Giant-atom quantum acoustodynamics in hybrid superconducting-phononic integrated circuits

Abstract

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.

Figures (4)

• Figure 1: Device for giant-atom quantum acoustodynamics.a, Schematic of the hybrid superconducting-phononic system, consisting of a transmon qubit (white) coupled to a phononic waveguide (light green) via two interdigital transducers (IDTs). b, False-color optical photograph of the device. c, False-color scanning electron micrograph of an IDT. d, Dispersion curves of phononic modes in the phononic waveguide. Solid lines: guided modes; dashed lines: slab modes. Inset: displacement field of the fundamental quasi-Love mode. e, Simulated IDT coupling strength $\gamma$ versus qubit frequency for different numbers of finger pairs. f, Simulated electric field (red and blue colors) and displacement field (deformations) of the fundamental quasi-Love mode excited by an IDT with five pairs of fingers at 5 GHz.
• Figure 2: Giant atom relaxation dynamics.a, Qubit readout. Upper panel: optical photograph of the circuit for qubit manipulation and readout. Lower panel: Spectrum of the measured qubit excitation probability $P_\mathrm{e}$ versus the bias current applied through the Z control line. b, Measured effective qubit relaxation rate $\gamma_e$ as a function of qubit frequency. Red and blue lines: fitted upper and lower envelopes. Inset: zoomed-in view of the modulation of $\gamma_e$; lines are fits. c, Qubit excitation decay dynamics at four qubit frequencies: $4.8887$ (blue), $4.8894$ (sky blue), $4.8899$ (yellow), and $4.8904\,\mathrm{GHz}$ (red). Solid curves: fits; yellow dashed line: extrapolation of the initial exponential decay for $0<t<T$ (shaded region) before interference occurs. Inset: expanded view of the modulation of $\gamma_e$ over the corresponding qubit frequency range.
• Figure 3: Driven giant atom dynamics.a, Energy-level diagram of the driven qubit with Rabi frequency $\Omega$ and detuning $\Delta$. b, Steady-state qubit excitation probability $P_\mathrm{e}$ versus $\Omega$, with qubit frequency at $4.279\,\mathrm{GHz}$, $\Delta=0$, and a drive duration of $t=3.8\,\mathrm{\mu s}$ for long enough relaxation with $(\gamma_\mathrm{in}+\gamma)t\gg1$. c, Dynamics of $P_\mathrm{e}$ versus qubit frequency ($\Delta=0$) under a weak fixed $\Omega/2\pi = 0.2\,\mathrm{MHz}$. d, Corresponding steady-state $P_\mathrm{e}$. Solid line: prediction from the measured $\gamma_e$. e and f, Dynamic evolution (e) and steady-state $P_\mathrm{e}$ (f) under a strong drive with $\Omega/2\pi =2.5\,\mathrm{MHz}$.
• Figure 4: State preparation through relaxation of the driven giant atom.a and b, Measured steady-State qubit coherence $\langle \sigma_x \rangle$ versus drive Rabi frequency $\Omega$ and detuning $\Delta$ at qubit frequencies of $4.640\,\mathrm{GHz}$ (a) and $4.891\,\mathrm{GHz}$ (b). c and d, Calculated $\langle \sigma_x \rangle$ for the same frequencies. e and f, Measured steady-state purity at qubit frequencies of $4.640\,\mathrm{GHz}$ (e) and $4.891\,\mathrm{GHz}$ (f). g and h, calculated steady-state purity. i, Steady-state purity versus $\Omega$ at $\Delta/2\pi=-5\,\mathrm{MHz}$ for two qubit frequencies: $4.891\,\mathrm{GHz}$ (blue) and $4.887\,\mathrm{GHz}$ (orange). Solid lines: theoretical results.