arXiv subjects

Alexei Ourjoumtsev

Publications and source records attributed to Alexei Ourjoumtsev.

At least 19 recordsLinked to original sources

arXiv2026

Improving Single Excitation Fidelity in Rydberg Superatoms for Efficient Single Photon Emission

Deterministic single photon emission from a Rydberg ensemble coupled to an optical cavity requires high-fidelity preparation of collective single excitations. In such a setup imperfect Rydberg blockade can lead to unwanted double excitations, which degrade photon indistinguishability. In this work we adapt the Derivative Removal by Adiabatic Gate (DRAG) technique, originally developed for superconducting qubits, to shape optical pulses that suppress double excitations in this atomic platform. By combining analytical modeling with numerical optimization, DRAG provides an improvement over conventional sine-squared pulses. Further optimization of pulse duration and atomic ensemble size identifies a parameter regime, distinct from that used in [Nature Photonics 17, 688 (2023)], that enhances the single excitation probability from the previous theoretical benchmark of 77% to 91.9%, approaching the fundamental limits set by decoherence in the system. Benchmarking against GRAPE (Gradient Ascent Pulse Engineering) confirms that DRAG operates close to the optimal control limit, while maintaining smooth, experimentally feasible pulse shapes. These results demonstrate the effectiveness and cross platform adaptability of DRAG for a high-fidelity single photon source.

quant-ph↗

arXiv2026

Imperfect blockade in Rydberg superatoms

Ensembles of atoms interacting via their Rydberg levels, known as "superatoms" for their ability to encode qubits and to emit single photons, attract increasing attention as building blocks for quantum network nodes. Assessing their performance requires an accurate, physically informative and numerically scalable description of interactions in a large and disordered ensemble. We derive such a description from first principles and successfully test it against brute-force numerics and experimental data. This model proves essential to make quantitative predictions about gate fidelities or photon emission efficiencies, and to guide experiments towards large-scale superatom-based systems.

quant-ph↗

arXiv2025

Protecting collective qubits from non-Markovian dephasing

Collectively-encoded qubits, involving ensembles of atomic or solid-state emitters, present many practical advantages for quantum technologies. However, they suffer from uncontrolled inhomogeneous dephasing which couples them to a quasi-continuum of dark states. In most cases, this process cannot be encompassed in a standard master equation with time-independent coefficients, making its description either tedious or inaccurate. We show that it can be understood as a displacement in time-frequency phase space and accurately included in resource-efficient numerical simulations of the qubit's dynamics. This description unveils a regime where the qubit becomes protected from dephasing through a combination of strong driving and non-Markovianity. We experimentally investigate this regime using a Rydberg superatom and extend its coherent dynamics beyond the inhomogeneous-dephasing characteristic time by an order of magnitude.

quant-ph↗

arXiv2022

Deterministic Free-Propagating Photonic Qubits with Negative Wigner Functions

Engineering quantum states of free-propagating light is of paramount importance for quantum technologies. Coherent states ubiquitous in classical and quantum communications, squeezed states used in quantum sensing, and even highly-entangled cluster states studied in the context of quantum computing can be produced deterministically, but they obey quasi-classical optical field statistics described by Gaussian, positive Wigner functions. Fully harnessing the potential of many quantum engineering protocols requires using non-Gaussian Wigner-negative states, so far produced using intrinsically probabilistic methods. Here we describe the first fully deterministic preparation of non-Gaussian Wigner-negative free-propagating states of light, obtained by mapping the internal state of an intracavity Rydberg superatom onto an optical qubit encoded as a superposition of 0 and 1 photons. This approach allows us to reach a 60% photon generation efficiency in a well-controlled spatio-temporal mode, while maintaining a strong photon antibunching. By changing the qubit rotation angle, we observe an evolution from quadrature squeezing to Wigner negativity. Our experiment sets this new technique as a viable method to deterministically generate non-Gaussian photonic resources, lifting several major roadblocks in optical quantum engineering.

quant-ph↗

arXiv2021

An Intracavity Rydberg Superatom for Optical Quantum Engineering: Coherent Control, Single-Shot Detection and Optical $π$ Phase Shift

We demonstrate a new versatile building block for optical quantum technologies, based on an intracavity Rydberg-blockaded atomic ensemble acting as a single two-level superatom. We coherently control its state and optically detect it in a single shot with a $95\%$ efficiency. Crucially, we demonstrate a superatom-state-dependent $π$ phase rotation on the light reflected from the cavity. Together with the state manipulation and detection, it is a key ingredient for implementing deterministic photonic entangling gates and for generating highly non-classical light states.

quant-ph↗

arXiv2020

High-Resolution Imaging of Cold Atoms through a Multimode Fiber

We developed an ultra-compact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of $230\,μ$m, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold Rb atoms with a $\sim 1\,μ$m resolution over a $100 \times 100\,μ$m$^2$ field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum engineering platforms where cold atoms are combined with optical cavities, superconducting circuits or optomechanical devices restricting the optical access.

physics.atom-ph↗

arXiv2020

Production and applications of non-Gaussian quantum states of light

This review covers recent theoretical and experimental efforts to extend the application of the continuous-variable quantum technology of light beyond "Gaussian" quantum states, such as coherent and squeezed states, into the domain of "non-Gaussian" states with negative Wigner functions. Starting with basic Gaussian nonclassicality associated with single- and two-mode vacuum states produced by means of parametric down-conversion and applying a set of standard tools, such as linear interferometry, coherent state injection, and conditional homodyne and photon number measurements, one can implement a large variety of optical states and processes that are relevant in fundamental quantum physics as well as quantum optical information processing. We present a systematic review of these methods, paying attention to both fundamental and practical aspects of their implementation, as well as a comprehensive overview of the results achieved therewith.

