(g) Inside the mirror band, the IDT modifies the qubit coupling strength g m to the evenly spaced cavity modes. (f) When the qubit is tuned outside the mirror bandwidth (unshaded), the IDT launches phonons with a frequency dependence determined by the Fourier transform of the IDT geometry, creating fringes in the qubit loss rate Γ 1. (e) Antenna paddles couple the qubit to a copper waveguide cavity at 5.9 GHz for readout and control. Insets show enlarged images (c) of the Bragg mirror and (d) the IDT, whose innermost fingers are extended to shunt the junctions. The imaged device has half the cavity length of the measured device. (b) The qubit IDT (purple) is shunted by Josephson junctions and located within an acoustic cavity of width W = 16 μ m. (b)–(e) False-color SEM micrographs of a representative device. Two distributed Bragg arrays (green) confine phonons in a narrow frequency band to create acoustic resonances (gray curve). A transmon qubit resembling a double-slit interferometer is formed by shunting two halves of an interdigitated transducers (IDT, red and blue) with a pair of Josephson junctions (yellow). Moreover, our strategy can take full advantage of the high mode density of acoustic systems to access a giant phase space with a compact device, a prized resource in quantum information processing.ĭouble-slit qubit concept and device. Resolving these excitation peaks represents a significant milestone in universal quantum control over acoustic modes, leading to nondestructive quantum measurement of phonons and the creation of multimode phononic entangled states. By populating several of the acoustic resonances, we resolve multiple peaks in the qubit excitation spectrum that correspond to an integer number of quanta in the acoustic cavity. We leverage this frequency dependence to realize a strong coupling to a multimode cavity without sacrificing the ability to control the qubit. In close analogy to double-slit diffraction, this separation leads to self-interference effects that generate sharp fringes in the frequency dependence of the qubit-cavity coupling strengths. Crucial to harnessing the multimode nature of the cavity is coupling the qubit to the phonons at two locations separated by many acoustic wavelengths. We observe the quantized mechanical excitations of a surface acoustic wave resonator through its strong coupling to a superconducting qubit. We investigate such a regime by exploiting the slowness of sound to strongly couple a compact mechanical resonator to an artificial atom. Furthermore, leveraging the unique properties of mechanical systems would enable the realization of physical regimes with technological applications that are inaccessible to light-based platforms. Inspired by these accomplishments, researchers have engineered artificial atoms to interact strongly with mechanical excitations. The study of interactions between light and atoms has generated numerous fundamental insights and innovative quantum technologies. By exciting several detuned yet strongly coupled phononic modes and measuring the resulting qubit spectrum, we observe that, for two modes, the device enters the strong dispersive regime where single phonons are spectrally resolved. We use this sharp frequency structure to resolve single phonons by tuning the qubit to a frequency of destructive interference where all acoustic interactions are dispersive. We observe this frequency structure both in the coupling rate to multiple cavity modes and in the qubit spontaneous emission rate into unconfined modes. In analogy to double-slit diffraction, the resulting interference generates high-contrast frequency structure in the qubit-phonon interaction. Crucial to this resolution is the sharp frequency dependence in the qubit-phonon interaction engineered by coupling the qubit to surface acoustic waves in two locations separated by ∼ 40 acoustic wavelengths. We resolve phonon number states in the spectrum of a superconducting qubit coupled to a multimode acoustic cavity.
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