Travailler au LKB

Scientific context
Rydberg atom arrays are powerful platforms for the quantum simulation of spin models. Ground-state atoms trapped in optical tweezers are arranged in an arbitrary geometry before being transferred into low-angular momentum Rydberg states using laser pulses. However, the duration of the simulation is currently limited by the relatively short lifetime of laser-accessible Rydberg states. Circular Rydberg states, i.e. Rydberg states with maximum angular momentum, have an extremely long radiative lifetime, and open the way to studying quantum dynamics on much longer timescales [1]. 

In our group, we are building a quantum simulator based on circular states of strontium. Strontium is a two-valence-electron atom, and its ionic core remains optically active when one of the valence electrons is excited to a circular Rydberg state. The ionic-core transitions can be driven without autoionizing the atoms [2], which enables laser slowing of atoms directly in the Rydberg state [3] as well as optical manipulation of the Rydberg state via its electrostatic interactions with the ionic core [4]. In the experiment, we prepare individual strontium ground-state atoms in optical tweezer arrays inside a cryostat and excite them to circular Rydberg states. The circular atoms are trapped in the same tweezers as the ground-state atoms using the polarizability of the ionic core. Thanks to the suppressed blackbody radiation in our cryogenic environment, we have demonstrated a circular state lifetime of about 2 ms, roughly 40 times longer than that of laser-excited Rydberg states [5]. We are currently studying the interactions between pairs of circular Rydberg atoms.

PhD Project
The purpose of the PhD is to pursue our ongoing effort toward building a strontium circular state quantum simulator. The long lifetime of the circular state, combined with the ability to trap the atom with very low power thanks to the polarizability of the ionic core, opens the way to performing quantum simulation over very long timescales with a large number of atoms, even in a cryogenic environment. In the coming years, we plan to develop innovative methods to prepare defect-free 2D arrays of circular atoms that will give access to a rich landscape of spin models, including frustrated systems that can host exotic, highly entangled ground states. This will open the way to the preparation and study of quantum spin liquids. Circular states of strontium also offer interesting prospects in quantum optics. The interaction between the Rydberg electron and its ionic core opens a way to directly couple Rydberg atoms to optical photons.

Master Project
We have recently purchased a new closed-cycle cryostat. During the master’s internship, the student will focus on building a new laser system to prepare Rydberg atoms in this setup.

References
[1] T. L. Nguyen et al., Towards quantum simulation with circular Rydberg atoms, Phys. Rev. X 8, 011032 (2018) – https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.011032
[2] R. C. Teixeira et al., Preparation of long-lived, non-autoionizing circular Rydberg states of strontium, Phys. Rev. Lett. 125, 263001 (2020) – https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.263001
[3] L. Lachaud et al., Slowing down a coherent superposition of circular Rydberg states of strontium, Phys. Rev. Lett. 133, 123202 (2024) – https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.123202
[4] A. Muni et al., Optical coherent manipulation of alkaline-earth circular Rydberg states, Nat. Phys. 18, 502-505 (2022) – https://www.nature.com/articles/s41567-022-01519-w
[5] B. Muraz et al., Measuring the Sr+ 5s1/2 Landé g-factor using singlet-triplet oscillations in a circular Rydberg state of strontium, arXiv:2608.26784 (2026) – https://arxiv.org/abs/2608.26784                     

Subject
The Multimode Quantum Optics group at LKB has carried out leading research in the experimental generation of squeezed and continuous-variable entangled states for the implementation of quantum information protocols. The activities in this area range from the generation of large entangled networks multiplexed both in time and frequency [1,2], to the implementation and verification of non-Gaussian operations in all-optical setups [3]. 

While, the generation of large-scale non-Gaussian states is a prerequisite for continuous-variable quantum computing, typical heralding-based methods such as photon subtraction are hard to implement at such scale. In other platforms with bosonic degrees of freedom, such as trapped ions [4] or superconducting microwave resonators [5], non-Gaussian states are generated by making the bosonic modes interact with a finite-dimensional systems (e.g. a qubit). In optics, a similar route is followed when making light interact with Rydberg atoms [6]. From a theory point of view, these are all hybrid approach that intertwine discrete and continuous degrees of freedom. Within this postdoctoral project, we aim to develop a general mathematical formalism to describe these systems in an intuitive way.

