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Strong and ultra-strong light-matter coupling are remarkable phenomena of quantum electrodynamics occurring when the interaction between a matter excitation and the electromagnetic field cannot be described by usual perturbation theory. This is generally achieved by coupling an excitation with large oscillator strength to the confined electromagnetic mode of an optical microcavity. In this work we demonstrate that strong/ultra-strong coupling can also take place in the absence of optical confinement. We have studied the non-perturbative spontaneous emission of collective excitations in a dense two-dimensional electron gas that superradiantly decays into free space. By using a quantum model based on the input-output formalism, we have derived the linear optical properties of the coupled system and demonstrated that its eigenstates are mixed light-matter particles, like in any system displaying strong or ultra-strong light-matter interaction. Moreover, we have shown that in the ultra-strong coupling regime, i.e. when the radiative broadening is comparable to the matter excitation energy, the commonly used rotating-wave and Markov approximations yield unphysical results. Finally, the input-output formalism has allowed us to prove that Kirchhoffs law, describing thermal emission properties, applies to our system in all the light-matter coupling regimes considered in this work.
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With the introduction of superconducting circuits into the field of quantum optics, many novel experimental demonstrations of the quantum physics of an artificial atom coupled to a single-mode light field have been realized. Engineering such quantum systems offers the opportunity to explore extreme regimes of light-matter interaction that are inaccessible with natural systems. For instance the coupling strength $g$ can be increased until it is comparable with the atomic or mode frequency $omega_{a,m}$ and the atom can be coupled to multiple modes which has always challenged our understanding of light-matter interaction. Here, we experimentally realize the first Transmon qubit in the ultra-strong coupling regime, reaching coupling ratios of $g/omega_{m}=0.19$ and we measure multi-mode interactions through a hybridization of the qubit up to the fifth mode of the resonator. This is enabled by a qubit with 88% of its capacitance formed by a vacuum-gap capacitance with the center conductor of a coplanar waveguide resonator. In addition to potential applications in quantum information technologies due to its small size and localization of electric fields in vacuum, this new architecture offers the potential to further explore the novel regime of multi-mode ultra-strong coupling.
We present and analyze a method where parametric (two-photon) driving of a cavity is used to exponentially enhance the light-matter coupling in a generic cavity QED setup, with time-dependent control. Our method allows one to enhance weak-coupling systems, such that they enter the strong coupling regime (where the coupling exceeds dissipative rates) and even the ultra-strong coupling regime (where the coupling is comparable to the cavity frequency). As an example, we show how the scheme allows one to use a weak-coupling system to adiabatically prepare the highly entangled ground state of the ultra-strong coupling system. The resulting state could be used for remote entanglement applications.
Localized-surface plasmon resonance is of importance in both fundamental and applied physics for the subwavelength confinement of optical field, but realization of quantum coherent processes is confronted with challenges due to strong dissipation. Here we propose to engineer the electromagnetic environment of metallic nanoparticles (MNPs) using optical microcavities. An analytical quantum model is built to describe the MNP-microcavity interaction, revealing the significantly enhanced dipolar radiation and consequentially reduced Ohmic dissipation of the plasmonic modes. As a result, when interacting with a quantum emitter, the microcavity-engineered MNP enhances the quantum yield over 40 folds and the radiative power over one order of magnitude. Moreover, the system can enter the strong coupling regime of cavity quantum electrodynamics, providing a promising platform for the study of plasmonic quantum electrodynamics, quantum information processing, precise sensing and spectroscopy.
In this experiment, we couple a superconducting Transmon qubit to a high-impedance $645 Omega$ microwave resonator. Doing so leads to a large qubit-resonator coupling rate $g$, measured through a large vacuum Rabi splitting of $2gsimeq 910$ MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies $omega$, placing our system close to the ultra-strong coupling regime ($bar{g}=g/omega=0.071$ on resonance). Combining this setup with a vacuum-gap Transmon architecture shows the potential of reaching deep into the ultra-strong coupling $bar{g} sim 0.45$ with Transmon qubits.
A primary motivation for studying topological matter regards the protection of topological order from its environment. In this work, we study a topological emitter array coupled to an electromagnetic environment. The photon-emitter coupling produces nonlocal interactions between emitters. Using periodic boundary conditions for all ranges of environment-induced interactions, chiral symmetry inherent to the emitter array is preserved and protects the topological phase. A topological phase transition occurs at a critical photon-emitter coupling which is related to the energy spectrum width of the emitter array. It produces a band touching with parabolic dispersion, distinct to the linear one without considering the environment. Interestingly, the critical point nontrivially changes dissipation rates of edge states, yielding dissipative topological phase transition. In the protected topological phase, edge states suffer from environment-induced dissipation for weak photon-emitter coupling. However, strong coupling leads to dissipationless edge states. Our work presents a way to study topological criticality in open quantum systems.
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