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arXiv:1912.09980 · Physical Review A

Beyond spontaneous emission: Giant atom bounded in the continuum

Shangjie Guo, Yidan Wang, Thomas Purdy, Jacob M. Taylor

Published Sep 8, 2020• 87 citations
arXivHTMLPDF

Abstract

The quantum coupling of individual superconducting qubits to microwave photons leads to remarkable experimental opportunities. Here we consider the phononic case where the qubit is coupled to an electromagnetic surface acoustic wave antenna that enables supersonic (electromagnetic) propagation of the qubit oscillations. This can be considered as a giant atom that is many phonon wavelengths long. We study an exactly solvable toy model that captures these effects, and find that this non-Markovian giant atom has a suppressed relaxation, so long as an effective vacuum coupling exists between a qubit excitation and a localized wave packet of sound, even in the absence of a cavity for the sound waves. Finally, we consider practical implementations of these ideas in current surface acoustic wave devices.

DOI
10.1103/physreva.102.033706

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The quantum coupling of individual superconducting qubits to microwave photons leads to remarkable experimental opportunities. Here we consider the phononic case where the qubit is coupled to an electromagnetic surface acoustic wave antenna that enables supersonic (electromagnetic) propagation of the qubit oscillations. This can be considered as a giant atom that is many phonon wavelengths long. We study an exactly solvable toy model that captures these effects, and find that this non-Markovian giant atom has a suppressed relaxation, so long as an effective vacuum coupling exists between a qubit excitation and a localized wave packet of sound, even in the absence of a cavity for the sound waves. Finally, we consider practical implementations of these ideas in current surface acoustic wave devices.
We demonstrate a giant atom by coupling a superconducting transmon qubit to a lithium niobate phononic waveguide at two points separated by about 600 acoustic wavelengths, with a propagation delay of 125 ns. The giant atom yields non-Markovian relaxation dynamics characterized by phonon backflow and a frequency-dependent effective decay rate varying four-fold over merely 4 MHz, corresponding to a Purcell factor exceeding 40. Exploiting this frequency-dependent dissipation, we prepare quantum superposition states with high purity. Our results establish phononic integrated circuits as a versatile platform for giant-atom physics, providing highly tunable quantum devices for advanced quantum information processing.
We investigate the quantum dynamics of a single transmon qubit coupled to surface acoustic waves (SAWs) via two distant connection points.Since the acoustic speed is five orders of magnitude slower than the speed of light, the traveling time between the two connection points needs to be taken into account.Therefore, we treat the transmon qubit as a giant atom with a deterministic time delay.We find that the spontaneous emission of the system, formed by the giant atom and the SAWs between its connection points, initially decays polynomially in the form of pulses instead of a continuous exponential decay behavior, as would be the case for a small atom.We obtain exact analytical results for the scattering properties of the giant atom up to two-phonon processes by using a diagrammatic approach.We find that two peaks appear in the inelastic (incoherent) power spectrum of the giant atom, a phenomenon which does not exist for a small atom.The time delay also gives rise to features in the reflectance, transmittance, and second-order correlation functions of the system.Furthermore, we find the short-time dynamics of the giant atom for arbitrary drive strength by a numerically exact method for open quantum systems with a finite-time-delay feedback loop.
Spontaneous emission of an excited atom in a featureless continuum of electromagnetic modes is a fundamental process in quantum electrodynamics associated with an exponential decay of the quantum emitter to its ground state accompanied by an irreversible emission of a photon. However, such a simple scenario is deeply modified when considering a "giant" atom, i.e., an atom whose dimension is larger than the wavelength of the emitted photon. In such an unconventional regime, non-Markovian effects and strong deviations from an exponential decay are observed owing to interference effects arising from nonlocal light-atom coupling. Here we suggest a photonic simulation of non-Markovian giant atom decay, based on light escape dynamics in an optical waveguide nonlocally coupled to a waveguide lattice. Major effects, such as nonexponential decay, enhancement, or slowing down of the decay, and formation of atom-field dark states can be emulated in this system.
A phonon (quantized vibration of a crystalline medium) is much shorter in wavelength than a photon of the same frequency. This length-scale reduction offers an opportunity to achieve higher density of quantum information storage, and may open a path to scalable implementations of superconducting quantum processors. However, the smallness of the acoustic wavelength also poses a challenge: It may result in fast qubit decoherence via an unintentional emission of phonons. The authors demonstrate how to circumvent such spontaneous emission and attain quantum coherent coupling of a qubit to an isolated phonon mode.
We investigate the single photon scattering in a phonon-photon hybrid system in the waveguide quantum electrodynamics (QED) scheme. In our consideration, an artificial giant atom, which is dressed by the phonons in a surface acoustic wave resonator, interacts with a coupled resonator waveguide (CRW) nonlocally via two connecting sites. Together with the interference effect by the nonlocal coupling, the phonon serves as a controller to the transport of the photon in the waveguide. On the one hand, the coupling strength between the giant atom and the surface acoustic wave resonator modulates the width of the transmission valley or window in the near resonant regime. On the other hand, the two reflective peaks induced by the Rabi splitting degrade into a single one when the giant atom is large detuned from the surface acoustic resonator, which implies an effective dispersive coupling. Our study paves the way for the potential application of giant atoms in the hybrid system.
Abstract Giant quantum emitters, whose effective size is comparable to the wavelength of the resonant radiation they couple to, give rise to interference effects and non-Markovian dynamics that lie beyond the scope of the standard Weisskopf–Wigner approximation. Here we study minimal models of giant emitters coupled to cavity–array waveguides and identify symmetric configurations where the dynamics can be solved exactly. This allows us to capture the emergence of bound states both inside and outside the photonic continuum, and to demonstrate the possibility of engineering their coherent superpositions. Our results provide analytical insight into non-Markovian light–matter interaction and suggest a feasible implementation using superconducting circuit platforms.