Phase space representations [7] form a natural starting point for this project. While these techniques were initially developed to describe continuous-variable systems, they have later also been translated to finite-dimensional systems. A particularly useful feature of such representations is that they can capture practically useful resources such as magic as negativity in the phase space representation [8]. Using the unifying language of group theory [9], we aim to develop a phase space description of hybrid systems, aiming to make their computational power apparent as a form of phase space negativity. The final goal of this project is to find an effective description to identify new methods for engineering resourceful quantum states, and to understand the computational potential of hybrid systems.

This project is part of the National Acceleration Strategies PEPR OQULUS and NISQ2LSQ:
https://www.pepr-quantique.fr/en/projet/oqulus/
https://www.pepr-quantique.fr/projet/nisq2lsq/

[1] Y. Cai, et al. Nature communications 8 (1), 15645 (2017)
[2] V. Roman-Rodriguez, et al. Physical Review Research 6 (4), 043113 (2024)
[3] YS Ra, et al. Nature Physics 16 (2), 144-147 (2020)
[4] S. Saner, et al. Phys. Rev. X 16, 021049 (2026)
[5] M. Kudra, et al. PRX Quantum 3, 030301 (2022)
[6] V. Magro, et al. Nat. Photon. 17, 688–693 (2023).
[7] M. Frigerio et al. Quantum 10, 2155 (2026)
[8] D. Gross J. Math. Phys. 47, 122107 (2006)
[9] C Brif and A Mann J. Phys. A: Math. Gen. 31 L9 (1998)

Environment
This project is conceived as an intense collaboration between the LKB and LIP6 at Sorbonne Université, and the QAT team at ENS. The successful candidate will be hosted primarily by the multimode quantum optics group at LKB to also reinforce a connection with experiments.
 
As a whole, the multimode quantum optics group has a tradition of working together with a diverse range of people from varied backgrounds. This diversity often leads to fruitful scientific input from different points of view, and it allows the group to explore new avenues. Furthermore, the moderate size of our group gives PhD students and postdocs the opportunity to discuss with PIs on a daily basis. This gives rise to a dynamical atmosphere with a lot of space for discussion.

Laboratoire Kastler Brossel is one of the worldwide leaders in the field of fundamental physics of quantum systems, covering a number of subjects spanning from fundamental tests of quantum theory to applications. It has an internationally recognized expertise throughout its 65 years history, including three Nobel Prize winners. Based in Sorbonne Université in the Pierre and Marie Curie Campus in the center of Paris, it is part of the Quantum Information Center (QICS) in Sorbonne, bringing together quantum physicists and computer scientists. It is also part of the Paris Centre for Quantum Technologies (PCQT), a research, training and innovation network involving 22 research laboratories in Paris Region, 35 start-ups and larger industry groups. 

Practical information 
Candidates must hold an internationally recognized PhD in a field related to theoretical quantum physics. A good background and past research track record in quantum information theory, quantum correlations, or phase space techniques is a strength. 

Application procedure: Inquiries and applications (incl. a detailed CV and two names of potential references) should be sent by email to Mattia Walschaers (mattia.walschaers@lkb.upmc.fr) – please include “[Application postdoc hybrid]” in the subject line

Salary: Monthly net salary (after-tax) between 2300€ and 3000€, depending on experience

Application deadline:  Preferentially apply before November 15^th 2027 

Starting date: flexible (earliest possible is January 2027)

Duration: up to 2 years 

Scientific context

Rydberg atoms, i.e., atoms excited to high-principal-quantum-number levels, are particularly well suited to the quantum simulation of condensed matter systems [1]. They can be prepared from arrays of single atoms, laser-trapped in optical tweezers, and their strong dipole-dipole interactions (in the MHz range, even at a few microns) combined with their few-100µs lifetimes enable the observation of quantum phase transitions and of quenched dynamics. 

Circular Rydberg atoms, i.e., Rydberg atoms with maximal angular momentum, have a 100-times longer lifetime. Their use would bring quantum simulations to unprecedented regimes, with the simulation of long-term dynamics, such as thermalization. They would also enable the simulation of large interacting spins, beyond spin-1/2 physics. 
However, their use has been so far limited because they have no optical transitions, hindering their individual detection and manipulation in an array. We propose to remove this limitation with a hybrid platform, combining an array of laser-trapped circular atoms of Rubidium with an auxiliary array of Rb ancilla atoms transiently excited to a regular low-angular momentum Rydberg level. We trap the circular Rydberg atoms in optical bottle beams [2] and the ancilla atoms in Gaussian optical tweezers. They interact through a resonant dipole-dipole interaction.
Using a resonant dipole-dipole interaction between the ancilla and circular atoms, we have recently demonstrated the quantum non-demolition measurement of the circular Rydberg atoms with the ancillae, as well as a spatially-resolved manipulation of their quantum states [3]. In addition, we have measured the dipole-dipole interaction between two circular Rydberg atoms, and observed the coupling between spin and motional degrees of freedom [4]. This now opens the way to genuine quantum simulation with circular Rydberg atoms.