quant-ph↗

arXiv2019

Manipulating cold atoms through a high-resolution compact system based on a multimode fiber

To manipulate cold atoms in spatially constrained quantum engineering platforms, we developed a lensless optical system with a $\sim$1 $μ$m resolution and a transverse size of only 225 $μ$m. We use a multimode optical fiber with a high numerical aperture, which directly guides light inside the ultra-high-vacuum system. Spatial light modulators allow us to generate control beams at the in-vacuum fiber end by digital optical phase conjugation. As a demonstration, we use this system to optically transport cold atoms towards the in-vacuum fiber end, to load them in optical microtraps and to re-cool them in optical molasses. This work opens new perspectives for setups combining cold atoms with other optical, electronic or opto-mechanical systems with limited optical access.

physics.atom-ph↗

arXiv2016

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.

quant-ph↗

arXiv2015

Photonic Controlled-Phase Gates Through Rydberg Blockade in Optical Cavities

We propose a novel scheme for high fidelity photonic controlled phase gates using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of Rydberg blockade and optical cavities effectively enhances the optical non-linearity created by the strong Rydberg interaction and thereby reduces the requirements for photonic quantum gates. The resulting gate can be implemented with cavities of moderate finesse which allows for highly efficient processing of quantum information encoded in photons. As a particular example of this, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.

quant-ph↗

arXiv2014

Homodyne tomography of a single photon retrieved on demand from a cavity-enhanced cold atom memory

We experimentally demonstrate that a non-classical state prepared in an atomic memory can be efficiently transferred to a single mode of free-propagating light. By retrieving on demand a single excitation from a cold atomic gas, we realize an efficient source of single photons prepared in a pure, fully controlled quantum state. We characterize this source using two detection methods, one based on photon-counting analysis, and the second using homodyne tomography to reconstruct the density matrix and Wigner function of the state. The latter technique allows us to completely determine the mode of the retrieved photon in its fine phase and amplitude details, and demonstrate its nonclassical field statistics by observing a negative Wigner function. We measure a photon retrieval efficiency up to 82% and an atomic memory coherence time of 900 ns. This setup is very well suited to study interactions between atomic excitations, and to use them in order to create and manipulate more sophisticated quantum states of light with a high degree of experimental control.

quant-ph↗

arXiv2013

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.

quant-ph↗

arXiv2012

Observation and measurement of "giant" dispersive optical non-linearities in an ensemble of cold Rydberg atoms

We observe and measure dispersive optical non-linearities in an ensemble of cold Rydberg atoms placed inside an optical cavity. The experimental results are in agreement with a simple model where the optical non-linearities are due to the progressive appearance of a Rydberg blockaded volume within the medium. The measurements allow a direct estimation of the "blockaded fraction" of atoms within the atomic ensemble.

physics.atom-ph↗

arXiv2012

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.

quant-ph↗

arXiv2012

Controlling the quantum state of a single photon emitted from a single polariton

We investigate in detail the optimal conditions for a high fidelity transfer from a single-polariton state to a single-photon state and subsequent homodyne detection of the single photon. We assume that, using various possible techniques, the single polariton has initially been stored as a spin-wave grating in a cloud of cold atoms inside a low-finesse cavity. This state is then transferred to a single-photon optical pulse using an auxiliary beam. We optimize the retrieval efficiency and determine the mode of the local oscillator that maximizes the homodyne efficiency of such a photon. We find that both efficiencies can have values close to one in a large region of experimental parameters.

quant-ph↗

arXiv2012

Observation of squeezed light from one atom excited with two photons

Single quantum emitters like atoms are well-known as non-classical light sources which can produce photons one by one at given times, with reduced intensity noise. However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example is the predicted ability for a single atom to produce quadrature-squeezed light, with sub-shot-noise amplitude or phase fluctuations. It has long been foreseen, though, that such squeezing would be "at least an order of magnitude more difficult" to observe than the emission of single photons. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms, but despite experimental efforts, single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity, several orders of magnitude larger than for usual macroscopic media. This produces observable quadrature squeezing with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons, the squeezed light stems from the quantum coherence of photon pairs emitted from the system. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters

quant-ph↗

arXiv2010

Feedback Cooling of a Single Neutral Atom

We demonstrate feedback cooling of the motion of a single rubidium atom trapped in a high-finesse optical resonator to a temperature of about 160 μK. Time-dependent transmission and intensity-correlation measurements prove the reduction of the atomic position uncertainty. The feedback increases the 1/e storage time into the one second regime, 30 times longer than without feedback. Feedback cooling therefore rivals state-of-the-art laser cooling, but with the advantages that it requires less optical access and exhibits less optical pumping.

quant-ph↗

arXiv2008

Sub-Rayleigh Quantum Imaging Using Single Photon Sources

We propose a technique capable of imaging a distinct physical object with sub-Rayleigh resolution in an ordinary far-field imaging setup using single-photon sources and linear optical tools only. We exemplify our method for the case of a rectangular aperture and two or four single-photon emitters obtaining a resolution enhanced by a factor of two or four, respectively.

quant-ph↗