Nowadays, dissipative phase transition (DPT) attracts a great deal of interest from the physics and quantum optics community. It is characterized by abrupt changes of the physical observables on the parameters of the system, such as a dissipation and drive rates as well as coupling constants. The increased attention to the DPT roots in significant development of a new research area of quantum optics, known as reservoir engineering, which allows to obtain a wide range of controllable nonequilibrium quantum systems. A prime example of such a system, which we will study here, is an optical oscillator with two-photon drive and two-photon dissipation (Fig 1(a)). The immediate practical application of such a model is the qubit platform based on the quantum Schrödinger cat states which is applicable for universal quantum computation [1]. However, the quantum and dissipative nature of such a system significantly complicates its consideration beyond the mean-field description. In statistical physics, one can apply the well-developed Landau theory, which is used for a wide range of equilibrium phenomena. Here, we rigorously demonstrate that the nonequilibrium DPT that arises in an optical oscillator with two-photon drive and two-photon dissipation can be mapped onto a nonlinear classical oscillator in a colored-noise environment in regions distant from the critical point. Furthermore, using Fokker-Plank approach, we develop a description of the quantum critical region near the critical point, where strong quantum fluctuations destroy mean-field treatment.
Nowadays, dissipative phase transition (DPT) attracts a great deal of interest from the physics and quantum optics community. It is characterized by abrupt changes of the physical observables on the parameters of the system, such as a dissipation and drive rates as well as coupling constants. The increased attention to the DPT roots in significant development of a new research area of quantum optics, known as reservoir engineering, which allows to obtain a wide range of controllable nonequilibrium quantum systems. A prime example of such a system, which we will study here, is an optical oscillator with two-photon drive and two-photon dissipation (Fig 1(a)). The immediate practical application of such a model is the qubit platform based on the quantum Schrödinger cat states which is applicable for universal quantum computation [1]. However, the quantum and dissipative nature of such a system significantly complicates its consideration beyond the mean-field description. In statistical physics, one can apply the well-developed Landau theory, which is used for a wide range of equilibrium phenomena. Here, we rigorously demonstrate that the nonequilibrium DPT that arises in an optical oscillator with two-photon drive and two-photon dissipation can be mapped onto a nonlinear classical oscillator in a colored-noise environment in regions distant from the critical point. Furthermore, using Fokker-Plank approach, we develop a description of the quantum critical region near the critical point, where strong quantum fluctuations destroy mean-field treatment.
Driven-dissipative phase transition in an open quantum systems nowdays is among the most rapidly developing fields of quantum optics. An example of such a system is depicted in Fig. 1 (a) , which shows a superconducting microwave cavity undergoing two-photon driving at g rate as well as two-photon dissipation at γ rate [1] . In this system, the presence of the two-photon dissipation does not destroy, but stabilizes the quantum state, also known as Schrödinger cat state, and it is possible to prepare a dynamically protected qubit on its basis [2] . Numerical methods are commonly applied to study such quantum systems. However, their use typically does not allow to uncover physical phenomena underlying the evolution of the system. In the limit of a large number of photons, a qualitative picture of the occurring phenomena can be obtained using the so-called semiclassical approximation. Nevertheless, this approach completely neglects the quantum fluctuations, which can play a significant role, even in the case of large average number of photons in the system.
Due to the variety of tools available to control atomic, molecular, and optical (AMO) systems, they provide a versatile platform for studying many-body physics, quantum simulation, and quantum computation. Although extensive efforts are employed to reduce coupling between the system and the environment, the effects of the environment can never fully be avoided, so it is important to develop a comprehensive understanding of open quantum systems. The system-environment coupling often leads to loss via dissipation, which can be countered by a coherent drive. Open quantum systems subject to dissipation and drive are known as driven-dissipative systems, and they provide an excellent platform for studying many-body nonequilibrium physics. The first part of this dissertation will focus on driven-dissipative phase transitions. Despite the nonequilibrium nature of these systems, the corresponding phase transitions tend to exhibit emergent equilibrium behavior. However, we will show that in the vicinity of a multicritical point where multiple phase transitions intersect, genuinely nonequilibrium criticality can emerge, even though the individual phase transitions on their own exhibit equilibrium criticality. These nonequilibrium multicritical points can exhibit a variety of exotic phenomena not possible for their equilibrium counterparts, including the emergence of complex critical exponents, which lead to discrete scale invariance and spiraling phase boundaries. Furthermore, the Liouvillian gap can take on complex values, and the fluctuation-dissipation theorem is violated, corresponding to an effective temperature which gets "hotter" and "hotter" at longer and longer wavelengths. The second part of this dissertation will focus on Rydberg atoms. In particular, we study how the spontaneous generation of contaminant Rydberg states drastically modifies the behavior of a driven-dissipative Rydberg system due to the resultant dipole-dipole interactions. These interactions lead to a complicated competition of both blockade and anti-blockade effects, leading to strongly enhanced Rydberg populations for far-detuned drive and reduced Rydberg populations for resonant drive.
