CMQED group

Bio-inspired superconducting sensors for sub-THz technologies

Alexis — 2026-01-13T10:29:53Z

The human cochlea is an extraordinarily sensitive sensor that decomposes sound into spatially separated frequencies across three decades with a dynamic range exceeding 120 dB. At its core, the human ear is based on a collection of sub-wavelength non-linear resonators. In the sub-THz frequency range, the losses become too high to realize efficient detectors, but superconductors exhibit minimal ohmic dissipation and a kinetic inductance that varies quadratically with current [1, 2]. For example, Fig. 1 a shows the dependance in temperature of a CPW resonator frequency due to this kinetic inductance.

The goal of this PhD project is to use this intrinsic nonlinearity to mimic the non-linear amplification occurring in the cochlea, providing tunability and enhanced spectral selectivity. At first, the candidate will characterize the response of a few subwavelength superconducting resonators at 100 GHz. The measurement of the self-Kerr non-linearity will give access to the quadratic dependance on the current I inside the superconducting resonator (see Fig. 1 b). This will give the opportunity to study non-reciprocal behavior in non-linear systems which may have crucial applications in the field of quantum technologies.

Figure 1 : a, NbN resonator measured bolow its critical temperature b, Nonreciprocal transmission within a planar superconducting metamaterial based on the nonlinear inductance of NbN. c, Rainbow trap geometry

The PhD student will then create a rainbow trap experiment in the sub-THz as depicted on Fig. 1 c. This structure introduces a radically new approach to sub-THz spectroscopy, inspired by the acoustic behavior of the human cochlea. Unlike conventional TDS systems relying on bulky optics and timegating, it aims for a single-shot, fully integrated superconducting on-chip spectrometer that directly maps frequencies in space. Its originality stems from three main elements : bio-inspired active graded designs, superconducting metamaterials with minimal dissipation and intrinsic kinetic-inductance nonlinearities, and operation in the underexplored sub-THz regime, promising for molecular and quantum sensing. By exploiting the synergy between biological efficiency with quantum-graded materials, we aim to establish a new class of compact, high-performance spectroscopic sensors.

[1] B. Ho Eom, P. K. Day, H. G. LeDuc, and J. Zmuidzinas, “A wideband, low-noise superconducting amplifier with high dynamic range,” Nature Physics, vol. 8, no. 8, pp. 623–627, 2012.
[2] J. Luomahaara, V. Vesterinen, L. Gr¨onberg, and J. Hassel, “Kinetic inductance magnetometer,” Nature Communications, vol. 5, no. 1, p. 4872, 2014.

Contact : alexis.jouan@espci.fr and fabrice.lemoult@espci.fr

Vortex-based Digital Superconducting Diode

2025-10-22T15:22:58Z

Last several decades the importance of creating effective superconducting electronics arises again for two main reasons. The first one is the energy consumption of modern computers based on semiconductors because of its large resistance, a dissipated heat in large-scale circuits becomes a significant problem.
This resistance also limits the possible operation speed. Superconductors have zero resistance and can work in THz range, that is two order of magnitude faster than current semiconductor processors. The second reason is the progress in quantum computing [1]. Qubits are very sensitive and can work only at very low temperature, and the question of having some preliminary operating circuit, that can de-couple
qubits from room-temperature computers, while making some classical computation efficiently, arises naturally.
Recent improvements in cryogenic and nanofabrication allow us not only show concepts of superconducting electronics but open a path to a real competition against semiconducting one. One of a basic element of any circuits is the diode — the element, that can provide signal flow only in one direction [2, 3]. It is used in rectifiers, AC–DC converters, and antennas for detecting electromagnetic signals. One of possible realizations of the superconducting diode is based on the operation of Abrikosov vortices — magnetic flux quantum [4]. The main advantages of this type is a relative fabrication simplicity. But there are still a lot of optimization in terms of design and operating protocols. Coupling vortices with external periodic drive can lead to effect of their synchronous motion [5].
Using that effect, it is possible to make a digital version of the superconducting diode : vortices move synchronously “1” or asynchronously “0”. In the Fig.1 these two states and transition from one to another are presented. Realization of this basic device with high-efficiency leads to more complex superconducting integrated circuits.

In this internship we propose to study properties of a vortex-based superconducting diode and optimizing its parameters for the best efficiency. It consists of three main parts : fabrication of different diode designs, transport measurements at cryogenic temperatures and numerical modelling based on time-dependent Ginzburg-Landau formalism for an optimal performance. The intern will acquire skills in nanofabrication, cryogenics, low-noise measurements and numerical modelling, as well as knowledge
in the field of superconductivity and condensed matter physics in general.

