arXiv subjects

Helmut Ritsch

Publications and source records attributed to Helmut Ritsch.

At least 19 recordsLinked to original sources

arXiv2026

Input-Output Analysis of Quantum Dot SUPER Excitation

The Swing-UP of the quantum EmitteR population (SUPER) two-color pulsed excitation scheme allows for robust and close to 100\% excitation of a two-level quantum emitter using only red detuned light. We analyze the underlying counterintuitive dynamics using a full quantum input-output description of the in- and outgoing light pulses for the case of two coherent free-space input pulses at realistic photon numbers. At the microscopic level, the SUPER mechanism exhibits its nonlinear three-photon Raman-type character, leading to a net photon-number change of $-2$ in one mode and $+1$ in the other, as was first guessed from cavity-enhanced model descriptions at low photon numbers. We confirm that in free space, a sufficiently high pulse photon number, much larger than one, is required to achieve high-fidelity inversion. To treat the large coherent-state amplitudes (photon numbers) relevant for SUPER, we extend the quantum input-output formalism to include a cumulant expansion approach. With an interaction-picture formulation, the few exchanged photons that govern the nontrivial dynamics enable direct full-quantum calculations in truncated Hilbert spaces, including treatments in a displacement frame and for initial Fock-state pulses.

quant-ph↗

arXiv2026

Tunable-Size Unruh-DeWitt Detector in a Multimode Cavity

Quantum simulators based on Bose-Einstein condensates (BECs) provide a powerful platform to study relativistic quantum-field phenomena in the lab and in particular emulate relativistic analog particle detectors. However, such detectors typically couple to excitations beyond the acoustic phonon regime, so that the nonlinear Bogoliubov dispersion introduces corrections that cause the effective relativistic description to break down. We propose to overcome this limitation using an intrinsically momentum-selective Unruh-DeWitt detector model in which a tweezer-trapped atom couples to the condensate through a multimode optical cavity. Although the atom is effectively pointlike, the cavity-mediated interaction kernel causes it to sample density fluctuations over a controllable finite region of the BEC equivalent to a finite-size detector. The effective detector size is tunable via the cavity mode structure determining a controllable momentum cutoff that filters out nonphononic excitations. We analytically calculate the response of such a detector for uniform linear acceleration and uniform circular motion and show that this momentum selectivity can enhance the signal at experimentally accessible accelerations. Compatible with existing cavity-QED platforms our scheme thus provides a versatile setting for analog quantum-field measurements or relativistic quantum-information protocols.

quant-ph↗

arXiv2026

Collective-State Preparation in a Subwavelength Triangular Trimer Using SUPER Excitation

The Swing-UP of quantum EmitteR population (SUPER) scheme has recently been proposed as a deterministic method for the preparation of collective radiative states in two strongly dipole-coupled quantum emitters (Phys. Rev. Res. \textbf{8}, 013179 (2026)). Here, we extend this approach to an equilateral subwavelength triangular trimer of dipole-coupled two-level quantum emitters (QEs), loosely inspired by biological light-harvesting ring geometries. Using tailored, time-overlapping, red-detuned ultrashort SUPER pulses, we numerically investigate the selective preparation of collective target states. We find that both the state selectivity and the preparation efficiency depend strongly on the inter-emitter spacing. In particular, at deep-subwavelength separations, the symmetric collective state can be deterministically prepared with near-unity efficiency, whereas the inversion efficiency and state selectivity are significantly lower at larger inter-emitter separations. Furthermore, this state preparation technique inherits a certain degree of robustness against reasonable static position imperfections and on-site frequency inhomogeneities of the individual QEs. Our results demonstrate that deep-subwavelength triangular trimers and, more broadly, highly compact ring geometries are excellent candidates for the deterministic preparation of collective radiative states via SUPER excitation. These predictions could be realized with solid-state emitters and molecules. Our findings offer a route toward the direct probing of the `pure' electromagnetic layer of interaction in biological and bio-inspired synthetic nanophotonic ring configurations, with possible relevance in photonics, quantum information processing, and metrology.

