Collective dissipation can generate useful quantum correlations, while ubiquitous individual decay destroys them. We study the interplay between these two competing processes considering a driven system of many spins (``atoms") undergoing both collective and individual dissipation (``radiation"). In steady state and depending on drive, we find that the system exhibits a first-order phase transition and quantum bistability: its quantum state is a mixture of two many-body states associated with the two competing decay processes. Accordingly, one of these states closely resembles a correlated ``coherently radiating spin state" (CRSS) -- the solution of purely collective dissipation -- exhibiting spin-squeezing entanglement. We predict dynamical switching between the two stable states, manifest as many-body quantum jumps in the various observables of spin and radiation. Macroscopically, the switching rate tends to vanish and the system can reside in a correlated CRSS for long times. This reveals how correlated dissipative physics emerges at the presence of decorrelating individual decay, opening a path for unlocking collective dissipation phenomena in realistic quantum platforms and applications. We discuss consequences for experiments in collective radiation.
Collective dissipation can generate useful quantum correlations, while ubiquitous individual decay destroys them. We study the interplay between these two competing processes considering a driven system of many spins (atoms") undergoing both collective and individual dissipation (radiation"). In steady state and depending on drive, we find that the system exhibits a first-order phase transition and quantum bistability: its quantum state is a mixture of two many-body states associated with the two competing decay processes. Accordingly, one of these states closely resembles a correlated ``coherently radiating spin state" (CRSS) -- the solution of purely collective dissipation -- exhibiting spin-squeezing entanglement. We predict dynamical switching between the two stable states, manifest as many-body quantum jumps in the various observables of spin and radiation. Macroscopically, the switching rate tends to vanish and the system can reside in a correlated CRSS for long times. This reveals how correlated dissipative physics emerges at the presence of decorrelating individual decay, opening a path for unlocking collective dissipation phenomena in realistic quantum platforms and applications. We discuss consequences for experiments in collective radiation.
Quantum mechanics is fundamentally governed by the Schrödinger equation, which is linear in the wave function. Nevertheless, quantum systems can exhibit highly nonlinear behavior in their observable dynamics. Such nonlinearities can emerge from many-body interactions, nonlinear coupling mechanisms, or the interplay between coherent evolution and dissipation. In the collective limit, microscopic quantum interactions can give rise to qualitatively new macroscopic behavior. Open quantum systems provide a rich setting for such phenomena, as environmental coupling introduces dissipation and fluctuations that compete with coherent dynamics and can alter the steady states and dynamical phases of a system. Combined with nonlinear interactions, this competition can produce behavior with no direct counterpart in conventional linear light--matter systems. In this thesis, I investigate such collective phenomena through nonlinear two-photon light--matter interactions and engineered quantum reservoirs. I first investigate the driven--dissipative two-photon Dicke model and demonstrate the emergence of a dissipative superradiant phase transition. The quadratic light--matter interaction introduces distinct stability requirements in the presence of dissipation. In particular, two-photon loss stabilizes the superradiant phase, allowing its steady states and critical behavior to be characterized. I next investigate photon blockade and transient quadrature-squeezed radiation in the two-photon Tavis--Cummings model. I show that two-photon interactions collectively enhance photon blockade and squeezing, in contrast to the conventional one-photon model, where the effective nonlinearity diminishes in the collective limit. Finally, I consider collective behavior induced by reservoir engineering. Two spatially separated open quantum systems without direct coherent interaction can undergo a nonlocal quantum phase transition when coupled to correlated quantum reservoirs. Remarkably, the transition is hidden from local observables and emerges only through nonlocal correlations and collective observables. Together, these results demonstrate how nonlinear interactions and engineered dissipation can reshape collective behavior in open quantum systems.