M2 Internship
Our experiments are performed in a UHV chamber operated at room-temperature. To fully benefit from the long lifetimes of the circular Rydberg atoms, we need to transfer the existing setup to a cryogenic environment. The intern will actively participate to the development of the cryogenic platform. In particular, he/she will participate to the adaptation of the optics, microwave and radiofrequency setups to the cryogenic platform. His or her work will, thus, constitute, a decisive contribution to our quantum simulations project.
 
PhD Thesis
The first months of the PhD work will be devoted to the operation of the new cryostat and its optimization. We will benefit from the enhanced lifetimes of the Rydberg levels to improve the performances of the existing room-temperature setup. During the PhD work, the proposed quantum simulator will then be operated. Quantum simulation of spin-1/2 Hamiltonians will first be performed, with the observation of long-term dynamics, out of the reach of existing simulators. We will then focus on the simulation of the interaction between spin 1s or larger. We will in particular use the ancillae to tune the interaction between the circular Rydberg atoms, using techniques developed in on-going collaborations with theoretical teams

References
[1] T. L. Nguyen et al., PRX 8, 011032 (2018)
[2] B. Ravon et al., PRL 131, 093401 (2023)       
[3] P. Méhaignerie et al., PRX Quantum 6, 010353 (2025)                               
[4] Y. Machu et al., arXiv :2509.24691 (2025)

Continuous-variable encoding of quantum information allows for the deterministic generation of entangled networks, where multimode capabilities provide scalability of Gaussian quantum resources. Tailored non-Gaussian quantum operations can be implemented via heralded non-linear optical processes or through collective interactions with atomic systems.
Positions are available on several of these research topics. The experimental work will be carried out in collaboration with the in-house theoretical team, as well as with French and international consortia, including both theoretical and experimental groups, and industrial partners in the quantum sector. These collaborations will support the expansion of your scientific network and the establishment of new international partnerships.
 
List of current funded projects related to the proposed work:
EIC Pathfinder VeriQub (European Innovation Council): https://veriqub.eu
EIC Pathfinder PANDA (European Innovation Council): https://www.pasqal.com/fr/project-panda/
National Acceleration Strategy PEPR OQULUS:
https://www.pepr-quantique.fr/en/projet/oqulus/
 
[1] V. Roman-Rodriguez, D. Fainsin, G.L. Zanin, N. Treps, E. Diamanti, V. Parigi Multimode squeezed state for reconfigurable quantum networks at telecommunication wavelengths Physical Review Research 6 (4), 043113 (2024)
[2] T. Kouadou, F. Sansavini, M. Ansquer, J. Henaff, N. Treps, V. Parigi  Spectrally shaped and pulse-by-pulse multiplexed multimode squeezed states of light APL Photonics  8  (2023)
[3] Y. Cai, J. Roslund, G. Ferrini, F. Arzani, X. Xu, C. Fabre, N. Treps Multimode entanglement in reconfigurable graph states using optical frequency combs  Nature communications 8 (1), 15645 (2017)
[4] YS Ra, A Dufour, M Walschaers, C Jacquard, T Michel, C Fabre, N Treps Non-Gaussian quantum states of a multimode light fieldNature Physics 16 (2), 144-147 (2020)
[5] U. Chabaud, G. Roeland, M. Walschaers, F. Grosshans, V. Parigi, D. Markham, N. Treps Certification of non-Gaussian states with operational measurements PRX Quantum 2 (2), 020333 (2021)
[6] M. Moreno-Cardoner, D. Goncalves and D. E. Chang Quantum Nonlinear Optics Based on Two-Dimensional Rydberg Atom Arrays Phys. Rev. Lett. 127, 263602 (2021)

 
 
Environment
 
As a whole, the group has a tradition of working together with a diverse range of people from varied backgrounds. This diversity often leads to fruitful scientific input from different points of view, and it allows the group to explore new avenues. Furthermore, the moderate size of our group gives PhD students and postdocs the opportunity to discuss with PIs on a daily basis. This gives rise to a dynamical atmosphere with a lot of space for discussion.
 