physics.optics↗

arXiv2026

Continuous Narrow-Linewidth Superradiance in Waveguide QED

Superradiant lasers promise continuous, narrow-linewidth coherent emission at the bare atomic transition frequency, enabling frequency references of exceptional precision. Recent experiments have advanced the field, but achieving truly continuous operation remains technically challenging. Here we propose an alternative route to an active optical frequency reference with fewer emitters using all-to-all dipole-dipole interactions mediated by a nanophotonic waveguide. We show that selectively pumping only a sub-ensemble of emitters, rather than the full ensemble, substantially improves emission characteristics. The collective interactions with unpumped emitters provide narrowband frequency selection and establish an effective feedback mechanism analogous to the role of a macroscopic cavity. We find directional superradiant emission with strongly phase-synchronized emitter correlations and a narrow output spectrum close to the bare emitter resonance. Our results demonstrate a strong metrological gain from selective partial pumping of quantum emitters with the second-order intensity correlation $g^{(2)}(0)\simeq 1$, indicating reduced equal-time intensity fluctuations, and open a route to waveguide-based optical frequency references using small clock-atom ensembles for chip-scale precision metrology.

quant-ph↗

arXiv2026

Environment-Enhanced Single-Photon Absorption in a Nano-Ring of Dipole-Coupled Quantum Emitters

Decoherence is mostly detrimental in quantum information and quantum optics applications. However, the interplay between environment-induced incoherent dynamics and unitary evolution can give rise to novel quantum many-body phenomena that can be harnessed as a useful resource. As is well known, in dense subwavelength atomic arrays only a single collective eigenmode in the single-excitation manifold couples strongly to free-space radiation, exhibiting superradiant spontaneous emission. Most of the remaining eigenstates form a manifold of weakly radiative modes, giving rise to long-lived subradiant excitations. Here we demonstrate that populating these subradiant modes via additional decoherence mechanisms, such as dephasing or coupling to phonons, can significantly enhance single-photon absorption in a nanoring of quantum emitters. Such nanoring geometry is particularly appealing due to its unique optical properties and its resemblance to natural light-harvesting complexes, which serve as efficient antennas in photosynthesis. Our findings may shed light on fundamental aspects of energy absorption in nature; despite the much greater complexity of biological systems, they may nonetheless operate according to similar underlying optical principles.

quant-ph↗

arXiv2025

The Cumulant Expansion Approach: The Good, The Bad and The Ugly

The configuration space, i.e. the Hilbert space, of compound quantum systems grows exponentially with the number of its subsystems: its dimensionality is given by the product of the dimensions of its constituents. Therefore a full quantum treatment is rarely possible analytically and can be carried out numerically for fairly small systems only. Fortunately, in order to obtain interesting physics, approximations often very well suffice. One of these approximations is given by the cumulants expansion, where expectation values of products of operators are approximated by products of expectation values of said operators, neglecting higher-order correlations. The lowest order of this approximation is widely known as the mean field approximation and used routinely throughout quantum physics. Despite its ubiquitous presence, a general criterion for applicability and convergence properties of higher order cumulants expansions remains to be found. In this paper, we discuss two problems in quantum electrodynamics and quantum information, namely the collective radiative dissipation of a dipole-dipole interacting chain of atoms and the factorization of a bi-prime by annealing in an adiabatic quantum simulator. In the first case we find smooth, convergence behavior, where the approximation performs increasingly better with higher orders, while in the latter going beyond mean field turns out useless and, even for small system sizes, we are puzzled by numerically challenging and partly non-physical solutions.

quant-ph↗

arXiv2025

Frequency shifts heralding ground state squeezing and entanglement of two coupled harmonic oscillators

It is often argued that two linearly coupled quantum harmonic oscillators, even when cooled to their ground state, display no inherently quantum features beyond quantized energy levels. Here, we challenge this view by showing that their classical observables encode genuinely quantum features. In particular, we demonstrate that the characteristic frequency shifts observed in coupled oscillators signal non-classical correlations and ground-state entanglement at zero temperature corresponding to two-mode squeezing between the uncoupled modes. From a complementary perspective, these two effects, frequency shifts and squeezing, represent the same underlying phenomenon but expressed in different mode bases. What appears as a spectral renormalization in one description manifests itself as entanglement in the other. Frequency shifts therefore constitute an entanglement witness accessible via standard spectroscopy. While the underlying squeezing is not directly measurable, it can be exploited to enhance the signal-to-noise ratio in precision frequency measurements of individual oscillators without requiring squeezed quantum noise. This uncovers a new route to quantum-enhanced sensing within systems traditionally regarded as classical, offering fresh insight into how signatures of quantumness persists across the quantum-to-classical boundary.