Prerequisite : A strong background in superconductivity and a taste for simulations and Python coding are recommended. If you are interested : please contact cheryl.feuilletpalma@espci.fr and sergei.kozlov@espci.fr.

Reference :
[1] F. Arute, K. Arya, R. Babbush, D. Bacon, J. C. Bardin, R. Barends, R. Biswas, S. Boixo, F. G. S. L. Brandao, D. A. Buell, B. Burkett, Y. Chen, Z. Chen,
B. Chiaro, R. Collins, W. Courtney, A. Dunsworth, E. Farhi, B. Foxen, A. Fowler, C. Gidney, M. Giustina, R. Graff, K. Guerin, S. Habegger, M. P.
Harrigan, M. J. Hartmann, A. Ho, M. Hoffmann, T. Huang, T. S. Humble, S. V. Isakov, E. Jeffrey, Z. Jiang, D. Kafri, K. Kechedzhi, J. Kelly, P. V.
Klimov, S. Knysh, A. Korotkov, F. Kostritsa, D. Landhuis, M. Lindmark, E. Lucero, D. Lyakh, S. Mandra, J. R. McClean, M. McEwen, A. Megrant, `
X. Mi, K. Michielsen, M. Mohseni, J. Mutus, O. Naaman, M. Neeley, C. Neill, M. Y. Niu, E. Ostby, A. Petukhov, J. C. Platt, C. Quintana, E. G. Rieffel,
P. Roushan, N. C. Rubin, D. Sank, K. J. Satzinger, V. Smelyanskiy, K. J. Sung, M. D. Trevithick, A. Vainsencher, B. Villalonga, T. White, Z. J. Yao,
P. Yeh, A. Zalcman, H. Neven, and J. M. Martinis, “Quantum supremacy using a programmable superconducting processor,” Nature, vol. 574,
pp. 505–510, Oct. 2019.
[2] P. J. W. Moll and V. B. Geshkenbein, “Evolution of superconducting diodes,” Nature Physics, vol. 19, pp. 1379–1380, Oct. 2023.
[3] M. Nadeem, M. S. Fuhrer, and X. Wang, “The superconducting diode effect,” Nature Reviews Physics, vol. 5, pp. 558–577, Sept. 2023.
[4] D. Margineda, A. Crippa, E. Strambini, Y. Fukaya, M. T. Mercaldo, M. Cuoco, and F. Giazotto, “Sign reversal diode effect in superconducting
Dayem nanobridges,” Communications Physics, vol. 6, p. 343, Nov. 2023.
[5] S. Kozlov, J. Lesueur, D. Roditchev, and C. Feuillet-Palma, “Dynamic metastable vortex states in interacting vortex lines,” Communications Physics,
vol. 7, pp. 1–8, June 2024.

Dynamical Coulomb blockade with NbN metamaterial resonators

2025-10-22T15:15:38Z

The ability to confine light at very small volumes is of paramount importance for enhancing light-matter interactions, both for devices and fundamental studies [1]. The Terahertz and sub-THz spectral domains are particularly prominent for building metallic resonators with ultra-sub-wavelength mode volumes. Indeed, the corresponding wavelengths are large (= 1mm – 100µm), one can leverage from nanofabrication techniques with nanometer resolution, and metals feature low losses, and even superconducting materials such as NbN are available [2]. The resonant architectures of choice are either double-metal cavities [3] or metamaterial resonators that can be engineered into 3D geometries [4], that are well mastered by our group.
In the present project, we will exploit such resonators made of NbN in order to realize and study an elementary system for both electronic transport and light-matter interaction : a semiconductor tunnel junction coupled with an ultra-subwavelength metamaterial resonator. This structure can operate in the Dynamical Coulomb Blockade, where the tunneling of electrons is coupled to the electromagnetic fluctuations of the resonator, providing thus a probe for the its quantum state. This concept was pioneered by M. Devoret, 2025 Nobel prize winner [5], and can even be used to study light-matter coupling systems in the extreme interaction regime known as Ultra-strong coupling [6].
Figure : Metamaterial resonators combined with semiconductor tunnel junctions realized in the CMQED team.
As an intern, the candidate will model, fabricate and characterize electromagnetic resonators in the 100 GHz range made from NbN layers. The internship will then be pursued as a PhD project funded by the ANR project HyQD100 where the resonators will be integrated with semiconductor tunnel junctions for the study of the regime of Dynamical Coulomb blockade, (Figure), for various applications both in the THz and sub-THz ranges. In particular, these junctions will be used for non-demolition quantum measurements of the 100 Qbits that will be produced in HyQD100. These studies open exciting possibilities for new types of devices which benefit from both concepts of semiconductor optoelectronics and superconducting quantum circuits.
The PhD candidate will receive a full training on nanofabrication techniques in the Paris Center cleanroom, and will acquire strong experience in the domains of quantum technologies and condensed matter physics, as well as advanced electromagnetism.

