# Publications by Frank Schlawin

## Mott polaritons in cavity-coupled quantum materials

New Journal of Physics IOP Publishing **21** (2019) 073066

We show that strong electron-electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with USD g_{\text{eff}}\propto\sqrt{2L} USD, where USD L USD is the number of electronic sites, and the maximal achievable value for USD g_{\text{eff}} USD is determined by the volume of the unit cell of the crystal. We find that USD g_{\text{eff}} USD for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

## Cavity-Mediated Unconventional Pairing in Ultracold Fermionic Atoms.

Physical review letters **123** (2019) 133601-

We investigate long-range pairing interactions between ultracold fermionic atoms confined in an optical lattice which are mediated by the coupling to a cavity. In the absence of other perturbations, we find three degenerate pairing symmetries for a two-dimensional square lattice. By tuning a weak local atomic interaction via a Feshbach resonance or by tuning a weak magnetic field, the superfluid system can be driven from a topologically trivial s wave to topologically ordered, chiral superfluids containing Majorana edge states. Our work points out a novel path towards the creation of exotic superfluid states by exploiting the competition between long-range and short-range interactions.

## Cavity-Mediated Electron-Photon Superconductivity.

Physical review letters **122** (2019) 133602-

We investigate electron paring in a two-dimensional electron system mediated by vacuum fluctuations inside a nanoplasmonic terahertz cavity. We show that the structured cavity vacuum can induce long-range attractive interactions between current fluctuations which lead to pairing in generic materials with critical temperatures in the low-kelvin regime for realistic parameters. The induced state is a pair-density wave superconductor which can show a transition from a fully gapped to a partially gapped phase-akin to the pseudogap phase in high-T_{c} superconductors. Our findings provide a promising tool for engineering intrinsic electron interactions in two-dimensional materials.

## Optical control of the current-voltage relation in stacked superconductors

PHYSICAL REVIEW B **100** (2019) ARTN 134510

## Manipulating quantum materials with quantum light (vol 99, 085116, 2019)

PHYSICAL REVIEW B **99** (2019) ARTN 099907

## Manipulating quantum materials with quantum light

Physical Review B American Physical Society **99** (2019) 085116-

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.

## Coherence turned on by incoherent light

NEW JOURNAL OF PHYSICS **20** (2018) ARTN 113040

## Entangled Two-Photon Absorption Spectroscopy.

Accounts of chemical research **51** (2018) 2207-2214

The application of quantum states of light such as entangled photons, for example, created by parametric down conversion, has experienced tremendous progress in the almost 40 years since their first experimental realization. Initially, they were employed in the investigation of the foundations of quantum physics, such as the violation of Bell's inequalities and studies of quantum entanglement. They later emerged as basic platforms for quantum communication protocols and, in the recent experiments on single-photon interactions, in photonic quantum computation. These applications aim at the controlled manipulation of the photonic degrees of freedom, and therefore rely on simple models of matter, where the analysis is simpler. Furthermore, quantum imaging with entangled light can achieve enhanced resolution, and quantum metrology can overcome the shot noise limit for classical light. This Account focuses on an entirely different emerging class of applications using quantum light as a powerful spectroscopic tool to reveal novel information about complex molecules. These applications utilize two appealing properties of quantum light: its distinct intensity fluctuations and its nonclassical bandwidth properties. These give rise to new and surprising behavior of nonlinear optical signals. Nonclassical intensity fluctuations can enhance nonlinear optical signals relative to linear absorption. For instance, the two-photon absorption of entangled photon pairs scales linearly (rather than quadratically) in the photon flux, just like a single photon absorption. This enables nonlinear quantum spectroscopy of photosensitive, for example, biological, samples at low light intensities. We will discuss how the two-photon absorption cross section becomes a function of the photonic quantum state, which can be manipulated by properties of the entangled photon pairs. In addition, the quantum correlations in entangled photon states further influence the nonlinear signals in a variety of ways. Apart from affecting the signal's scaling with intensity, they also constitute an entirely new approach to shaping and controlling excitation pathways in molecular aggregates in a way that cannot be achieved with shaped classical pulses. This is because between the two absorption events in entangled two-photon absorption, the light and material system are entangled. Classical constraints for the simultaneous time and frequency resolution can thus be circumvented, since the two are not Fourier conjugates. Here we review the simplest manifestation of quantum light spectroscopy, two-photon absorption spectroscopy with entangled photons. This will allow us to discuss exemplarily the impact of quantum properties of light on a nonlinear optical signal and explore the opportunities for future applications.