In open quantum systems, dissipative phase transitions (DPTs) emerge from the interplay between unitary evolution, drive, and dissipation. While second-order DPTs have been predominantly investigated theoretically, first-order DPTs have been observed in single-photon-driven Kerr resonators. We present here an experimental and theoretical analysis of both first and second-order DPTs in a two-photon-driven superconducting Kerr resonator. We characterize the steady state at the critical points, showing squeezing below vacuum and the coexistence of phases with different photon numbers. Through time resolved measurements, we study the dynamics across the critical points and observe hysteresis cycles at the first-order DPT and spontaneous symmetry breaking at the second-order DPT. Extracting the timescales of the critical phenomena reveals slowing down across five orders of magnitude when scaling towards the thermodynamic limit. Our results showcase the engineering of criticality in superconducting circuits, advancing the use of parametric resonators for critically-enhanced quantum information applications.
Open quantum systems exhibit a range of novel out-of-equilibrium behavior due to the interplay between coherent quantum dynamics and dissipation. Of particular interest in these systems are driven, dissipative transitions, the emergence of dynamical phases with novel broken symmetries, and critical behavior that lies beyond the conventional paradigm of Landau-Ginzburg phenomenology. Here, we consider a parametrically driven two-mode system in the presence of non-Markovian system-reservoir interactions. We show that the non-Markovian dynamics modifies the phase diagram of this system, resulting in the emergence of a broken symmetry phase in a universality class that has no counterpart in the corresponding Markovian system. This emergent phase is accompanied by enhanced two-mode entanglement that remains robust at finite temperatures. Such reservoir-engineered dynamical phases can potentially shed light on universal aspects of dynamical phase transitions in a wide range of nonequilibrium systems, and aid in the development of techniques for the robust generation of entanglement and quantum correlations at finite temperatures with potential applications to quantum control, state preparation, and metrology.
The physics of systems out of equilibrium is a topic of great interest, mainly due to the possibility of exploring phenomena that can not be observed in equilibrium systems. Driven-dissipative phase transitions open the opportunity of studying phases with no classical counterparts, and these can be experimentally realized in quantum optical platforms. Since these transitions occur in systems kept out of equilibrium, they are characterized by a finite entropy production rate. However, due to technical difficulties regarding the zero temperature limit and the non-gaussianity of such models, very little is known about how entropy production behaves around criticality. Using a quantum phase-space method, based on the Husimi Q-function, we put forth a framework that allows for the complete characterization of the entropy production in driven-dissipative transitions. This new theoretical framework is tailored specifically to describe photon loss dissipation, which is effectively a zero temperature process for which the standard theory of entropy production breaks down. It makes no assumptions about Gaussianity about the model or the state. It works for both, steady-states as well as the dynamics and as an application, we study both situations in the paradigmatic driven-dissipative Kerr model, which presents a discontinuous phase transition. For general driven-dissipative critical systems, where one can define a thermodynamic limit, we find that the entropy production rate and flux naturally split into two contributions: an extensive one and a contribution due to quantum fluctuations only. Moreover, we identify a contribution to the entropy production due to unitary dynamics, and we find that the behavior of this contribution at the non-equilibrium steady-state (NESS) matches the behavior of entropy production rate observed in classical systems. The quantum contributions are found to diverge at the critical point.
We investigate nonequilibrium criticality and chaos in a two-photon-driven optomechanical system. The parametric drive preserves a discrete Z2 symmetry of the optical field, while radiation-pressure coupling transfers the resulting nonlinear dynamics to a mechanical oscillator. Combining semiclassical stability analysis, exact Liouvillian spectra, and stochastic quantum trajectories, we show that this driven-dissipative optomechanical model supports both first- and second-order dissipative phase transitions. At negative detuning a second-order transition yields spontaneous breaking of the cavity-parity symmetry in the thermodynamic limit. At positive detuning the same symmetry breaking coexists with a first-order transition, signaled by metastability and by an additional symmetric Liouvillian mode. At stronger pump power the mean-field dynamics loses all stable fixed points and develops limit cycles and chaotic attractors with positive Lyapunov exponent. Quantum trajectories in this regime display chaotic-like motion, enhanced steady-state entropy, and delocalization over many entropic Liouvillian modes. These results establish two-photon-driven optomechanics as a platform where dissipative criticality, symmetry breaking, and quantum signatures of chaos emerge within the same experimentally accessible setting.