached.