Ultrastrong Light-matter Coupling Research Articles

Dynamical generation and transfer of nonclassical states in strongly interacting light-matter systems in cavities

Abstract We propose leveraging strong and ultrastrong light-matter coupling to efficiently generate and exchange nonclassical light and quantum matter states. Two initial conditions are considered: (a) a displaced quadrature-squeezed matter state, and (b) a coherent state in a cavity. In both scenarios, polaritons mediate the dynamical generation and transfer of nonclassical states between light and matter. By monitoring the dynamics of both subsystems, we uncover the emergence of cavity-induced beatings in the collective matter oscillations. The beating period depends on the particle density through the vacuum Rabi splitting and peaks sharply under light-matter resonance conditions. For initial condition (a), nonclassicality is efficiently transferred from matter to photons under strong and ultrastrong coupling. However, for initial condition (b), nonclassical photonic states are generated only in the ultrastrong coupling regime due to the counter-rotating terms, highlighting the advantages of ultrastrong coupling. Furthermore, in the ultrastrong coupling regime, distinctive asymmetries relative to cavity detuning emerge in dynamical observables of both light and matter. The nonclassical photons can be extracted through a semi-transparent cavity mirror, while nonclassical matter states can be detected via time-resolved spectroscopy. This work highlights that polariton states may serve as a tool for dynamically generating and transferring nonclassical states, with potential applications in quantum technology.

Breaking the angular dispersion limit in thin film optics by ultra-strong light-matter coupling

Thin film interference is integral to modern photonics, e.g., allowing for precise design of high performance optical filters, photovoltaics and light-emitting devices. However, interference inevitably leads to a generally undesired change of spectral characteristics with angle. Here, we introduce a strategy to overcome this fundamental limit in optics by utilizing and tuning the exciton-polariton modes arising in ultra-strongly coupled microcavities. We demonstrate optical filters with narrow pass bands that shift by less than their half width (< 15 nm) even at extreme angles. By expanding this strategy to strong coupling with the photonic sidebands of dielectric multilayer stacks, we also obtain filters with high extinction ratios and up to 98% peak transmission. Finally, we apply this approach in flexible filters, organic photodiodes, and polarization-sensitive filtering. These results illustrate how strong coupling provides additional degrees of freedom in thin film optics that will enable exciting new applications in micro-optics, sensing, and biophotonics.

Comparing parameterized and self-consistent approaches to abinitio cavity quantum electrodynamics for electronic strong coupling.

Molecules under strong or ultra-strong light-matter coupling present an intriguing route to modify chemical structure, properties, and reactivity. A rigorous theoretical treatment of such systems requires handling matter and photon degrees of freedom on an equal quantum mechanical footing. In the regime of molecular electronic strong or ultra-strong coupling to one or a few molecules, it is desirable to treat the molecular electronic degrees of freedom using the tools of abinitio quantum chemistry, yielding an approach referred to as abinitio cavity quantum electrodynamics (ai-QED), where the photon degrees of freedom are treated at the level of cavity QED. We analyze two complementary approaches to ai-QED: (1) a parameterized ai-QED, a two-step approach where the matter degrees of freedom are computed using existing electronic structure theories, enabling the construction of rigorous ai-QED Hamiltonians in a basis of many-electron eigenstates, and (2) self-consistent ai-QED, a one-step approach where electronic structure methods are generalized to include coupling between electronic and photon degrees of freedom. Although these approaches are equivalent in their exact limits, we identify a disparity between the projection of the two-body dipole self-energy operator that appears in the parameterized approach and its exact counterpart in the self-consistent approach. We provide a theoretical argument that this disparity resolves only under the limit of a complete orbital basis and a complete many-electron basis for the projection. We present numerical results highlighting this disparity and its resolution in a particularly simple molecular system of helium hydride cation, where it is possible to approach these two complete basis limits simultaneously. In this same helium hydride system, we examine and compare the practical issue of the computational cost required to converge each approach toward the complete orbital and many-electron bases limit. Finally, we assess the aspect of photonic convergence for polar and charged species, finding comparable behavior between parameterized and self-consistent approaches.