quant-ph↗

arXiv2025

Selective Preparation of Collective States in Coupled Quantum Emitters Using the SUPER Excitation Scheme

The efficient preparation of collective eigenstates of subwavelength-spaced optical dipoles is a prerequisite for observing their signature radiative properties and for their applications in quantum information processing. We theoretically investigate the deterministic preparation of superradiant and subradiant states of two dipole-coupled two-level quantum emitters at deep-subwavelength separation using the Swing-UP of Quantum Emitter Population (SUPER) excitation scheme. Utilizing suitable pulse parameters for two red-detuned, time-overlapping Gaussian pulses, the SUPER scheme enables close-to-unity population inversion in the targeted collective eigenstates. Furthermore, a tunable optical phase in the SUPER scheme enables the simultaneous inversions in both pure super- and subradiant states with finite populations, thereby resulting in the preparation of hybrid collective states. These results are possible to realize with or without an optical cavity. Our approach to populating the collective eigenstates in a cavity environment paves the way for the efficient preparation of these states in the presence of environmental decoherence. Our scheme enables single-photon generation, which is measured using the second-order correlation function. We also discuss in detail possible experimental realizations, in particular using solid-state emitters and molecules.

quant-ph↗

arXiv2025

Nanoscale Mirrorless Superradiant Lasing

We predict collective 'free-space' lasing in a dense nanoscopic emitter arrangement where dipole-dipole coupled atomic emitters synchronize their emission and exhibit lasing behavior without the need for an optical resonator. At the example of a subwavelength-spaced linear emitter chain with varying fractions of pumped and unpumped emitters, we present a comprehensive study of this mirrorless lasing phenomenon. The total radiated power transitions from subradiant suppression under weak pumping to superradiant enhancement at stronger pumping, while exhibiting directional emission confined to a narrow spatial angle. At the same time multiple independent spectral emission lines below the lasing threshold merge towards a single narrow spectral line at high pump power. The most substantial enhancement and line narrowing occur when a fraction of unpumped atoms is present. We show that this leads to superradiant lasing near the bare atomic frequency, making the system a promising candidate for a minimalist active optical frequency reference.

quant-ph↗

arXiv2024

Conditional Entanglement Amplification via Non-Hermitian Superradiant Dynamics

Due to the inherently probabilistic nature of quantum mechanics, each experimental realization of a dynamical quantum system may yield a different measurement outcome, especially when the system is coupled to an environment that causes dissipation. Although it is in principle possible that some quantum trajectories lead to exotic highly entangled quantum states, the probability of observing these trajectories is usually extremely low. In this work, we show how to maximize the probability of generating highly entangled states, including maximally entangled cat states, in an ensemble of atoms experiencing superradiant decay. To this end, we analyze an effective non-Hermitian Hamiltonian which governs the dynamics between the quantum jumps associated with photon emission. A key result of our study is that, in order to maximally enhance the probability of cat state generation, the initial state needs to be non-classical. This can be achieved e.g. with one-axis twisting in a cavity-QED system.

quant-ph↗

arXiv2024

Symmetry based efficient simulation of dissipative quantum many-body dynamics in subwavelength quantum emitter arrays

We propose an efficient method to numerically simulate the dissipative dynamics of large numbers of quantum emitters in ordered arrays in the presence of long-range dipole-dipole interactions mediated by the vacuum electromagnetic field. Using the spatial symmetries of the system, we rewrite the equations of motion in a collective spin basis and subsequently apply a higher-order cumulant expansion for the collective operators. By truncating the subradiant collective modes with a heavily suppressed decay rate and keeping only the effect from the radiating collective modes, we reduce the numerical complexity significantly. This allows to efficiently compute the dissipative dynamics of the observables of interest for a linear, ring-shaped and planar arrays of quantum emitters. In particular, we characterize the excited population, the total photon emission rate and the second order intensity correlation function $g^{(2)}(\tau =0)$, which are challenging to compute for large systems with traditional cumulant expansion methods based on the individual spins.

