MULTIPHOTONICS (2025)

Multiphotonics (2025)

The second MULTIPHOTONICS meeting will take place in Madrid, on 8–9 October (2025) with the support of ICMM—CSIC and IFF—CSIC. It follows the very successful MULTIPHOTONICS (2024) first edition. The workshop will likewise discuss the physics of multiphoton correlations.

If you liked Munich, you'll love Madrid!

Topics

Multiphoton generation: Single and $N$-photon emission.
Quantum light generation with properties such as entanglement or squeezing.
Frequency filtering, statistics, coherence and correlation measurements.
Quantum optics, cavity-QED, light-matter interaction and nanophotonics.

Venue

ICMM-CSIC on the Cantoblanco Campus:

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Hotel accommodation

External attendants will be provided with a two-nights (Tue 7 & Wed 9) hotel room with breakfast at the VP Jardín De Tres Cantos in Tres Cantos. This is a quiet, modern urban-planning city at the north of Madrid, well connected to the site of the meeting and to the Spanish capital itself.

Organizers

The event is supported by a joint ICMM‒IFF effort:

• Fabrice Laussy from ICMM‒CSIC.
• Carlos Sánchez Muñoz from IFF‒CSIC.

Contact the organizers.

Participants

1. Serge Reynaud
   • Laboratoire Kastler Brossel, Paris
   • Abstract
2. Sven Höfling
   • University of Würzburg, Würzburg
   • Abstract
3. Jesper Mørk
   • Technical University of Denmark, Copenhagen
   • Abstract
4. Zhiliang Yuan
   • Academy of Quantum Information Sciences, Beijing
   • Abstract
5. Kai Müller
   • Technische Universität München, München
   • Abstract
6. Tim Thomay
   • University at Buffalo, New York
   • Abstract
7. Ahsan Nazir
   • University of Manchester, Manchester
   • Abstract
8. Jake Iles-Smith
   • University of Sheffield, Sheffield
   • Abstract
9. Philipp Schneeweiß
   • Humboldt-Universität zu Berlin, Berlin
   • Abstract
10. Vincenzo D'Ambrosio
    • Università di Napoli Federico II, Napoli
    • Abstract
11. Владислав Шишков (Vladislav Shishkov)
    • Aalto University, Espoo
    • Abstract
12. Juan Camilo López Carreño
    • University of Warsaw, Warsaw
    • Abstract
13. 王雅君 (Yajun Wang)
    • Shanxi University, Taiyuan
    • Abstract
14. Alexandros Spiliotis
    • IESL-FORTH, Heraklion
    • Abstract
15. Natalia Armaou
    • Westlake University, Hangzhou
    • Abstract
16. Moritz Meinecke
    • University of Würzburg, Würzburg
    • Abstract
17. Carlos Antón Solanas
    • Universidad Autónoma de Madrid
    • Abstract
18. Johannes Feist
    • Universidad Autónoma de Madrid
    • Abstract
19. Antonio Isaac Fernandez Dominguez
    • Universidad Autónoma de Madrid
    • Abstract
20. Alejandro González Tudela
    • IFF-CSIC, Madrid
    • Abstract
21. Eduardo Zubizarreta Casalengua
    • Technische Universität München, München
    • Abstract
22. Sang Kyu Kim
    • Technische Universität München, München
    • Abstract
23. Joaquin Guimbao Gaspar
    • ICMM-CSIC, Madrid
    • Abstract
24. Jacob Ngaha
    • ICMM-CSIC, Madrid
    • Abstract
25. Lukas Hanschke
    • Technische Universität München, München
    • Abstract
26. Elena del Valle
    • Universidad Autónoma de Madrid
    • Abstract
27. Carlos Sánchez
    • IFF-CSIC, Madrid
    • Abstract
28. Fabrice Laussy
    • ICMM-CSIC, Madrid
    • Abstract

Tentative Program

Tuesday 7 October

Participants arrive.

Wednesday 8 October (Day 1)

Morning Session

Opening (by Fabrice Laussy)

8:50-9:00

Serge Reynaud

Quantum jumps in single-atom resonance fluorescence — 9:00-9:30

Quantum jumps have been part of the theoretical understanding of interaction processes between atoms and photons since early quantum theory. They play a key role in the description of emission of photons by a single atom excited by a laser tuned near resonance. I will discuss quantum jumps in this resonance fluorescence process for a single two-level atom (with broadband detection).