Laboratoire Kastler Brossel is one of the main worldwide leaders in the field of fundamental physics of quantum systems, covering a number of subjects spanning from fundamental tests of quantum theory to applications. It has an internationally recognized expertise throughout its 65 years history, including three Nobel Prize winners. The multimode quantum optics team explore both fundamental science and applications in CV quantum information. Based in Sorbonne Université in the Pierre and Marie Curie Campus in the center of Paris, it is part of the Quantum Information Center (QICS) in Sorbonne, hosting both quantum physicists and computer scientists. It is also part of the Paris Centre for Quantum Technologies (PCQT), a research, training and innovation network involving 22 research laboratories in Paris Region, 35 start-ups and larger industry groups. 
 
Practical information 
 
Candidates must hold an internationally recognized PhD in a field related to experimental quantum physics. A good background and past research track record in experimental optics, and quantum physics is required. 
Application procedure: Inquiries and applications (incl. a detailed CV and two names of potential referees) should be sent by email to Nicolas Treps (nicolas.treps@lkb.upmc.fr) and Valentina Parigi(valentina.parigi@lkb.upmc.fr). 
Salary: Monthly net salary (after-tax) between 2300€ and 3000€, depending on experience
Application deadline:  Preferentially apply before December 1^st 2025 
Starting date: flexible
Duration: up to 2 years (with possible renewal of 1 year)

The multimode quantum optics group pioneered the modal approach to quantum metrology [6] and obtained unparalleled sensitivity in source separation estimation [2] using a unique technology invented in the group [3]. The group has a strong experimental focus, but is also engaged in purely theoretical activities aiming at developing quantum optics in the continuous variable (CV) framework.
 
In a nutshell 

Why to apply:
 -you are passionate about fundamental quantum science and/or technological applications 
 -you will benefit from an international team of researchers who are experts in Continuous Variable quantum optics and metrology (theory and experiment) 
– you will be in a group at the core of many national and international research networks
 
Our specific view:
– Experiments and theory should go together to implement practical parameter estimation at the quantum limit
– Modal approach to quantum metrology allows both for quantum limited parameter estimation and to derive theoretical bounds closely related 

Environment
As a whole, the group has a tradition of working together with a diverse range of people from varied backgrounds. This diversity often leads to fruitful scientific input from different points of view, and it allows the group to explore new avenues. Furthermore, the moderate size of our group gives PhD students and postdocs the opportunity to discuss with PIs on a daily basis. This fosters a dynamic atmosphere with a lot of space for discussion.
 
Your work will be supported by the CEA-Audace project Lumiere, which aims to provide support and build collaborations. Notably, the project will enable you to gain access to groundbreaking multi-photon detectors with very high sensitivity and fast response time, promising significant advancements in metrology and tracking applications.
The project is inherently interdisciplinary, bridging quantum optics with fields such as signal processing, machine learning, and advanced instrumentation. It also benefits from strong collaborations with national and international teams, ensuring frequent exchanges of ideas and expertise. 
 
 
Practical information  
Candidates must hold an internationally recognized PhD in a field related to experimental quantum physics. A good background and past research track record in experimental optics, and quantum physics is required. 
Application procedure: Inquiries and applications should be sent by email to Nicolas Treps(nicolas.treps@lkb.upmc.fr). Applications should include a detailed CV and two names of potential referees
Salary: Monthly net salary (after-tax) between 2200€ and 2900€, depending on experience
Application deadline:  Preferentially apply before 16 May 2025
Starting date: flexible
Duration: 2 years (with possible renewal of 1 year)
 
References
 
[1] Tsang, M., Nair, R., & Lu, X. M. (2016). Quantum theory of superresolution for two incoherent optical point sources. Physical Review X, 6(3), 031033.
[2] Rouvière, C., Barral, D., Grateau, A., Karuseichyk, I., Sorelli, G., Walschaers, M., & Treps, N. (2024). Ultra-sensitive separation estimation of optical sources. Optica, 11(2), 166-170.
[3] G. Labroille, BertrDenolle, P. Jian, P. Genevaux, N. Treps, and J.-F. Morizur, Efficient and mode selective spatial mode multiplexer based on multi-plane light conversion, Opt Express 22, 15599 (2014).
[4] Gessner, M., Treps, N., & Fabre, C. (2023). Estimation of a parameter encoded in the modal structure of a light beam: a quantum theory. Optica, 10(8), 996-999.
[5] G. Sorelli, M. Gessner, M. Walschaers, and N. Treps, Optimal Observables and Estimators for Practical Superresolution Imaging, Phys. Rev. Lett. 127, 123604 (2021).
[6] C. Fabre and N. Treps, Modes and States in Quantum Optics, Rev. Mod. Phys. 92, 035005 (2020).