quant-ph↗

arXiv2024

Efficient excitation transfer in an LH2-inspired nanoscale stacked ring geometry

Subwavelength ring-shaped structures of quantum emitters exhibit outstanding radiation properties and are useful for antennas, excitation transport, and storage. Taking inspiration from the oligomeric geometry of biological light-harvesting 2 (LH2) complexes, we study here generic examples and predict highly efficient excitation transfer in a three-dimensional (3D) subwavelength concentric stacked ring structure with a diameter of 400 $nm$, formed by two-level atoms. Utilizing the quantum optical open system master equation approach for the collective dipole dynamics, we demonstrate that, depending on the system parameters, our bio-mimicked 3D ring enables efficient excitation transfer between two ring layers. Our findings open prospects for engineering other biomimetic light-matter platforms and emitter arrays to achieve efficient energy transfer.

physics.optics↗

arXiv2024

Self-ordering, cooling, and lasing in an ensemble of clock atoms

Active atomic clocks are predicted to provide far better short-term stability and robustness against thermal fluctuations than typical feedback-based optical atomic clocks. However, continuous laser operation using an ensemble of clock atoms still remains an experimentally challenging task. We study spatial self-organization in a transversely driven ensemble of clock atoms inside an optical resonator and coherent light emission from the cavity. We focus on the spectral properties of the emitted light in the narrow atomic linewidth regime, where the phase coherence providing frequency stability is stored in the atomic dipoles rather than the cavity field. The atoms are off-resonantly driven by a standing-wave coherent laser transversely to the cavity axis allowing for atomic motion along the cavity axis as well as along the pump. In order to treat larger atom numbers we employ a second-order cumulant expansion which allows us to calculate the spectrum of the cavity light field. We identify the self-organization threshold where the atoms align themselves in a checkerboard pattern, thus maximizing light scattering into the cavity, which simultaneously induces cooling. For a larger driving intensity, more atoms are transferred to the excited state, reducing cooling but increasing light emission from the excited atoms. This can be enhanced via a second cavity mode at the atomic frequency spatially shifted by a quarter wavelength. For large enough atom numbers we observe laser-like emission close to the bare atomic transition frequency.

quant-ph↗

arXiv2024

Collectively enhanced ground-state cooling in subwavelength atomic arrays

Subwavelength atomic arrays feature strong light-induced dipole-dipole interactions, resulting in subradiant collective resonances characterized by narrowed linewidths. In this work, we present a sideband cooling scheme for atoms trapped in subwavelength arrays that utilizes these narrow collective resonances. Working in the Lamb-Dicke regime, we derive an effective master equation for the atomic motion by adiabatically eliminating the internal degrees of freedom of the atoms, and validate its prediction with numerical simulations of the full system. Our results demonstrate that subradiant resonances enable the cooling of ensembles of atoms to temperatures lower than those achievable without dipole interactions, provided the atoms have different trap frequencies. Remarkably, narrow collective resonances can be sideband-resolved even when the individual atomic transition is not. In such scenarios, ground-state cooling becomes feasible solely due to light-induced dipole-dipole interactions. This approach could be utilized for future quantum technologies based on dense ensembles of emitters, and paves the way towards harnessing many-body cooperative decay for enhanced motional control.

quant-ph↗

arXiv2024

Subradiance and Superradiant Long Range Excitation Transport among Quantum Emitter Ensembles in a Waveguide

