Session P07: Emerging Quantum Systems

Cavity Induced Topology in Graphene

Strongly coupling materials to cavity modes can affect their electronic properties altering the phases of matter. We study a setup where graphene electrons are coupled to chiral photons with both left and right circularly polarized photons, and time-reversal symmetry is broken due to a phase shift between them. We develop a many-body perturbative theory, and derive cavity mediated electronic interactions induced in graphene. This theory leads to a gap equation which predicts a sizable topological band gap at Dirac nodes in vacuum and when the cavity is prepared in an excited Fock state. Remarkably, band gaps also open in light-matter hydbrization points away from the Dirac nodes giving rise to topological photo-electron bands with high Chern numbers. We reveal that the physical mechanism behind this phenomenon lies on the exchange of chiral photons with electronic matter at the hybridization points, and the number and polarization of exchanged photons determine the Chern number. This is a generic microscopic mechanism for the photo-electron band topology. Our theory shows that graphene-based materials, with no need of Floquet engineering and hence protected from the heating effects, host high Chern insulator phases when coupled to chiral cavity fields.   

Degenerate Multimode Cavities for Topological Quantum Optics with Rydberg Polaritons

Quantum simulation experiments provide a unique opportunity to realize, manipulate, and understand fractional Quantum Hall states with a high degree of controllability. 

Initial progress towards this long-standing goal has been demonstrated with our realization of a two-photon Laughlin state [1] and of a two-atom Laughlin state in an optical lattice [2], stimulating the quest to prepare bigger systems where true many-body properties, like exotic anyonic excitations, are accessible. 

Our experimental platform hybridizes photons in a twisted optical cavity with atomic Rydberg excitations to create a system of itinerant, strongly interacting particles in an artificial gauge field.

We will present first results using our aspheric lens-based degenerate twisted  multimode cavity realizing a lowest Landau level for a mesoscopic number of photons. With increasing Rydberg interaction strength, we can access a series of interesting states from the mean field regime towards strongly correlated states such as the Laughlin state. Combined with new imaging capabilities, we show the first data of spatially-resolved interacting polaritons on the way to larger Quantum Hall states.

[1] Logan W Clark, Nathan Schine, Claire Baum, Ningyuan Jia and Jonathan Simon, "Observation of Laughlin states made of light" Nature 582, 41-45, (2020)

[2] Léonard, J., Kim, S., Kwan, J. et al. Realization of a fractional quantum Hall state with ultracold atoms. Nature 619, 495–499 (2023)   

Strong Coupling between Warm Rubidium Vapor and Microcavities on an Integrated Photonic Chip

Second-generation quantum technologies in quantum information processing, quantum communication, and quantum sensing require the ability to coherently control the interaction between light and matter. We have developed a practical and scalable platform interfacing warm rubidium vapor with an integrated photonic microcavity to realize the strong coupling regime of cavity QED. We observed normal mode splitting, signaling the hybridization of a microring cavity mode with rubidium D-lines at a vapor temperature of 100 ^oC, which is equivalent to ~ 50 atoms interacting with the cavity mode (on average). We will present further studies of various cavity geometries such as slot-rings and photonic crystal rings that maximize the fraction of the cavity mode that interacts with the vapor as well as reduce the cavity mode volume, which together pave the way towards strongly-coupled single atom-single photon operation.   

Optomechanical Dicke phase transition with 20 tweezed atoms

We report on the realization of a mesoscopic, optomechanical Dicke phase transition using a 1D tweezer array in an optical cavity. We use the tweezers to place up to 20 atoms precisely on the nodes of the cavity field at a spacing of 5.5 wavelengths and expose them to a transverse pump field. Above a critical pump strength, the atoms self-organize onto the cavity antinodes with the same phase, leading to a bifurcation with Z₂ symmetry. Using heterodyne detection, we observe the Dicke bifurcation in the cavity field phase as a function of atom number, pump power, and detunings of the atoms and pump to the cavity. We measure the mechanical susceptibility of this system by introducing a bias in the tweezer positions and show that the susceptibility diverges when the system is self-organized. Lastly, we examine the switching dynamics of the cavity field phase due to atom thermal motion.   

Correlated quantum states of light generated by using phase modulated vacuum fields

Propagation of quantum field interacting with single two-level or three-level atoms has been studied. Using the Gaussian quantum mode functions, we calculate evolution of the quantum state that includes atomic and field variable. We demonstrated the phase acquired by the single photon propagation that can be of great importance for long quantum communications. The results can be used for controlling quantum field propagation, and for design of optical elements such as a quantum prism and a quantum lens. 

We consider a Lambda-type three-level atom in a QED cavity. One atomic transition is driven by a classical field and the other transition is driven by the "vacuum" field. The vacuum field can be strongly modified by the cavity, and in particular, the "vacuum" field can have a chirped frequency modulation that it can be a part of adiabatic rapid passage together with classical drive field. The action of the classical and "vacuum" or quantum field can result in the generation of the quantum fields with controllable parameters. Such QED cavity can be used to generate strongly correlated quantum fields that can be of interest for quantum sensing, spectroscopy at the single photon level, quantum teleportation, and other applications for quantum information.    