Kai Müller

Unlocking multiphoton emission from a single-photon source through mean-field engineering — 9:30-10:00

Multiphotons are generally regarded as accidental in the context of single photon sources. However, multiphoton emission can turn out to be even more fundamental and interesting than the single-photon emission, since in a coherently driven system, the multiphoton suppression arises from quantum interferences between virtual multiphoton fluctuations and the mean field in a Poisson superposition of all number states. Here, we demonstrate how one can control the multiphoton dynamics of a two-level system by disrupting these quantum interferences through a precise and independent homodyne control of the mean field. We show that, counterintuitively, quantum fluctuations always play a major qualitative role, even and in fact especially, when their quantitative contribution is vanishing as compared to that of the mean field.

Joaquin Guimbao Gaspar

10:00-10:30

Juan Camilo López Carreño

10:30-11:00

Coffee break

Post-coffee

Jesper Mørk

Quantum noise and squeezing in nanolasers. — 11:30-12:00

We present a recently developed method for simulating quantum noise in nanolasers. Based on a simple stochastic interpretation of rate equations, the approach accurately reproduces quantum master equation results for few-emitter lasers and aligns with Langevin equations in macroscopic regimes. Notably, it bridges the intermediate mesoscopic regime previously inaccessible to existing models. We apply this method to analyze amplitude squeezing in nanolasers using novel cavities with extreme light confinement, which strongly enhance light-matter interaction.

Elena del Valle

12:00-12:30

Ahsan Nazir

A Markovian approach to $N$-photon correlations beyond the quantum regression theorem — 12:30-13:00

We introduce a Markovian framework for computing $N$-photon frequency-resolved correlation functions of quantum emitters in vibrational environments, overcoming the limitations of the quantum regression theorem (QRT). Applying our approach to a driven semiconductor quantum dot renders the investigation of the phonon impact on dot fluorescence tractable beyond the single-photon spectrum. Our method accurately captures the emergence of the phonon sideband—missed by conventional QRT treatments—and reveals rich phonon-induced structure in the filtered two-photon spectrum. Surprisingly, we find that photons emitted via the phonon sideband inherit the second-order coherence properties of the Mollow triplet.

Владислав Шишков

Spectral theory and statistical properties of integrated single-photon sources. — 13:00-13:30

Single-photon light sources are microscopic objects with sizes much smaller than the wavelength of the light they emit. Due to their small sizes, such emitters have a wide emission pattern, requiring special approaches to integrate them into photonic circuits. One of the most common approaches for a single-photon source integration into a photonic integrated circuit is coupling via a resonator in the large Purcell factor regime. This integration method, however, leads to a change in the statistical properties of the resulting light. Indeed, a resonator acts as a spectral filter that distorts the spectrum of the light and, thus, changes its statistical properties. We analyse the applicability of the spectral filtering theory for determining the statistical properties of light emitted by a single-photon source coupled to photonic integrated circuits via resonators in the large Purcell factor regime.

Lunch at El Goloso — 13:30-15:00

Group photo

Afternoon Session

Philipp Schneeweiß

Tailoring photon statistics with an atom-based two-photon interferometer — 15:00-15:30

Based on Ref. .

Johannes Feist

15:30-16:00

Lukas Hanschke

16:00-16:30

Jacob Ngaha

16:30-17:00

Coffee break

Evening Session

Jake Iles-Smith

Beyond the Quantum Regression Theorem: Tensor Network Methods for Solid-State Quantum Optics — 17:30-18:00

Correlation functions are a fundamental theoretical tool in quantum optics; they are used to calculate emission spectra, photon coherence properties, and to assess how well a quantum emitter will operate within a quantum technology. However, calculating these quantities is not always straightforward. While the quantum regression theorem can describe the emission properties of isolated atomic systems, it fails in solid-state environments, where strong coupling to the surrounding lattice leaves crucial features of the optical response unaccounted for.

In this talk, I will introduce a new family of methods for computing multi-time correlation functions of quantum emitters beyond atomic systems. By combining the process tensor formalism with tensor networks, these techniques provide a numerically exact framework for extracting the emission characteristics of solid-state emitters. I will illustrate their use in quantifying how electron–phonon interactions shape optical coherence, considering both localized and bulk phonon modes. Finally, I will show how these tools can be applied to place rigorous bounds on the indistinguishability and efficiency of quantum emitters in low-dimensional materials, including quantum dots in transition metal dichalcogenides (e.g. WSe₂) and atomic defects in hexagonal boron nitride.