Superradiant phase transitions in ultrastrong coupling regime

Abstract One of the long-standing problems in quantum optics and laser physics is the observation of both equilibrium and non-equilibrium (Dicke) superradiant states in two-level (qubit) systems interacting with the quantized electromagnetic (EM) field. We demonstrate that the superradiant phase transition (PT) at thermal equilibrium in such a system is close to ultrastrong light–matter coupling (USC) condition, which is currently available for superconducting devices. We examine the USC regime for EM fields interacting with two-level systems located in highly connected graph nodes of photonic networks beyond the rotating wave approximation. We find that a critical matter–field coupling parameter reduces ⟨ k ⟩ times due to network peculiarities providing suitable conditions for equilibrium PT observation; ⟨ k ⟩ is a network average node degree. We study the non-equilibrium (Dicke) superradiance for the same network system and show that superradiance appears as a non-equilibrium PT to the superradiant laser operating within a bad cavity limit. We show that population inversion in this case plays the same role as the reciprocal temperature inherent to the equilibrium superradiant PT. Our results open new perspectives for experimental observation of superradiance in cavity-free systems in the gigahertz photonic regime and beyond.

Scalable ultra-strong light–matter coupling at THz frequencies using graded alloy parabolic quantum wells

Scalable ultra-strong light–matter coupling at THz frequencies using graded alloy parabolic quantum wells

The orientation dependence of cavity-modified chemistry.

Recent theoretical studies have explored how ultra-strong light-matter coupling can be used as a handle to control chemical transformations. Ab initio cavity quantum electrodynamics calculations demonstrate that large changes to reaction energies or barrier heights can be realized by coupling electronic degrees of freedom to vacuum fluctuations associated with an optical cavity mode, provided that large enough coupling strengths can be achieved. In many cases, the cavity effects display a pronounced orientational dependence. Here, we highlight the critical role that geometry relaxation can play in such studies. As an example, we consider a recent work [Pavošević et al., Nat. Commun. 14, 2766 (2023)] that explored the influence of an optical cavity on Diels-Alder cycloaddition reactions and reported large changes to reaction enthalpies and barrier heights, as well as the observation that changes in orientation can inhibit the reaction or select for one reaction product or another. Those calculations used fixed molecular geometries optimized in the absence of the cavity and fixed relative orientations of the molecules and the cavity mode polarization axis. Here, we show that when given a chance to relax in the presence of the cavity, the molecular species reorient in a way that eliminates the orientational dependence. Moreover, in this case, we find that qualitatively different conclusions regarding the impact of the cavity on the thermodynamics of the reaction can be drawn from calculations that consider relaxed vs unrelaxed molecular structures.

Phonon pumping by modulating the ultrastrong vacuum

The vacuum (i.e., the ground state) of a system in ultrastrong light-matter coupling contains particles that cannot be emitted without any dynamical perturbation and is thus called virtual. We propose a protocol for inducing and observing real mechanical excitations of a mirror enabled by the virtual photons in the ground state of a tripartite system, where a resonant optical cavity is ultrastrongly coupled to a two-level system (qubit) and, at the same time, optomechanically coupled to a mechanical resonator. Real phonons are coherently emitted when the frequency of the two-level system is modulated at a frequency comparable to that of the mechanical resonator and, therefore much lower than the optical frequency. We demonstrate that this hybrid effect is a direct consequence of the virtual photon population in the ground state. Within a classical physics analogy, attaching a weight to a spring only changes its resting position, whereas dynamically modulating the weight makes the system oscillate. In our case, however, the weight is the vacuum itself. We propose and accurately characterize a hybrid superconducting-optomechanical setup based on available state-of-the-art technology, where this effect can be experimentally observed.

