We study quantum features of electromagnetic radiation propagating in a waveguide built of an infinite array of super-conducting qubits (SCQ) placed within a resonator comprised of two massive superconductors. Such engineered media, so-called superconducting quantum metamaterials (SCQMM), have a great potential for quantum information processing and communications and devise a comprehensive study of the nature of the “light”-matter interaction. We predict the emergence of novel in-terference effects with possible practical applications. For exam-ple, photons may exhibit a nontrivial dispersion relation such as band edges and band gaps. In this way, QMMs may be viewed as photonic crystals. This, in turn, enriches their potential for practi-cal applications and provides novel means for devising compre-hensive studies of both practical and fundamental aspects of the interaction of artificial atoms and the EM field.
We study quantum features of electromagnetic radiation propagating in a waveguide built of an infinite array of super-conducting qubits (SCQ) placed within a resonator comprised of two massive superconductors. Such engineered media, so-called superconducting quantum metamaterials (SCQMM), have a great potential for quantum information processing and communications and devise a comprehensive study of the nature of the “light”-matter interaction. We predict the emergence of novel in-terference effects with possible practical applications. For exam-ple, photons may exhibit a nontrivial dispersion relation such as band edges and band gaps. In this way, QMMs may be viewed as photonic crystals. This, in turn, enriches their potential for practi-cal applications and provides novel means for devising compre-hensive studies of both practical and fundamental aspects of the interaction of artificial atoms and the EM field.
Superconducting quantum metamaterials extend the idea of their classical counterpart to a regime where their constituent meta-atoms are quantum objects, which can hold their quantum coherence for longer than the propagation time of light through the medium. In this work, we have realized a quantum metamaterial consisting of eight individually controllable superconducting transmon qubits, which are coupled to the mode continuum of a one-dimensional coplanar waveguide. This system can be described within the framework of waveguide-quantum electrodynamics, which predicates that the mutual interaction of the qubits with the waveguide gives rise to long-range interactions of the qubits. In spectroscopic measurements we observe the formation of super- and subradiant collective metamaterial excitations, as well as the emergence of a polaritonic band gap and study their dependence on the number of participating resonant qubits. We utilize the collective Autler-Townes splitting of the metamaterial to demonstrate control over its band gap. Furthermore, we exploit the control over the band structure for a first realization of slowly propagating light in the metamaterial. Our findings show that superconducting quantum metamaterials are a suitable platform to study fundamental excitations in solids and pave way to applications in quantum information processing like quantum memories.
We study quantum features of electromagnetic radiation propagating in a one-dimensional superconducting quantum metamaterial composed of an infinite chain of charge qubits placed within two stripe massive superconducting resonators. The quantum-mechanical model is derived assuming weak fields and that, at low temperatures, each qubit is either unoccupied or occupied by a single Cooper pair. We demonstrate the emergence of two bands of single-photon qubit bound states with the energies lying outside the photon continuum---one is above and the second slightly below the linear photon band. The higher energy band varies slowly with the qubit-photon center of mass quasimomentum. It becomes practically flat provided that the electromagnetic energy is far below the Josephson energy when the latter is small compared to the charging energy. The dispersion of the lower band is practically identical to that of free photons. The emergence of bound states may cause radiation trapping indicating possible applicability for the control of photon transport in superconducting qubit-based artificial media.