References :
[1] M. Fox, “Quantum Optics : An Introduction” (Oxford Master Series in Physics, 2006)
[2] H.T. Cheng et al., “Tuning the Resonance in High-Temperature Superconducting Terahertz Metamaterials”, Phys. Rev. Lett. 105, 247402 (2026)
[3] C. Feuillet-Palma et al., “Extremely sub-wavelength THz metal-dielectric wire microcavities”, Optics Express Vol. 20, Issue 27, pp. 29121-29130 (2012).
[4] M. Jeannin, et al. “Ultrastrong light–matter coupling in deeply subwavelength THz LC resonators “, ACS Photonics 6 (5), 1207-1215 (2019).
[5] M. H. Devoret, et al., “Effect of the electromagnetic environment on the Coulomb blockade in ultrasmall tunnel junctions”, Phys. Rev. Lett. 64, 1824 (1990).
[6] U. Iqbal, C. Mora, Y. Todorov,” Dynamical Coulomb blockade : An all-electrical probe of the ultrastrong light-matter coupling regime”, Physical Review Research 6 (3), 033097 (2024).

Quantum devices in the ultra-strong light-matter coupling regime

2025-10-22T15:07:48Z

The absorption and emission of light in an optoelectronic device are often considered as perturbative phenomena, which are treated in a single-particle picture. When the light-matter coupling energy, ћW_R, exceeds the dissipation rates of the system then the light-matter interaction is no longer a perturbative process, but instead energy is periodically exchanged with the microcavity at a frequency W_R, The system enters the strong coupling regime, where the cavity mode is split into two light-matter coupled (polariton) states separated by energy 2ћW_R. The last decade has seen the emergence of yet stronger interaction regime, where the coupling constant WR becomes comparable to the frequency of the matter excitation, W_m. This regime with W_R/W_m 1 is known as “ultra-strong” light-matter coupling and sets new frontiers for cavity quantum electrodynamics [1]. This regime can be realized with quantum heterostructures that interact with far infrared photons (TeraHertz, = 30µm-300µm and Mid-Infrared,lambda= 3µm-30µm domains) [2]. A very interesting topic is the possibility of observing the signatures of ultra-strong coupling in the electronic transport of devices such as infrared detectors [3] and tunnel junctions [4]. Such devices could enable the readout of the quantum properties of light in
the MIR and THz frequencies, thus opening a new field of application for quantum technologies.

As an intern, the candidate will characterize optoelectronic devices that have been already fabricated in clean room. She/he will thus acquire advanced training in infrared spectroscopy and electrical measurements of quantum devices, including in cryogenic conditions.

This activity will be followed by a PhD project, where the aim is to explore non-linear quantum devices operating in the ultra-strong light-matter coupling regime. We will study devices where semiconductor quantum wells are integrated into optical resonators featuring deep sub-wavelength electromagnetic confinement [5] (Figure). This activity will be guided by recent theoretical work from our group which studies non-linear optical conversion in the ultra-strong coupling regime [6]. For this project, the Ph.D. student will actively participate in the conception and fabrication of the nano-devices, starting from 3D numerical modeling, through clean-room processing and optical characterization of the structures. She/he will acquire not only strong scientific expertise in solid state devices and quantum optics, but also in nanofabrication techniques. The PhD project will be funded by an ANR project which is experimental collaboration with IEMN Lille.
References
[1] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B 72, 115303 (2005).
[2] Y. Todorov, et al., Phys. Rev. Lett. 105, 196402 (2010).
[3] F. Pisani et al., Nature Comm. 14, 3914 (2023).
[4] U. Iqbal, C. Mora, Y. Todorov, Phys. Rev. Research 6, 033097 (2024).
[5] M. Jeannin et al. ACS Photonics 6, (5) 1207-1215 (2019).
[6] T. Krieguer, Y. Todorov, Phys. Rev. B 111, 165304 (2025).