Dynamical Coulomb blockade: An all electrical probe of the ultrastrong light-matter coupling regime

The regime of ultrastrong light-matter coupling (USC) has attracted considerable interest recently, as it predicts fascinating quantum phenomena such as the presence of virtual photons in the ground state. Here we propose theoretically an approach to probe the quantum correlations in USC systems based on an electrical transport experiment. Namely, we consider the case in which the light-matter coupled system constitutes the electromagnetic environment of an ultrasmall tunnel junction operating in the regime of a dynamical Coulomb blockade (DCB). Unlike optical spectroscopy, such a probe can be seen as a nondemolition measurement of the USC system ground state. We have developed two approaches in order to evaluate the effect of the light-matter coupled states on the tunneling process. The first is a generalized, multimode Peierls substitution for the tunneling Hamiltonian. The second is based on the P(E) theory describing a DCB, where the USC system is treated as an equivalent circuit. The two approaches are consistent, whereas the second one enables us explicitly to take into account the presence of dissipation. This study has been performed for the zero-temperature case, which is pertinent for the terahertz spectral range where excitation energies are typically much higher compared to the microwave domain, where DCB was first studied. Published by the American Physical Society 2024

Self-Hybridized Vibrational-Mie Polaritons in Water Droplets

We study the self-hybridization between Mie modes supported by water droplets with stretching and bending vibrations in water molecules. Droplets with radii &gt;2.7 μm are found to be polaritonic on the onset of the ultrastrong light-matter coupling regime. Similarly, the effect is observed in larger deuterated water droplets at lower frequencies. Our results indicate that polaritonic states are ubiquitous and occur in water droplets in mists, fogs, and clouds. This finding may have implications not only for polaritonic physics but also for aerosol and atmospheric sciences. Published by the American Physical Society 2024

Lindblad Master Equation Capable of Describing Hybrid Quantum Systems in the Ultrastrong Coupling Regime.

Despite significant theoretical efforts devoted to studying the interaction between quantized light modes and matter, the so-called ultrastrong coupling regime still presents significant challenges for theoretical treatments and prevents the use of many common approximations. Here we demonstrate an approach that can describe the dynamics of hybrid quantum systems in any regime of interaction for an arbitrary electromagnetic (EM) environment. We extend a previous method developed for few-mode quantization of arbitrary systems to the case of ultrastrong light-matter coupling, and show that even such systems can be treated using a Lindblad master equation where decay operators act only on the photonic modes by ensuring that the effective spectral density of the EM environment is sufficiently suppressed at negative frequencies. We demonstrate the validity of our framework and show that it outperforms current state-of-the-art master equations for a simple model system, and then study a realistic nanoplasmonic setup where existing approaches cannot be applied.

Sculpting ultrastrong light-matter coupling through spatial matter structuring.

The central theme of cavity quantum electrodynamics is the coupling of a single optical mode with a single matter excitation, leading to a doublet of cavity polaritons which govern the optical properties of the coupled structure. Especially in the ultrastrong coupling regime, where the ratio of the vacuum Rabi frequency and the quasi-resonant carrier frequency of light, ΩR/ω c, approaches unity, the polariton doublet bridges a large spectral bandwidth 2ΩR, and further interactions with off-resonant light and matter modes may occur. The resulting multi-mode coupling has recently attracted attention owing to the additional degrees of freedom for designing light-matter coupled resonances, despite added complexity. Here, we experimentally implement a novel strategy to sculpt ultrastrong multi-mode coupling by tailoring the spatial overlap of multiple modes of planar metallic THz resonators and the cyclotron resonances of Landau-quantized two-dimensional electrons, on subwavelength scales. We show that similarly to the selection rules of classical optics, this allows us to suppress or enhance certain coupling pathways and to control the number of light-matter coupled modes, their octave-spanning frequency spectra, and their response to magnetic tuning. This offers novel pathways for controlling dissipation, tailoring quantum light sources, nonlinearities, correlations as well as entanglement in quantum information processing.

