Photon Condensation and Enhanced Magnetism in Cavity QED

A cavity-QED quantum simulator of dynamical phases of a BCS superconductor

We propose to simulate dynamical phases of a BCS superconductor using an ensemble of cold atoms trapped in an optical cavity. Effective Cooper pairs are encoded via internal states of the atoms and attractive interactions are realized via the exchange of virtual photons between atoms coupled to a common cavity mode. Control of the interaction strength combined with a tunable dispersion relation of the effective Cooper pairs allows exploration of the full dynamical phase diagram of the BCS model, as a function of system parameters and the prepared initial state. Our proposal paves the way for t…

On the phase transition of light in cavity QED lattices

Systems of strongly interacting atoms and photons, that can be realized wiring up individual cavity QED systems into lattices, are perceived as a new platform for quantum simulation. While sharing important properties with other systems of interacting quantum particles here we argue that the nature of light-matter interaction gives rise to unique features with no analogs in condensed matter or atomic physics setups. By discussing the physics of a lattice model of delocalized photons coupled locally with two-level systems through the elementary light-matter interaction described by the Rabi mod…

Photon pair condensation by engineered dissipation

Dissipation can usually induce detrimental decoherence in a quantum system. However, engineered dissipation can be used to prepare and stabilize coherent quantum many-body states. Here, we show that by engineering dissipators containing photon pair operators, one can stabilize an exotic dark state, which is a condensate of photon pairs with a phase-nematic order. In this system, the usual superfluid order parameter, i.e. single-photon correlation, is absent, while the photon pair correlation exhibits long-range order. Although the dark state is not unique due to multiple parity sectors, we dev…

Effective theory for matter in non-perturbative cavity QED

Starting from a general material system of $N$ particles coupled to a cavity, we use a coherent-state path integral formulation to produce a non-perturbative effective theory for the material degrees of freedom. We tackle the effects of image charges, the $A^2$ term and a multimode arbitrary-geometry cavity. The resulting (non-local) action has the photonic degrees of freedom replaced by an effective position-dependent interaction between the particles. In the large-$N$ limit, we discuss how the theory can be cast into an effective Hamiltonian where the cavity induced interactions are made exp…

Cavity QED with a Bose-Einstein condensate

Cavity quantum electrodynamics (cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure, and is providing a useful platform for developing concepts in quantum information processing. By using high-quality resonators, a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in. This has led to fundamental studies with both microwave and optical resonators. To meet the challenges posed by quantum state engineering and…

Duality Symmetry of Quantum Electrodynamics and Magnetic Charges

The duality symmetry between electricity and magnetism hidden in classical Maxwell equations suggests the existence of dual charges, which have usually been interpreted as magnetic charges and have not been observed in experiments. In quantum electrodynamics (QED), both the electric and magnetic fields have been unified into one gauge field, which makes this symmetry inconspicuous. Here, we recheck the duality symmetry of QED by introducing a dual gauge field and a dual symmetric Lagrangian. Within the framework of gauge-field theory, we show that the electric-magnetic duality symmetry cannot …

The Interspersed Spin Boson Lattice Model

We describe a family of lattice models that support a new class of quantum magnetism characterized by correlated spin and bosonic ordering [Phys. Rev. Lett. 112, 180405 (2014)]. We explore the full phase diagram of the model using Matrix-Product-State methods. Guided by these numerical results, we describe a modified variational ansatz to improve our analytic description of the groundstate at low boson frequencies. Additionally, we introduce an experimental protocol capable of inferring the low-energy excitations of the system by means of Fano scattering spectroscopy. Finally, we discuss the i…

Coupling quantum circuits to magnetic molecular qubits

This thesis explores the coupling of magnetic systems to quantum circuits in the context of quantum computing applications. In particular we study the coupling of superconducting coplanar waveguide resonators to Single Molecule Magnets (SMMs) . The combination of approaches from the fields of Cavity Quantum electrodynamics (QED) and Circuit QED with those from the field of molecular magnetism con provide unique opportunities for quantum computing. We investigate the necessary conditions for coupling single spins and spin ensembles to resonators and what characteristics SMMs should have in orde…

腔莫尔材料：利用量子光-物质相互作用控制磁阻挫

Cavity Moiré Materials: Controlling Magnetic Frustration with Quantum Light-Matter Interaction

该研究提出腔莫尔材料理论，证明腔 QED 中的电磁真空修饰可改变电子能与跃迁，并在扭转 TMD 异质双层中实现对磁阻挫及量子自旋液体等相的调控。

Dipolar Spin Models with Arrays of Superconducting Qubits

We propose a novel platform for quantum many body simulations of dipolar spin models using current circuit QED technology. Our basic building blocks are 3D Transmon qubits where we use the naturally occurring dipolar interactions to realize interacting spin systems. This opens the way toward the realization of a broad class of tunable spin models in both two- and one-dimensional geometries. We illustrate the potential offered by these systems in the context of dimerized Majumdar-Ghosh-type phases, archetypical examples of quantum magnetism, showing how such phases are robust against disorder a…

强相互作用里德堡气体中的耗散时间晶体

Dissipative time crystal in a strongly interacting Rydberg gas

本研究在室温里德堡原子气体中实验实现了耗散时间晶体，通过光子透射的持续振荡、非衰减自相关及对噪声的鲁棒性，证明了连续时间晶体的长程时间序。

含物质场的$\u2124_N$格点规范理论

$\mathbb{Z}_N$ lattice gauge theories with matter fields

该研究针对含物质场的$\u2124_N$格点规范理论，提出严格可解模型并分析其基态与激发特性，通过微扰研究相转变，指出有限$N$的$\u2124_N$量子模拟器仅能近似求解中等尺度的格点QED问题。