arXiv subjects

Kai Schwennicke

Publications and source records attributed to Kai Schwennicke.

7 recordsLinked to original sources

arXiv2026

Programmable generation of arbitrary continuous-variable anharmonicities and nonlinear couplings

Harmonic oscillators are promising continuous-variable (CV) quantum resources because their infinite-dimensional Hilbert spaces allow for resource-efficient quantum computing and simulation. To reach their full potential, CV platforms need to be able to efficiently implement non-Gaussian operations. Bosonic quantum-signal-processing schemes have emerged as attractive methods to construct arbitrary non-Gaussian operations; however, these schemes are restricted to single modes, i.e., the implementation of anharmonic potentials. Here, we introduce trigonometric-gate-implemented Fourier synthesis (TGIFS), a method for implementing arbitrary non-Gaussian operations applicable to both single- and multi-mode systems, allowing the generation of both anharmonicities and nonlinear multi-mode couplings. TGIFS synthesizes a target Hamiltonian by decomposing it into a Fourier series whose terms are implemented via bosonic quantum signal processing, which uses a discrete-variable (DV) system to induce a nonlinearity in the CV system. Our hybrid CV-DV protocol allows for the direct simulation of a broad range of CV phenomena (such as those in lattice gauge theory, chemical dynamics, and quantum chaos) and provides a richer toolbox for CV circuit compilation.

quant-ph↗

arXiv2026

Programmable quantum simulation of anharmonic dynamics

Continuous-variable-discrete-variable (CV-DV) quantum simulators offer a natural route to simulating bosonic dynamics relevant to many branches of physics and chemistry. However, programmable simulation of arbitrary dynamics is an outstanding challenge. In particular, simulating anharmonic dynamics, which is ubiquitous across the physical sciences, is challenging due to the highly harmonic nature of oscillators used in CV-DV simulators. Here, we experimentally demonstrate programmable CV-DV quantum simulation of anharmonic dynamics in a range of double-well potentials, implemented in a trapped-ion system. We synthesise the time-evolution operators using a bosonic-quantum-signal-processing subroutine, which allows the potential to be tuned between experiments by controlling classical experimental parameters. We observe coherent dynamics in various double-well potentials, where a wavepacket tunnels through the potential barrier, and we suppress this effect by programmatically introducing asymmetry.

quant-ph↗

arXiv2025

When do molecular polaritons behave like optical filters?

This review outlines several linear optical effects featured by molecular polaritons arising in the collective strong light-matter coupling regime. Under weak laser irradiation and when the single-molecule light-matter coupling can be neglected (often in the limit when the number of molecules per photon mode is large), we show that the excited-state molecular dynamics under collective strong coupling can be exactly replicated without the cavity using a shaped (or ``filtered'') laser, whose field amplitude is enhanced by the cavity quality factor, shining on the bare molecules. As a consequence, the absorption within a cavity can be understood as the overlap between the polariton transmission and the bare molecular absorption, suggesting that polaritons act in part as optical filters. This framework demystifies and provides a straightforward explanation for a large class of experiments and theoretical models in molecular polaritonics, highlighting that the same effects can be achieved without the cavity with shaped laser pulses. With a few modifications, this simple conceptual picture can also be adapted to understand the incoherent nonlinear response of polaritonic systems. This review establishes a clear distinction between polaritonic phenomena that can be fully explained through classical linear optics and those that require a quantum electrodynamics approach. It also highlights the need to differentiate between effects that necessitate polaritons (i.e., hybrid light-matter states) and those that can occur in the weak coupling regime. We further discuss that certain quantum optical effects like fluorescence can be partially described as optical filtering, whereas some others like cavity-induced Raman scattering go beyond this. Further exploration in these areas is needed to uncover novel polaritonic phenomena beyond optical filtering.

