On-Demand Microwave Single-Photon Source Based on Tantalum Thin Film

TL;DR

This work demonstrates a Ta(110) thin-film-based microwave single-photon source using a transmon qubit strongly coupled to a CPW resonator, achieving antibunched emission verified through $G^{(2)}(\tau)$ measurements. A traveling-wave parametric amplifier (TWPA) in the detection chain substantially improves signal-to-noise ratio, enabling rapid, high-fidelity second-order correlation verification with an estimated single-photon efficiency around $0.66$ and waveguide coupling fraction $\Gamma_{1}^{e}/\Gamma_{1} \approx 0.716$. The qubit transition is tunable up to $\approx 10.8$ GHz, and the system supports pulsed operation (e.g., a $\pi$-pulse) to generate near-pure single-photon states, with a state-preparation fidelity near $0.91$. The combination of tantalum thin films and TWPA-driven readout offers a robust platform for microwave quantum photonics, enabling faster characterization and paving the way for scalable microwave quantum networks and interferometer-based experiments.

Abstract

Single-photon sources are crucial for quantum information technologies. Here, we demonstrate a microwave single-photon source fabricated using a tantalum-based thin film, whose favorable material properties enable high-quality and stable photon emission. The antibunching behavior of the emitted radiation is revealed by second-order correlation measurements. Furthermore, traveling-wave parametric amplifiers are used as the pre-amplifier in the detection chains, we substantially improve the signal-to-noise ratio and thereby greatly reduce the acquisition time required for second-order correlation measurements. These results demonstrate the viability of tantalum-based superconducting devices as reliable platforms for microwave quantum photonics.

Figures (5)

• Figure 1: The thin film and device characterization: (a) AFM image of surface morphology. (b) XRD characterization of the film with red dashed lines indicating the (110) and (220) peak positions, the $\beta$-Ta (002) peak is marked by red arrow. (c) Measurement curve of film resistance vs temperature, with inset showing the superconducting transition temperature. (d) SEM characterization of the key components of the microwave single-photon source. The inset is a zoom-in image, which shows the SQUID of the qubit.
• Figure 2: Schematic diagram of the experimental setup, illustrating both the cryogenic and room-temperature components employed for frequency-domain and time-domain measurements. The cryogenic stage hosts the superconducting quantum device together with the associated input attenuation, DC flux-bias lines, and amplification chain. The room-temperature stage includes the microwave signal sources, DC sources, arbitrary waveform generators, analog-to-digital converters, and the signal-processing unit used for control and measurement.
• Figure 3: Qubit response under continuous-wave probing. (a) Reflection spectrum of the transmon qubit as a function of the applied flux bias. The “sweet-spot’’ transition frequency vanishes at integer multiples of the magnetic flux quantum $\Phi_{0}$ threading the SQUID loop. (b) Complex reflection coefficient $r$ of the waveguide measured at the qubit transition frequency $\omega_{01}/2\pi = 8.887~\mathrm{GHz}$, represented in the real–imaginary plane. Experimental data (dots) are normalized to the background response when $\omega_{01}$ is detuned far from the probing frequency. The probe power is varied from $-154$ to $-122~\mathrm{dBm}$ in steps of $2~\mathrm{dB}$.
• Figure 4: Rabi oscillation measurement of the emitted field. (a) Time-resolved quadrature amplitude of the emission field detected at one output port of the beam splitter as a function of the qubit preparation Rabi angle $\theta_{r}$. (b) A representative quadrature-amplitude trace at $\theta_{r} = \pi/2$, corresponding to the state $(\vert 0\rangle + \vert 1\rangle)/\sqrt{2}$ [blue dash–dotted line in (a)]. (c) Maximum quadrature amplitude extracted from each trace in (a) as a function of $\theta_{r}$ [red dash–dotted line in (a)]. (d) Time dependence of the cross power between the two output channels for the same set of Rabi angles $\theta_{r}$ as in (a). (e) A representative cross-power trace at $\theta_{r} = \pi$, corresponding to the state $\vert 1\rangle$ [blue dash–dotted line in (d)]. (f) Maximum cross power extracted from each trace in (d) as a function of $\theta_{r}$ [red dash–dotted line in (d)]. In all panels, the experimental data (blue dots) are compared with theoretical fits (solid red lines).
• Figure 5: Correlation function measurements. (a) Time dependence of the first-order correlation function $G^{(1)}(\tau)$ evaluated at the central peak ($\tau = 0$) and the side peaks ($\tau = n t_{p}$) as a function of the qubit preparation Rabi angle $\theta_{r}$. (b) Time dependence of the second-order correlation function $G^{(2)}(\tau)$ at $\tau = 0$ and $\tau = n t_{p}$ versus $\theta_{r}$. Blue and red error bars indicate the standard deviations of the mean values of $G^{(2)}(0)$ and $G^{(2)}(n t_{p})$, respectively. (c) Measured $G^{(2)}(\tau)$ for the single-photon Fock state $\vert 1\rangle$. (d) Measured $G^{(2)}(\tau)$ for the superposition state $(\vert 0\rangle + \vert 1\rangle)/\sqrt{2}$. (e) Measured $G^{(2)}(\tau)$ for a coherent state with amplitude $\vert \alpha \vert \approx 1$. In all panels, the experimental data (blue dots) are compared with theoretical fits (solid red lines).