In contrast to free space, in waveguides the dispersive and dissipative dipole-dipole interactions among quantum emitters exhibit a periodic behavior over remarkably long distances. We propose a novel setup exploiting this long-range periodicity in order to create highly excited subradiant states and facilitate fast controlled collective energy transport amongst far-apart ensembles coupled to a waveguide. For sufficiently large ensembles collective superradiant emission into the fiber modes dominates over its free space counterpart. We show that for a large number of emitters a fast transverse coherent pulse can create almost perfect subradiant states with up to $50\%$ excitation. On the other hand, for a coherent excitation of one sub-ensemble above an overall excitation fraction of $50\%$ we find a nearly lossless and fast energy transfer to the ground state sub-ensemble. This transport can be enhanced or suppressed by controlling the positions of the ensembles relative to each other, while it can also be realized with a random position distribution. In the optimally enhanced case this fast transfer appears as superradiant emission with subsequent superabsorption, yet, without a superradiant decay after the absorption. The highly excited subradiant states as well as the superradiant excitation transfer appear as suitable building blocks in applications like active atomic clocks, quantum batteries, quantum information protocols and quantum metrology procedures such as fiber-based Ramsey schemes.

quant-ph↗

arXiv2024

Nonequilibrium Nonlinear Effects and Dynamical Boson Condensation in a Driven-Dissipative Wannier-Stark Lattice

Driven-dissipative light-matter systems can exhibit collective nonequilibrium phenomena due to loss and gain processes on the one hand and effective photon-photon interactions on the other hand. As generic example we study a bosonic lattice system implemented via an array of driven-dissipative coupled nonlinear resonators with linearly increasing resonance frequencies across the lattice. The model also describes a driven-dissipative Bose-Hubbard model in a tilted potential without a particle-conservation constraint. We numerically predict a diverse range of stationary and non-stationary states resulting from the interplay of the tilt, tunneling, on-site interactions and loss and gain processes. Our key finding is that, under weak on-site interactions, the bosons mostly condense into a selected, single-particle Wannier-Stark state without exhibiting the expected Bloch oscillations. As the strength of the onsite interactions increase, a non-stationary regime emerges which, surprisingly, exhibits periodic Bloch-type oscillations. As a direct consequence of the driven-dissipative nature of the system we predict a highly nontrivial phase diagram including regular oscillating as well as chaotic dynamical regimes. While a straightforward photonic implementation using microwave or optical modes is possible, such dynamics might also be observable for an ultracold gas in a vertical lattice with gravity or a tilted external potential.

quant-ph↗

arXiv2023

Combining critical and quantum metrology

Critical metrology relies on the precise preparation of a system in its ground state near a quantum phase transition point where quantum correlations get very strong. Typically this increases the quantum Fisher information with respect to changes in system parameters and thus improves the optimally possible measurement precision limited by the Cram\'er-Rao bound. Hence critical metrology involves encoding information about the unknown parameter in changes of the system's ground state. Conversely, in conventional metrology methods like Ramsey interferometry, the eigenstates of the system remain unchanged, and information about the unknown parameter is encoded in the relative phases that excited system states accumulate during their time evolution. Here we introduce an approach combining these two methodologies into a unified protocol applicable to closed and driven-dissipative systems. We show that the quantum Fisher information in this case exhibits an additional interference term originating from the interplay between eigenstate and relative phase changes. We provide analytical expressions for the quantum and classical Fisher information in such a setup, elucidating as well a straightforward measurement approach that nearly attains the maximum precision permissible under the Cram\'er-Rao bound. We showcase these results by focusing on the squeezing Hamiltonian, which characterizes the thermodynamic limit of Dicke and Lipkin-Meshkov-Glick Hamiltonians.

quant-ph↗

arXiv2023

Harnessing quantum emitter rings for efficient energy transport and trapping

Efficient transport and harvesting of excitation energy under low light conditions is an important process in nature and quantum technologies alike. Here we formulate a quantum optics perspective to excitation energy transport in configurations of two-level quantum emitters with a particular emphasis on efficiency and robustness against disorder. We study a periodic geometry of emitter rings with subwavelength spacing, where collective electronic states emerge due to near-field dipole-dipole interactions. The system gives rise to collective subradiant states that are particularly suited to excitation transport and are protected from energy disorder and radiative decoherence. Comparing ring geometries with other configurations shows that that the former are more efficient in absorbing, transporting, and trapping incident light. Because our findings are agnostic as to the specific choice of quantum emitters, they indicate general design principles for quantum technologies with superior photon transport properties and may elucidate potential mechanisms resulting in the highly efficient energy transport efficiencies in natural light-harvesting systems.

quant-ph↗