Ultrasmall and tunable TeraHertz surface plasmon cavities at the ultimate plasmonic limit

The ability to confine THz photons inside deep-subwavelength cavities promises a transformative impact for THz light engineering with metamaterials and for realizing ultrastrong light-matter coupling at the single emitter level. To that end, the most successful approach taken so far has relied on cavity architectures based on metals, for their ability to constrain the spread of electromagnetic fields and tailor geometrically their resonant behavior. Here, we experimentally demonstrate a comparatively high level of confinement by exploiting a plasmonic mechanism based on localized THz surface plasmon modes in bulk semiconductors. We achieve plasmonic confinement at around 1 THz into record breaking small footprint THz cavities exhibiting mode volumes as low as {V}_{cav}/{lambda }_{0}^{3} sim 1{0}^{-7}-1{0}^{-8}, excellent coupling efficiencies and a large frequency tunability with temperature. Notably, we find that plasmonic-based THz cavities can operate until the emergence of electromagnetic nonlocality and Landau damping, which together constitute a fundamental limit to plasmonic confinement. This work discloses nonlocal plasmonic phenomena at unprecedentedly low frequencies and large spatial scales and opens the door to novel types of ultrastrong light-matter interaction experiments thanks to the plasmonic tunability.

Polaritonic linewidth asymmetry in the strong and ultrastrong coupling regime.

The intriguing properties of polaritons resulting from strong and ultrastrong light-matter coupling have been extensively investigated. However, most research has focused on spectroscopic characteristics of polaritons, such as their eigenfrequencies and Rabi splitting. Here, we study the decay rates of a plasmon-microcavity system in the strong and ultrastrong coupling regimes experimentally and numerically. We use a classical scattering matrix approach, approximating our plasmonic system with an effective Lorentz model, to obtain the decay rates through the imaginary part of the complex quasinormal mode eigenfrequencies. Our classical model automatically includes all the interaction terms necessary to account for ultrastrong coupling without dealing with the rotating-wave approximation and the diamagnetic term. We find an asymmetry in polaritonic decay rates, which deviate from the expected average of the uncoupled system's decay rates at zero detuning. Although this phenomenon has been previously observed in exciton-polaritons and attributed to their disorder, we observe it even in our homogeneous system. As the coupling strength of the plasmon-microcavity system increases, the asymmetry also increases and can become so significant that the lower (upper) polariton decay rate reduction (increase) goes beyond the uncoupled decay rates, γ - < γ 0,c < γ +. Furthermore, our findings demonstrate that polaritonic linewidth asymmetry is a generic phenomenon that persists even in the case of bulk polaritons.

Electronic transport driven by collective light-matter coupled states in a quantum device

In the majority of optoelectronic devices, emission and absorption of light are considered as perturbative phenomena. Recently, a regime of highly non-perturbative interaction, ultra-strong light-matter coupling, has attracted considerable attention, as it has led to changes in the fundamental properties of materials such as electrical conductivity, rate of chemical reactions, topological order, and non-linear susceptibility. Here, we explore a quantum infrared detector operating in the ultra-strong light-matter coupling regime driven by collective electronic excitations, where the renormalized polariton states are strongly detuned from the bare electronic transitions. Our experiments are corroborated by microscopic quantum theory that solves the problem of calculating the fermionic transport in the presence of strong collective electronic effects. These findings open a new way of conceiving optoelectronic devices based on the coherent interaction between electrons and photons allowing, for example, the optimization of quantum cascade detectors operating in the regime of strongly non-perturbative coupling with light.

Ultrastrong magnon-photon coupling, squeezed vacuum, and entanglement in superconductor/ferromagnet nanostructures

Ultrastrong light-matter coupling opens exciting possibilities to generate squeezed quantum states and entanglement. We propose achieving this regime in superconducting hybrid nanostructures with ferromagnetic interlayers. Strong confinement of the electromagnetic field between superconducting plates results in the existence of magnon-polariton (MP) modes with ultrastrong magnon-photon coupling, ultrahigh cooperativity, and colossal group velocities. These modes provide a numerically accurate explanation of recent experiments and have intriguing quantum properties. The MP quantum vacuum consists of the squeezed magnon and photon states with the degree of squeezing controlled in wide limits by the external magnetic field. The ground-state population of virtual photons and magnons is vast and can be used for generating correlated magnon and photon pairs. MP excitations contain bipartite entanglement between magnons and photons. Our results indicate that superconducting/ferromagnet nanostructures are very promising for quantum magnonics.