Building Integrated SNSPDs to Develop an hBN Quantum Photonic Platform Including Single-Photon Emitters.

2025-10-22T14:07:37Z

The rapid progress of quantum technologies has revealed two key challenges. First, we need to precisely control quantum systems without disturbing their sensitive states — meaning keeping their quantum coherence.
Second, we must find efficient ways to convert quantum information into classical signals, so that the results of quantum operations can be measured and used at large scale. In quantum photonics, information is carried by individual photons. This approach depends on two main building blocks : single-photon sources, which generate one photon at a time, and single-photon detectors, which can detect them individually. These components are essential for many areas of quantum information science, such as quantum computing and quantum simulation.
So far, the most advanced systems use either external single-photon emitters that are distributed across several channels (demultiplexed), or on-chip processes that create photons in a probabilistic way, along with external detectors. However, these methods are still limited in efficiency, which makes it very hard to scale up to experiments involving many photons.
This technological bottleneck mainly comes from the fact that we still lack reliable fabrication methods to integrate high-performance photon sources and detectors directly onto photonic chips.

This proposal focuses on the experimental realization of integrated superconducting nanowire single-photon detectors [1, 2, 3, 4](SNSPDs) in hexagonal boron nitride (hBN) photonic circuits. The project aims to achieve a fully integrated quantum photonic platform where single photons are generated, routed, and detected on the same chip with high efficiency.

The intern will develop NbN-based superconducting nanowire single-photon detectors (SNSPDs) integrated into hBN waveguides. This internship is expected to lead to a PhD project funded by the ANR BONI&CLIDE in collaboration with GEMac group and LPENS [5, 6, 7, 8].

Through three main research axes during this PhD project such as fabrication and optimization of SNSPDs, experimental characterization of detector performance, and integration into quantum photonic demonstrators, we will develop an hBN-based platform for on-chip quantum experiments.
The PhD candidate will receive a full training on nanofabrication techniques in the Paris Center cleanroom, and will acquire strong experience in the domains of quantum technologies and condensed matter physics, superconductivity as well as advanced electromagnetism.

Prerequisite : A strong background in quantum physics and/or solid state physics. A taste for nanofabrication and transport measurements under cryogenic environnement.
Contact cheryl.feuilletpalma@espci.fr and sergei.kozlov@espci.fr.

References :
[1] Iman Esmaeil Zadeh et al. Superconducting nanowire single-photon detectors : A perspective on evolution, state-of-the-art, future developments,
and applications. Applied Physics Letters, 118:190502, 05 2021.
[2] Cheryl Feuillet-Palma. Transport et interaction mati`ere–rayonnement
dans des mat´eriaux corr´el´es. Comptes Rendus. Physique, 26:129–180,
2025.
[3] Paul Amari et al. Scalable Nanofabrication of High-Quality YBCO
Nanowires for Single-Photon Detectors. Physical Review Applied,
20(4):044025, October 2023.
[4] Sergei Kozlov et al. Dynamic metastable vortex states in interacting vortex lines. Communications Physics, 7(1):1–8, June 2024.
[5] Clarisse Fournieret al. Position-controlled spes with reproducible wavelength in hbn. Nature Communications, 12(1):3779, 2021. [Open Access].
[6] Domitille G´erard et al. Quantum efficiency and vertical position of
quantum emitters in hbn determined by purcell effect in hybrid metaldielectric planar photonic structures. ACS Photonics, 11:5188, 2024.
[Open Access].
[7] Clarisse Fournier et al. Investigating the fast spectral diffusion of a quantum emitter in hbn using resonant excitation and photon correlations.
Physical Review B, 107:195304, 2023. [Open Access].
[8] Domitille G´erard et al. Crossover from inhomogeneous to homogeneous
response of a resonantly driven hbn quantum emitter. Physical Review B,
111:085304, 2025.

Hybridization of Terahertz Phonons and Magnons in Disparate and Spatially-Separated Material Specimens

2025-10-22T08:35:31Z

The interaction between condensed matter excitations and electromagnetic cavity fields serves as a rich playground for fundamental research and lies at the core of photonic and quantum technologies. Herein, the intriguing concept of composite states formed by distinct quasiparticles strongly coupled to the same optical cavity modes is experimentally and theoretically demonstrated. Specifically, magnons excited in a slab of an antiferromagnetic crystal and phonons excited in a distinct specimen of an insulating material are explored. The crystal slabs form an optical cavity with Fabry–Pérot oscillations in the terahertz range. Hybridized phonon–magnon polariton modes and their tunability by adjusting the distance between the slabs, showing that hybridization persists even at separations of up to several millimeters is demonstrated. The experimental results are interpreted using both classical and quantum electrodynamical models. The quantum description allows us to quantify the degree of hybridization linked to a topological behavior of the electric field phasor, in agreement with the classical electrodynamics expectations. The presented results are obtained at room temperature and cavities of millimeter size, paving the way for the engineering of realistic, frequency-tunable THz devices through the hybridization of electric (phononics) and magnetic (spintronics) elementary excitations of matter.

