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arXiv:2202.07034 · Applied Physics Letters

Slowing down light in a qubit metamaterial

Jan David Brehm, Richard Gebauer, Alexander Stehli, Alexander N. Poddubny, Oliver Sander, Hannes Rotzinger, Alexey Vladimirovich Ustinov

Published Nov 14, 2022• 19 citations
arXivHTMLPDF

Abstract

The rapid progress in quantum information processing leads to a rising demand for devices to control the propagation of electromagnetic wave pulses and to ultimately realize universal and efficient quantum memory. While in recent years, significant progress has been made to realize slow light and quantum memories with atoms at optical frequencies, superconducting circuits in the microwave domain still lack such devices. Here, we demonstrate slowing down electromagnetic waves in a superconducting metamaterial composed of eight qubits coupled to a common waveguide, forming a waveguide quantum electrodynamics system. We analyze two complementary approaches, one relying on dressed states of the Autler–Townes splitting and the other based on a tailored dispersion profile using the qubits tunability. Our time-resolved experiments show reduced group velocities of down to a factor of about 1500 smaller than in vacuum. Depending on the method used, the speed of light can be controlled with an additional microwave tone or an effective qubit detuning. Our findings demonstrate high flexibility of superconducting circuits to realize custom band structures and open the door to microwave dispersion engineering in the quantum regime.

DOI
10.1063/5.0122003

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The rapid progress in quantum information processing leads to a rising demand for devices to control the propagation of electromagnetic wave pulses and to ultimately realize universal and efficient quantum memory. While in recent years, significant progress has been made to realize slow light and quantum memories with atoms at optical frequencies, superconducting circuits in the microwave domain still lack such devices. Here, we demonstrate slowing down electromagnetic waves in a superconducting metamaterial composed of eight qubits coupled to a common waveguide, forming a waveguide quantum electrodynamics system. We analyze two complementary approaches, one relying on dressed states of the Autler–Townes splitting and the other based on a tailored dispersion profile using the qubits tunability. Our time-resolved experiments show reduced group velocities of down to a factor of about 1500 smaller than in vacuum. Depending on the method used, the speed of light can be controlled with an additional microwave tone or an effective qubit detuning. Our findings demonstrate high flexibility of superconducting circuits to realize custom band structures and open the door to microwave dispersion engineering in the quantum regime.
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.
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Waveguide quantum electrodynamics—the study of atomic systems interacting with propagating electromagnetic fields—is a powerful platform for understanding the complex interplay between light and matter. Qubit control is an indispensable tool in this field, and most experiments thus far have used narrowband control pulses to interact with qubits at specific frequencies. Dynamics can change significantly with fast, broadband pulses, since waveguide properties such as dispersion affect the pulse evolution and its impact on the qubit. Here we use dispersion to achieve spatial addressing of superconducting qubits separated by a subwavelength distance within a microwave waveguide. This approach relies on a self-focusing effect to create a position-dependent interaction between a chirped pulse and an individual qubit. This method emphasizes the importance of dispersion in the design and analysis of quantum experiments, and offers avenues for the rapid control of quantum states.
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.
Abstract Waveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits toward a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.
Recent progresses in Josephson-junction-based superconducting circuits have propelled quantum information processing forward. However, the lack of a metastable state in most superconducting artificial atoms hinders the development of photonic quantum memory in this platform. Here we use a single superconducting qubit-resonator system to realize a desired Λ -type artificial atom, and to demonstrate slow light with a group velocity of 3.6 km/s and the microwave storage with a memory time extending to several hundred nanoseconds via electromagnetically induced transparency. Our results highlight the potential of achieving microwave quantum memory, promising substantial advancements in quantum information processing within superconducting circuits.