Antonio Isaac Fernandez Dominguez

Preparation of quantum emitter states with nanophotonics tools — 18:00-18:30

In this talk, I will discuss different nanophotonics-based strategies to generate tailored states of quantum emitters. First, I will explore the creation of highly entangled multiqubit states by inverse engineering of their photonic environment. Next, I will address the preparation of topologically protected excitonic states in emitter chains that emulate a Su–Schrieffer–Heeger Hamiltonian (to appear in ACS Photonics). Finally, I will briefly examine the spatial dependence of photon correlations in pairs of quantum emitters placed near elementary nanophotonic structures, highlighting the conditions under which their suppression becomes independent of the internal state of the emitters themselve.

Yajun Wang (王雅君)

Squeezed light and lasing — 18:30-19:00

We focused on the study of technical and quantum noises of a laser source in interferometry. Recently, we have solved the key scientific and technical problems of laser noise characterization and stabilization beyond the quantum noise limit, explored the physical root of noise sources, and developed the key technologies of laser technical and quantum noises stabilization. Especially, we have experimentally stimulated a squeezed lasing in the reservoir-engineered optical parametric oscillator (OPO). It successfully circumvents the decoherence in the system, and eliminates the undesired noise of spontaneous photon emission in the OPO. As a result, the amplified parametric process simultaneously reserves the coherence and quantum properties in the OPO, and yields a -6.1 dB squeezed laser in optical domain with a 15 kHz linewidth and 2.6 mW optical power.

Physical mechanism of the parametric coupling with vacuum or squeezed vacuum reservoir. (a) Parametric down-conversion (PDC) in an above threshold OPO. (b) Mixture of two coherent states with coherence preservation and quantum characteristic deterioration. (c) PDC in a subthreshold OPO. (d) Squeezed vacuum state with quantum characteristic preservation and coherence deterioration. (e) PDC in a subthreshold OPO with squeezing vacuum reservoir. (f) Squeezed lasing with coherence and quantum characteristic preservation.

Carlos Sánchez

Quantum metrology through spectral measurements in quantum optics — 19:00-19:30

A central challenge in quantum metrology from open quantum optical systems is to identify measurement strategies that optimally extract information encoded in the complex quantum state of emitted radiation. Different measurement strategies effectively access distinct temporal modes of the emitted field, and the resulting choice of mode can strongly impact the information available for parameter estimation. While a ubiquitous approach in quantum optics is to select frequency modes through spectral filtering, the metrological potential of this technique has not yet been systematically quantified. We develop a theoretical framework to assess this potential by modeling spectral detection as a cascaded quantum system, allowing us to reconstruct the full density matrix of frequency-filtered photonic modes and to compute their associated Fisher information. We show how this approach allows us to identify optimal filtering strategies for metrology, in terms of frequency selection, detector linewidth, and metrological gain accessible through multiphoton frequency-resolved correlations and mean-field engineering.

20:30 Dinner

Thursday 9 October (Day 2)

Morning Session

Sven Höfling

9:00-9:30

Moritz Meinecke 

9:30-9:50

Tim Thomay

Higher-order Fock states for sensing applications. — 9:50-10:20

See Ref. .

Eduardo Zubizarreta Casalengua

10:20-10:50

Coffee break

Post-Coffee Session

Zhiliang Yuan

From Two-Photon Nowhere to a Two-Photon Emitter — 11:15-11:45

In a coherently driven two-level system, simultaneous two-photon emission is forbidden, yet strong two-photon correlations can still arise via quantum interference, as described by the pure- state model of resonance fluorescence and experimentally confirmed. However, the achievable brightness in this regime is fundamentally limited. Motivated by this constraint, we demonstrate a natural two-photon emitter using a semiconductor quantum dot with degenerate biexciton–exciton transitions embedded in a Purcell-enhanced micropillar cavity. By exploiting dark-state biexciton loading and leveraging single-photon stimulated emission, we achieve a record two-photon fraction of 98.3%, marking a significant advance toward on-demand, solid- state multiphoton quantum light sources.

Carlos Antón Solanas

Superposition and entanglement with vacuum-one-photon states — 11:45-12:15

A two-level system (such as a charged semiconductor quantum dot), excited under a pulsed resonant drive of area $\theta$, emits photon-number superposition states composed by vacuum and a single-photon in the form $\cos(\theta/2)\ket{0}+\sin(\theta/2)\ket{1}$. Additionally, a sequential two-pulse excitation can generate photon-number encoded time-entangled states of the form ${1\over\sqrt2}(\ket{0_e0_l}+\ket{1_e1_l})$ or ${1\over\sqrt2}(\ket{1_e0_l}+\ket{0_e1_l})$, where the subindex e and l refer to early and late time-bins, respectively. The expansion of this sequential excitation to a three-level system (such as one polarisation cascade of the biexciton-exciton decay) generates entanglement in time and energy, with prospects to allow secure quantum communication protocols.