Patch antenna enhanced charge-sensitive infrared phototransistors

2025-10-20T18:31:19Z

Charge-sensitive infrared phototransistors (CSIP) constitute an outstanding technology for mid-infrared detection with sensitivities demonstrated at the single photon level. Here, we report on the performances of CSIP detectors operating at a wavelength λ = 8.9 μm that are integrated into double-metal patch antenna resonators. In order to build such devices, we have developed a fabrication protocol that allows accommodating the phototransistor architecture with the double-metal geometry providing very strong electromagnetic field confinement. We observe minimal photon fluxes in the order of 7000 photons/s.μm^2 that are 10^3 smaller with respect to previous realizations of devices with similar absorbing regions in a mesa geometry. This work opens additional perspectives for building ultra-small area devices, as required for single photon counters, while keeping high quantum absorption efficiencies.

3D Meta-Atoms for High Confinement of Mid-IR Radiation

2025-10-20T18:27:18Z

This is our work on 3D meta-atoms for Mid-Infrared waves !

The ability to confine photons into structures with highly sub-wavelength volumes is extremely interesting for many applications such as sensing, nonlinear optics, and strong light-matter interactions. However, their realization is increasingly difficult as the wavelength becomes shorter, due to fabrication challenges and increased metal losses. In this work, the first experimental characterization of 3D circuit-like resonators operating in the mid-infrared is presented. Through a combination of simulations, reflectivity measurements, and scanning near-field optical microscopy, an analytical model capable of predicting the electromagnetic response of these structures based on their geometrical parameters is developed. The studied design offers a high degree of flexibility, enabling precise control over the resonant frequency of the various modes supported by the resonator, as well as independent control over radiative and non-radiative losses. Combined with the extreme field confinement demonstrated, these meta-atoms are highly promising for applications
in detectors, emitters, nonlinear processes, and strong light-matter coupling.

Image produced by Francesco Pisani @ 2024

Quantum theory for nonlinear optical effects in the ultrastrong light-matter coupling regime

2025-10-20T18:05:04Z

A theoretical work where we describe non-linear effects that occur in microcavity-coupled quantum well systems that operate in the ultra-strong light-matter coupling regime.

We present a microscopic quantum theory for the nonlinear optical phenomena in semiconductor quantum well heterostructures operating in the regime of ultrastrong light-matter coupling regime. This work extends the Power-Zienau-Wooley (PZW) formulation of quantum electrodynamics to account for nonlinear interactions based on a fully fermionic approach, without resorting to any bosonization approximation. It provides a unified description of the microcavity and the local field enhancement effects on the nonlinear optical response, thus encompassing the phenomena known as epsilon near zero (ENZ) effect. In particular, our theory describes the impact of the light-matter coupled states on the high-frequency generation process, relevant for recent experimental investigations with polaritonic metasurfaces. We unveil the limitations of traditional single-particle approaches and propose novel design principles to optimize nonlinear conversion efficiencies in dense, microcavity-coupled electronic systems. The theoretical framework developed here provides an efficient tool for the development of advanced quantum optical applications in the midinfrared and terahertz spectral domains. Furthermore, it establishes a foundation for exploring the quantum properties of the ultrastrong light-matter regime through frequency-converted polariton states.

Matéo Vernet

2025-10-20T07:28:49Z

Granular superconductors like Nb and NbN and how they can be used to create tunable quantum circuits.

Matéo Vernet has an engineering degree from ESPCI Paris and a master's degree in Physics of Complex Systems from Université Paris Cité. His research focuses on granular superconductors, such as niobium (Nb) and niobium nitride (NbN) thin films. Combining ion irradiation and electrostatic gating, he studies how geometrical disorder, magnetic impurities, and electrostatic fields change the micro- and macroscopic quantum behavior. In addition, his research encompasses a theoretical approach based on the localization landscape, which will offer novel insights into the spatial inhomogeneity and emergence of superconductivity in disordered and structured systems.