physics.chem-ph↗

arXiv2024

Extracting accurate light-matter couplings from disordered polaritons

The vacuum Rabi splitting (VRS) in molecular polaritons stands as a fundamental measure of collective light-matter coupling. Despite its significance, the impact of molecular disorder on VRS is not fully understood yet. This study delves into the complexities of VRS amidst various distributions and degrees of disorder. Our analysis provides precise analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule, offering a straightforward protocol for extracting accurate collective light-matter coupling values from experimental data. Importantly, our study cautions against directly translating large VRS to the onset of ultrastrong coupling regime. Furthermore, for rectangular disorder, we witness the emergence of narrow side bands alongside a broad central peak, indicating an extended coherence lifetime even in the presence of substantial disorder. These findings not only enhance our understanding of VRS in disordered molecular systems but also open avenues for achieving prolonged coherence lifetimes between the cavity and molecules via the interplay of collective coupling and disorder.

physics.chem-ph↗

arXiv2023

Chiral edge waves in a dance-based human topological insulator

Topological insulators are insulators in the bulk but feature chiral energy propagation along the boundary. This property is topological in nature and therefore robust to disorder. Originally discovered in electronic materials, topologically protected boundary transport has since been observed in many other physical systems. Thus, it is natural to ask whether this phenomenon finds relevance in a broader context. We choreograph a dance in which a group of humans, arranged on a square grid, behave as a topological insulator. The dance features unidirectional flow of movement through dancers on the lattice edge. This effect persists when people are removed from the dance floor. Our work extends the applicability of wave physics to the performance arts.

cond-mat.mes-hall↗

arXiv2022

Enantioselective Topological Frequency Conversion

Two molecules are enantiomers if they are non-superimposable mirror images of each other. Electric dipole-allowed cyclic transitions $|1\rangle\to|2\rangle\to|3\rangle\to|1\rangle$ obey the symmetry relation $\mathcal{O}^{R}=-\mathcal{O}^{S}$, where $\mathcal{O}^{R,S}=(μ_{21}^{R,S}E_{21})(μ_{13}^{R,S}E_{13})(μ_{32}^{R,S}E_{32})$, and $R,S$ label the two enantiomers. Here we generalize the concept of topological frequency conversion to an ensemble of enantiomers. We show that, within a rotating-frame, the pumping power between fields of frequency $ω_{1}$ and $ω_{2}$ is sensitive to enantiomeric excess, $\mathcal{P}_{2\to1}=\hbar\frac{ω_{1}ω_{2}C_{L}^{R}}{2π}(N_{R}-N_{S})$, where $N_{i}$ is the number of enantiomers $i$ and $C_{L}^{R}$ is an enantiomer-dependent Chern number. Connections with chiroptical microwave spectroscopy are made. Our work provides an underexplored and fertile connection between topological physics and molecular chirality.

quant-ph↗

arXiv2020

Optical Activity from the Exciton Aharonov-Bohm Effect: A Floquet Engineering Approach

Floquet engineering is a convenient strategy to induce nonequilibrium phenomena in molecular and solid-state systems, or to dramatically alter the physicochemical properties of matter, bypassing costly and time-consuming synthetic modifications. In this article, we investigate theoretically some interesting consequences of the fact that an originally achiral molecular system can exhibit nonzero circular dichroism (CD) when it is driven with elliptically polarized light. More specifically, we consider an isotropic ensemble of small cyclic molecular aggregates in solution whose local low-frequency vibrational modes are driven by an elliptically polarized continuous-wave infrared pump. We attribute the origin of a nonzero CD signal to time-reversal symmetry breaking due to an excitonic Aharonov-Bohm (AB) phase arising from a time-varying laser electric field, together with coherent interchromophoric exciton hopping. The obtained Floquet engineered excitonic AB phases are far more tunable than analogous magnetically-induced electronic AB phases in nanoscale rings, highlighting a virtually unexplored potential that Floquet engineered AB phases have in the coherent control of molecular processes and simultaneously introducing new analogues of magneto-optical effects in molecular systems which bypass the use of strong magnetic fields.

quant-ph↗