Session W52: Invited Session: The Upside of Noise: Engineered Dissipation in Quantum Many-Body Physics and Quantum Computing

Preparation and measurement of strongly interacting states of photons

Photons has been considered as a promising medium to implement quantum simulators. However, most phenomena that are interesting from quantum simulation perspective involve thermalization and a controllable chemical potential, as a key parameter in phase diagrams, which are both absent for photons. More specifically, on the one hand, photonic systems are dissipative which means that the chemical potential is zero, and on the other hand, due to the weakness of inelastic scatterings, photons do not naturally thermalize. I will discuss various externally driven schemes to prepare manybody states of photons in the presence of dissipation. In fact, such driven-dissipative nature of these systems is the crucial reason of their interest. Specifically, I investigate driven fractional quantum Hall and Bose-Hubbard models. Furthermore, I describe how to characterize and measure various manybody features of correlated states of photons.   

Confining the state of light to a quantum manifold by engineered two-photon loss

Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.   

Blueprint for an analog quantum code fabric

A physical realization of self correcting quantum code would be profoundly useful for constructing a quantum computer. In this theoretical talk, we provide a partial solution to major challenges preventing self correcting quantum code from being engineered in realistic devices. We consider a variant of Kitaev's toric code coupled to propagating bosons, which induce a long-ranged interaction between anyonic defects. By coupling the primary quantum system to an engineered dissipation source through resonant energy transfer, we demonstrate a ``rate barrier" which leads to a potentially enormous increase in the system's quantum state lifetime through purely passive quantum error correction, even when coupled to an infinite temperature bath. While our mechanism is not scalable to infinitely large systems, the maximum effective size can be very large, and it is fully compatible with active error correction schemes. Our model uses only on-site and nearest-neighbor interactions, and could be implemented in superconducting qubits.   

Topology by Dissipation in Atomic Fermion Systems

Controlled dissipation can be used as a resource to drive a many-body system into quantum mechanically ordered states from an arbitrary initial one. We discuss this concept in the context of atomic fermions, highlighting a dissipatively induced pairing mechanism, which is operative in the absence of attractive forces. We show how this targeted cooling can be utilized to cool atomic fermions into topologically non-trivial states in one dimension by quasi-local dissipative operations, and point out a possible physical implementation. This realizes a dissipative analog of the ground state of Kitaev's quantum wire. In higher dimensions, the analogy to Hamiltonian ground states breaks down due to a fundamental incompatibility of topology and locality. We present a new quasi-local dissipative mechanism for the preparation of Chern insulators, which bypasses these obstacles by making use of the intrinsic open system character of the preparation process, with no Hamiltonian counterpart. This greatly extends the scope of efficiently attainable topological symmetry classes via tailored dissipation.   

Real-time observation of fluctuations in a driven-dissipative quantum many-body system undergoing a phase transition

A Bose-Einstein condensate whose motional degrees of freedom are coupled to a high-finesse optical cavity via a transverse pump beam constitutes a dissipative quantum many-body system with long range interactions. These interactions can induce a structural phase transition from a flat to a density-modulated state. The transverse pump field simultaneously represents a probe of the atomic density via cavity- enhanced Bragg scattering. By spectrally analyzing the light field leaking out of the cavity, we measure non-destructively the dynamic structure factor of the fluctuating atomic density while the system undergoes the phase transition. An observed asymmetry in the dynamic structure factor is attributed to the coupling to dissipative baths. Critical exponents for both sides of the phase transition can be extracted from the data. We further discuss our progress in adding strong short-range interactions to this system, in order to explore Bose-Hubbard physics with cavity-mediated long-range interactions and self-organization in lower dimensions.