Single-photon transport properties in periodic and position-disordered coupled atom-cavity arrays

We analyze the transport properties of single photons in one-dimensional lattices of a waveguide-coupled atom-cavity system [arXiv:2401.15231]. By investigating various parameter regimes, we find that when atoms are decoupled from the cavities or are coupled (either weakly or strongly), the transmission and reflection spectra exhibit the formation of a frequency comb pattern. In particular, in the strong coupling regime, these frequency combs are superimposed on the frequency doublet spectrum originating from the Rabi splitting. We further took the infinitely long extension of the setup. We study the photonic dispersion properties, which we found to be engineerable through the atom-cavity detuning, cavity backscattering, and strong coupling regime of cavity quantum electrodynamics. Finally, when the cavities' position (microring resonators in our model) is subjected to disorder, obeying a Gaussian distribution, we find that the frequency combs are smoothed over and that the general single atom-cavity spectral behavior becomes dominant. The study of quantum many-body physics in optical systems and quantum networking can be the two targeted areas of application of this work.    

Status of an on-demand single-photon source using four-wave mixing in a hot vapor cell.

Fast coherent control of Rydberg excitations is essential for quantum logic gates and on-demand single-photon sources based on the Rydberg blockade as demonstrated for room-temperature rubidium atoms in a micro-cell. During our ongoing development of the next generation of this single-photon source we employ state-of-the-art 1010nm pulsed fiber amplifiers to drive a Rydberg excitation via the 6P intermediate state.

We report on the current status of the project and show time resolved observations of nanosecond pulsed four-wave mixing and GHz Rabi cycling involving the 32S Rydberg state. Our results demonstrate coherent control of the Rydberg state and the the final 6P state on the sub-nanosecond time scale.

The MHz repetition rates and significantly higher photon yields allow us to study and optimize the antibuching through elaborate pulse shaping motivated by numerical simulations. Such excitation timescales also pave the way towards fast optimal control methods for high fidelity Rydberg logic gates.   

Simulating open quantum systems with giant atoms

Open quantum many-body systems are of both fundamental and applicational interest. However, it remains an open challenge to simulate and solve such systems, both with state-of-the-art classical methods and with quantum-simulation protocols. In this talk, we introduce a versatile platform for quantum simulation of open systems: giant atoms, i.e., atoms (possibly artificial), that couple to a waveguide at multiple points, which can be wavelengths apart. We first show that a single giant atom can be used to simulate the dynamics of a driven dissipative qubit; various parameter regimes can be simulated by simply properly tuning the frequency of the giant atom. This example highlights the tunability of giant atoms, their versatility for performing post-selection, and their stability against realistic imperfections. We further show that two giant atoms can simulate the dynamics of a protected qubit coupled to a dissipative qubit. This demonstration highlights the connectivity of giant atoms mediated by the waveguide, which allows two-qubit gates to be performed without additional couplers between qubits - a unique property of giant atoms, which has been demonstrated with superconducting qubits. The tunability and connectivity of giant atoms enables the generalization of our protocol to larger system sizes, allowing to perform quantum simulation on open quantum many-body systems (in particular, dissipative spin systems). We discuss the challenges that may arise when scaling up our quantum-simulation protocol in this way.   

Dissipative stabilization and bias-preserving operations for dark-cat operations in atomic structures

Neutral atoms become one of the most promising platforms for quantum information and simulation purposes. Finding hardware-efficient ways to encode quantum information and performing error correction is still an important problem for this system. Our work shows how decoherence-free qubits can be efficiently encoded in the large spin hyperfine ground or metastable states of atoms. In particular, they are encoded in the dark states of a Raman-coupled hyperfine structure. This encoding resembles cat code structure in bosonic systems. For the encoded qubits, readily available laser coupling methods are used to construct any single-qubit gates in holonomic manners including those that preserve error bias, while laser coupling to Rydberg states is employed to create bias-preserving entangling gates among qubits. The bias-preserving operation set is sufficient for universal quantum computing on the concatenated repetition code level. When encoded in metastable levels, certain errors during gates can be treated as erasure errors. Also, with continuous syndrome monitoring and fresh atom replacement these errors can be converted to biased type. Those features of errors will be beneficial for further quantum error correcting strategies with reduced resource overhead and improved threshold.   

Modified fluorescence decay and other collective effects in cold atoms coupled to a hollow-core photonic-bandgap fiber

Collective and correlated states of atoms and light have applications in metrology and quantum information processing that include superradiant lasing, generation of non-classical light, and quantum memories for photons. A shared optical mode can enhance the collective response of an atomic ensemble coupled to it by mediating interactions between the atoms [1]. Here, we experimentally study the radiative dynamics of laser-cooled caesium atoms confined inside a photonic-bandgap fiber with a ~7.5 um diameter hollow core. The atoms are excited with pulses (ranging in length from ~1 to ~100 ns) near the cycling transition of 6S1/2, F = 4 to 6P3/2 , F’ = 5. We observe a variety of atom-number and excitation pulse-detuning dependent effects in the forward and backward direction with respect to the propagation of the excitation pulse, including significantly reduced fluorescence lifetimes of the atomic ensemble.  

[1]: J. Ruostekoski et al. Emergence of correlated optics in one-dimensional waveguides for classical and quantum atomic gases, PRL 117, 143602 (2016) 

*This research was undertaken in part thanks to funding from the Canada First Research Excellence Fund's Transformative Quantum Technologies (TQT) initiative.