## Nonlinear optical molecular spectroscopy with quantum light in microcavities

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY **256** (2018)

## Entangled photon spectroscopy

Journal of Physics B: Atomic, Molecular and Optical Physics IOP Publishing **50** (2017) 203001

This tutorial outlines the theory of nonlinear spectroscopy with quantum light, and in particular with entangled photons. To this end, we briefly review molecular quantum electrodynamics, and discuss the approximations involved. Then we outline the perturbation theory underlying nonlinear spectroscopy. In contrast to the conventional semiclassical theory, our derivation starts from Glauber's photon counting formalism, and naturally includes the semiclassical theory as a special case. Finally, we review previous work, which we sort into work depending on the unusual features of quantum noise, and work relying upon quantum correlations in entangled photons. This work naturally draws from both quantum optics and chemical physics. Even though it is impossible to provide a comprehensive overview of both fields in one tutorial, this text aims to be self-contained. We refer to specialised reviews, where we cannot provide details. We do not attempt to provide an exhaustive review of all the literature, but rather focus on specific examples intended to elucidate the underlying physics, and merely cite the remaining publications.

## Theory of coherent control with quantum light

New Journal of Physics IOP Publishing **19** (2017) 1-11

We develop a coherent control theory for multimode quantum light. It allows us to examine a fundamental problem in quantum optics: What is the optimal pulse form to drive a two-photon-transition? In formulating the question as a coherent control problem, we show that - and quantify how much - the strong frequency quantum correlations of entangled photons enhance the transition compared to shaped classical pulses. In ensembles of collectively driven two-level systems, such enhancement requires non-vanishing interactions.

## Terahertz field control of interlayer transport modes in cuprate superconductors

PHYSICAL REVIEW B **96** (2017) ARTN 064526

## Nonlinear optical signals and spectroscopy with quantum light

Reviews of Modern Physics American Physical Society (2016)

Conventional nonlinear spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies or time delays. Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and through the variation of photon statistics by coupling to matter. We present an intuitive diagrammatic approach for calculating ultrafast spectroscopy signals induced by quantum light, focusing on applications involving entangled photons with nonclassical bandwidth properties - known as “time-energy entanglement”. Nonlinear optical signals induced by quantized light fields are expressed using time ordered multipoint correlation functions of superoperators in the joint field plus matter phase space. These are distinct from Glauber’s photon counting formalism which use normally ordered products of ordinary operators in the field space. One notable advantage for spectroscopy applications is that entangled photon pairs are not subjected to the classical Fourier limitations on the joint temporal and spectral resolution. After a brief survey of properties of entangled photon pairs relevant to their spectroscopic applications, different optical signals, and photon counting setups are discussed and illustrated for simple multi-level model systems.

## Pump-probe spectroscopy using quantum light with two-photon coincidence detection

PHYSICAL REVIEW A **93** (2016) ARTN 023807

## Quantum Transport on Disordered and Noisy Networks: An Interplay of Structural Complexity and Uncertainty

ANNUAL REVIEW OF CONDENSED MATTER PHYSICS, VOL 7 **7** (2016) 223-248

## A Non time Ordered Pulse Scanning Protocol for Multidimensional Spectroscopy with Entangled Light

ULTRAFAST PHENOMENA XIX **162** (2015) 436-439

## Nonlinear spectroscopy of trapped ions (vol 90, 023603, 2014)

PHYSICAL REVIEW A **92** (2015) ARTN 039903

## Nonlinear spectroscopy of controllable many-body quantum systems (vol 16, 092001, 2014)

NEW JOURNAL OF PHYSICS **17** (2015) ARTN 019601

## Probing polariton dynamics in trapped ions with phase-coherent two-dimensional spectroscopy.

The Journal of chemical physics **142** (2015) 212439-

We devise a phase-coherent three-pulse protocol to probe the polariton dynamics in a trapped-ion quantum simulation. In contrast to conventional nonlinear signals, the presented scheme does not change the number of excitations in the system, allowing for the investigation of the dynamics within an N-excitation manifold. In the particular case of a filling factor one (N excitations in an N-ion chain), the proposed interaction induces coherent transitions between a delocalized phonon superfluid and a localized atomic insulator phase. Numerical simulations of a two-ion chain demonstrate that the resulting two-dimensional spectra allow for the unambiguous identification of the distinct phases, and the two-dimensional line shapes efficiently characterize the relevant decoherence mechanism.

## Nonlinear spectroscopy of controllable many-body quantum systems

NEW JOURNAL OF PHYSICS **16** (2014) ARTN 092001