In this talk, I will revisit these works and will discuss ongoing experiments on ⑴ the photon-number emission from the Ramsey sequence and its connection to energy exchanges between a two-level system and the light field, and ⑵ the constrained metrology performance of photon-number encoded Fibonacci states and alternatives to reach Heisenberg scaling in metrology protocols.

Sang Kyu Kim  

12:15-12:35

Alejandro González Tudela

Controlling propagating photons with chiral, multi-mode waveguide QED — 12:35-13:05

Controlling propagating quantum states of light is essential for the development of photonic quantum technologies. In this talk, I will first review how qualitatively new photon-mediated interactions emerge when emitters or resonators couple to chiral, multi-mode waveguides. I will then show how these interactions enable the implementation of controlled photon-photon phase gates using only two-level systems. In both cases, I will discuss potential implementations in the microwave using circuit QED setups.

Lunch

Afternoon Session

Vincenzo D'Ambrosio

Tailoring spatial correlations with structured light — 14:00-14:30

Alexandros Spiliotis

14:30-15:00

Natalia Armaou 

Spatial correlations of opposite OAM states of light — 15:00-15:20

Fabrice Laussy

Liquid time and time liquids — 15:20-15:50

The basic quantum-optical emitter—the two-level system—is already much more complicated than one could reasonably expect, and to this day, its thorough characterization remains to be completed. Here, I will jump to the case of the $N$-level system, and survey the amazing phenomenology that immediately shouts out from this simplest extension of the brick of quantum optics. A first surprise is that the $N$-level system, not the two-level one, is the most suitable to implement perfect single-photon sources. Furthemore, a good single-photon source acquires features that differ considerably from those usually wanted for that purpose. For instance, instead of merely suppressing two-photon coincidences at $\tau=0$, a good single-photon emitter is one that develops long-time oscillations as a result of self-organizing its photon streams to all orders in photon counting, differing from the basic case in a way similar to how a liquid differs from a gas. This calls for revisiting our understanding of single-photon sources, and raise fascinating questions on how they relate, in time, to exotic phase of matters. I will also describe how such a picture extends into multiphotonics, and how one could observe such effects experimentally.

Closing (by Carlos Sánchez)

Goodbye coffee & Merienda

Participants depart

Friday 10 October (After)

Prof. S. Reynaud will also give an INC (Instituto Nicolás Cabrera) Colloquium, «Vacuum fluctuations and Casimir forces», at 12:00 in the seminar room of Module 4, Faculty of Sciences (01.04.SS.500). All details can be found in this page.

Q&A(bstracts)

At this occasion, we shall try to revive an old format of archiving Scientific debates: instead of publishing proceedings, we will publish the abstract and the (edited) Questions & Answers sessions, which contains information nowhere else to be found.

One picture is worth a thousand words

For the Multiphotonics (2024), each participant contributed a formula, meaningful and/or inspiring for them, characteristic of their contribution to the field or merely illustrating their talk. Interestingly, there was no degeneracy: Rempe provided the Jaynes-Cummings Hamiltonian, someone went for the mere harmonic oscillator (but wasn't Dirac saying it was enough to understand this?), I (Fabrice) offered the dissipative Jaynes-Cumming ladder formula, which I still haven't found in any publication earlier to mine, Eduardo provided the two-photon spectrum of resonance fluorescence, ${(\omega_1+\omega_1^2+\omega_2+\omega_2^2)^2\over(1+\omega_1)^2(1+\omega_2)^2(\omega_1+\omega_2)^2}$ which produces the logo of the meeting (the blue circle in the numerator, the reddish glowing triangle in the denominator). A pretty formula indeed.

For this edition, we'd like to try the same thing but with a figure instead of a formula. This could be a graph, a density plot, the sketch of a concept (artistic or scientific), a diagram, the setup of an experiment, etc., with the same intent of providing a picture—call that a vision if you like—to illustrate the participants' understanding of the topic, ideally with a connection to their talk, even if a remote one. Out of this medley of visual cues to what light-matter interactions is about, we will build the logo of the 2025 meeting.