Positronium density measurements using polaritonic effects

Recent experimental advances in Positronium (Ps) physics have made it possible to produce dense Ps ensembles in which Ps-Ps interactions may occur, leading to the production of Ps$_2$ molecules and paving the way to the realization of a Ps Bose-Einstein Condensate (BEC). In order to achieve this latter goal it would be advantageous to develop new methods to measure Ps densities in real-time. Here we describe a possible approach to do this using polaritonic methods: using realistic experimental parameters we demonstrate that a dense Ps gas can be strongly coupled to the photonic field of a distributed Bragg reflector microcavity. In this strongly coupled regime, the optical spectrum of the system is composed of two hybrid positronium-polariton resonances separated by the vacuum Rabi splitting, which is proportional to the square root of the Ps density. Given that polaritons can be created on a sub-cycle timescale, a spectroscopic measurement of the vacuum Rabi splitting could be used as an ultra-fast Ps density measurement in regimes relevant to Ps BEC formation. Moreover, we show how positronium-polaritons could potentially enter the ultrastrong light-matter coupling regime, introducing a radically novel platform to explore its non-perturbative phenomenology.

Controlling the strong light-matter coupling in metal-dielectric optical resonators using spin-crossover molecules

We report the observation of (ultra)strong light-matter coupling, in the UV spectral region, between optical modes of a metal/dielectric bilayer nanocavity and the electronic excitations of spin-crossover (SCO) molecules. By thermally switching the SCO molecules between their low-spin and high-spin states, we demonstrate the possibility of fine-tuning the light-molecule hybridization strength, allowing a reversible switching between strong- (with Rabi splitting values of up to 550 meV) and weak-coupling regimes within a single photonic resonator. As a result, we show that spin-crossover molecular compounds constitute a novel, promising class of active nanomaterials in the context of tuneable and reconfigurable polaritonic devices.

Spontaneous Scattering of Raman Photons from Cavity-QED Systems in the Ultrastrong Coupling Regime.

We show that spontaneous Raman scattering of incident radiation can be observed in cavity-QED systems without external enhancement or coupling to any vibrational degree of freedom. Raman scattering processes can be evidenced as resonances in the emission spectrum, which become clearly visible as the cavity-QED system approaches the ultrastrong coupling regime. We provide a quantum mechanical description of the effect, and show that ultrastrong light-matter coupling is a necessary condition for the observation of Raman scattering. This effect, and its strong sensitivity to the system parameters, opens new avenues for the characterization of cavity QED setups and the generation of quantum states of light.

Fast Generation of 2N‐Photon Fock States using Shortcuts to Adiabaticity and Ultrastrong Light–Matter Coupling

AbstractBy using shortcuts to adiabatic (STA) method, the proposal is to implement a fast multi‐photon down‐conversion, which can rapidly create 2N photons from the quantum vacuum based on the counter‐rotating effect of an ultrastrong light–matter coupling. The energy for the produced photons is given by a high‐frequency pump field. The STA method is used to design the driving fields to induce a rapid population transfer. Such an accelerated evolution can restrain the influence of decoherence during the evolution, so as to generate Fock states from vacuum with high fidelities.

Reduced-density-matrix-based ab initio cavity quantum electrodynamics

A reduced-density-matrix (RDM)-based approach to {\em ab initio} cavity quantum electrodynamics (QED) is developed. The expectation value of the Pauli-Fierz Hamiltonian is expressed in terms of one- and two-body electronic and photonic RDMs, and the elements of these RDMs are optimized directly in polynomial time by semidefinite programming techniques, without knowledge of the full wave function. QED generalizations of important ensemble $N$-representability conditions are derived and enforced in this procedure. The resulting approach is applied to the description of classic ground-state strong electron correlation problems, augmented by the presence of ultrastrong light-matter coupling. First, we assess cavity-induced changes to the singlet-triplet energy gap of the linear oligoacene series; for a heptacene molecule, this gap can change by as much as 1.9 kcal mol$^{-1}$ (or 15\%) when the molecule is aligned along the cavity mode polarization axis. We also explore the metal-insulator transition in linear hydrogen chains and demonstrate that strong electron-photon interactions increase the insulating character of these systems under all coupling strengths considered.