In the last decade, the field of Quantum Optics has been enriched with a completely new platform: superconducting quantum circuits, also known as circuit-QED. This platform builds on the original designs of superconducting qubits, that is few-level systems made of superconducting islands and Josephson junctions, but adds to it a new ingredient, microwave photons. In this talk I will review circuit-QED with a focus on two complementary lines of research: ultrastrong light-matter coupling and strongly correlated propagating photons. The first topic deals with novel designs of superconducting qubits, engineering their coupling to the microwave radiation field to reach Rabi frequencies that approach the qubit and photon frequencies. In this regime, known as ”ultrastrong coupling”, the rotating wave approximation from Quantum Optics is no longer valid, the vacuum may spontaneously polarize, producing photons that get bound to the qubit, and novel physics is expected to emerge. I will review recent experiments on this topic, together with various theoretical proposals to tame this ultrastrong coupling. The second topic revolves around the notion of ”propagating” photons in these circuits. Most experiments in circuit-QED so far adopted the typical configuration of artificial atoms (qubits) interacting with artificial cavities (LC resonators). This proved useful in order to reach the strong coupling regime, demonstrating Rabi oscillations and developing a variety of protocols for quantum information, tomograhy, entanglement, etc. However, recent experiments have shown that it is possible to also work with propagating photons, replacing the LC resonators with open transmission lines, and studying the scattering properties of those artificial atoms. In this context we have worked on developing new technology to detect and control these photons, engineering single-photon detectors, quantum metamaterials and photonic crystals, or even new nonlinear media based on coupled cavities.
Recent years have seen rapid advances in our ability to harness, manipulate, or enhance quantum effects within artificial nanostructures and electronic metamaterials. Synthetic materials, including nanopatterned electronic structures, Moir´e materials, and quantum hetero-structures, can enable the bottom-up functional engineering of electronic band structure, electron-phonon interactions, thermal transport, kinetic nonlinearity, and other material properties. In turn, such functional materials engineering, when applied to superconducting nanoelectronics, can lead to enhanced functionalities and new operating regimes in devices ranging from SC qubits for quantum information science, single-photon detectors, quantum-limited parametric amplifiers, and millimeterwave quantum technologies. We propose to develop a new class of quantum-limited parametric amplifiers, based on superconducting (SC) Josephson metamaterials with strong and tailor-able nonlinearity in the microwave regime. Our innovation is in the establishment of a fundamentally new light-matter interacting system, through the unique combination of SC Josephson nonlinear metamaterials and surface magnon-polariton resonators.
Metamaterials are a class of artificially engineered materials with periodic structures possessing exceptional properties not found in conventional materials. This definition can be extended when we introduce a degree of freedom by adding quantum elements such as quantum dots, cold atoms, Josephson junctions, and molecules, making metamaterials highly valuable for various quantum applications. Metamaterials have been used to achieve invisibility cloaking, super-resolution, energy harvesting, and sensing, among other applications. Most of these applications are performed in the classical regime. Metamaterials have gradually made their way into the quantum regime since the advent of quantum computing and quantum sensing and imaging. Quantum metamaterials are a relatively new technology, and their use in quantum information processing has proliferated. We restrict this study to quantum state manipulation and control, quantum entanglement, single photon generation, quantum state switching, quantum state engineering, quantum key distribution, quantum algorithms, orbital angular momentum, and quantum imaging. Considering these developments, we examine the theory, fabrication, and applications contributing to quantum information processing and how quantum metamaterials contribute to this field. We find that the ability to harness the unique properties of metamaterials to drive these applications is of great importance, as they have the potential to unlock new possibilities for revolutionizing quantum information processing, bringing the world closer to practical quantum technologies with unprecedented capabilities. We conclude by suggesting possible future research directions.
Abstract Embedding tunable quantum emitters in a photonic bandgap structure enables control of dissipative and dispersive interactions between emitters and their photonic bath. Operation in the transmission band, outside the gap, allows for studying waveguide quantum electrodynamics in the slow-light regime. Alternatively, tuning the emitter into the bandgap results in finite-range emitter–emitter interactions via bound photonic states. Here, we couple a transmon qubit to a superconducting metamaterial with a deep sub-wavelength lattice constant (λ/60). The metamaterial is formed by periodically loading a transmission line with compact, low-loss, low-disorder lumped-element microwave resonators. Tuning the qubit frequency in the vicinity of a band-edge with a group index of ng = 450, we observe an anomalous Lamb shift of −28 MHz accompanied by a 24-fold enhancement in the qubit lifetime. In addition, we demonstrate selective enhancement and inhibition of spontaneous emission of different transmon transitions, which provide simultaneous access to short-lived radiatively damped and long-lived metastable qubit states.