When atoms are excited to high-lying Rydberg states they interact strongly with dipolar forces. The resulting state-dependent level shifts allow us to study many-body systems displaying intriguing nonequilibrium phenomena, such as constrained spin systems, and are at the heart of numerous technological applications, e.g., in quantum simulation and computation platforms. Here, we show that these interactions also have a significant impact on dissipative effects caused by the inevitable coupling of Rydberg atoms to the surrounding electromagnetic field. We demonstrate that their presence modifies the frequency of the photons emitted from the Rydberg atoms, making it dependent on the local neighborhood of the emitting atom. Interactions among Rydberg atoms thus turn spontaneous emission into a many-body process which manifests, in a thermodynamically consistent Markovian setting, in the emergence of collective jump operators in the quantum master equation governing the dynamics. We discuss how this collective dissipation-stemming from a mechanism different from the much studied superradiance and subradiance-accelerates decoherence and affects dissipative phase transitions in Rydberg ensembles.
We theoretically investigate a possibility to establish multi-qubit quantum correlations in one-dimensional chains of qubits. We combine a reservoir engineering strategy with coherent dynamics to generate multi-qubit entangled states. We find that an interplay between the coherent and incoherent dynamics result in the generation of stable (time-independent) many-body entangled steady states. Our results will be relevant in the context of the dissipative generation of quantum states, with applications in short-distance quantum computation and for exploring the emergence of collective phenomena in many-body open quantum systems.
When multiple quantum emitters couple to a common electromagnetic environment, interference in their collective radiative dynamics gives rise to superradiance and subradiance. In regimes where coherent interactions and collective dissipation compete, the microscopic many-body dynamics and quantum correlations among the emitters that underlie superradiance and subradiance are theoretically challenging and remain experimentally elusive, even though collective emission has been observed in many physical systems. Here, we realize a superconducting qubit array coupled to a common microwave waveguide that mediates collective dissipation, with simultaneous access to coherent interactions and microscopic measurements of many-body dynamics. Engineered qubit-waveguide couplings with tunable amplitude and phase enable control of collective interference and the resulting super- and subradiant states. Leveraging site-resolved control and readout, we directly observe the microscopic decay dynamics of multi-qubit states across different excitation manifolds and track the evolution of populations and tunable quantum correlations. We reveal collective decay in regimes beyond the ideal Dicke model, where strong qubit-qubit interactions stabilize superradiance and subradiance against local dephasing and reshape decay pathways through spatially and spectrally structured many-body eigenstates. Our results establish a flexible platform for exploring collective phenomena in many-body quantum optics and driven-dissipative approaches to robust quantum information processing.
We study the dynamics of lattice models of quantum spins one-half, driven by a coherent drive and subject to dissipation. Generically the mean-field limit of these models manifests multistable parameter regions of coexisting steady states with different magnetizations. We introduce an efficient scheme accounting for the corrections to mean field by correlations at leading order, and benchmark this scheme using high-precision numerics based on matrix-product operators in one- and two-dimensional lattices. Correlations are shown to wash the mean-field bistability in dimension one, leading to a unique steady state. In dimension two and higher, we find that multistability is again possible, provided the thermodynamic limit of an infinitely large lattice is taken first with respect to the longtime limit. Variation of the system parameters results in jumps between the different steady states, each showing a critical slowing down in the convergence of perturbations towards the steady state. Experiments with trapped ions can realize the model and possibly answer open questions in the nonequilibrium many-body dynamics of these quantum systems, beyond the system sizes accessible to present numerics.
We investigate the fate of dissipative phase transitions in quantum many-body systems when the individual constituents are qudits (d-level systems) instead of qubits. As an example system, we employ a permutation- invariant XY model of N infinite-range interacting d-level spins (Lipkin-Meshkov-Glick model) undergoing individual and collective dissipation. In the mean-field limit, we identify a dissipative phase transition, whose critical point is independent of d after a suitable rescaling of parameters. When the decay rates between all adjacent levels are identical and d ≥ 4, the critical point expands, in terms of the ratio between dissipation and interaction strengths, to a critical region in which two phases coexist and which increases as d grows. In addition, a larger d leads to a more pronounced change in spin expectation values at the critical point. Numerical investigations for finite N reveal symmetry breaking signatures in the Liouvillian spectrum at the phase transition. The phase transition is furthermore marked by maximum entanglement negativity and a significant purity change of the steady state, which become more pronounced as d increases. Considering qudits instead of qubits thus opens new perspectives on accessing rich phase diagrams in open many-body systems.