Transmon Architecture for Emission and Detection of Single Microwave Photons

TL;DR

This work demonstrates a transmon emitter/detector (TED) built on a double transmon coupler (DTC) that mediates tunable, transition-selective interactions between a long-lived data transmon and a waveguide, enabling emission, detection, and metrology of single microwave photons. The authors implement source and detector TEDs with identical parameters, couple them via a coax circulator, and validate a pitch-detect protocol that shows controlled Fock-state emission and high-probability detection in a realistic network. Key findings include ~60% photon detection with inferred ~95% detection probability at the detector TED, ~2 μs reset and emission/detection times, and ~300 MHz waveguide tunability that relaxes interconnect requirements for heterogeneous quantum processors. The results establish TEDs as compact, reconfigurable quantum communication interfaces for quantum networking, capable of unconditional fast reset and metrology while preserving data-qubit coherence.

Abstract

We showcase the recently developed double transmon coupler (DTC) circuit as a compact, drop-in, tunable and transition-selective link between an otherwise coherent transmon and the continuum of modes in a waveguide. We use these transmon-DTC devices as transmon emitter/dectectors (TEDs) for microwave photons. We highlight the flexibility of these devices by sending photons from a source TED to a measurement TED using a meter of coaxial cable and a circulator, each TED with nominally identical circuit parameters. We detect $60\,\%$ of the photons using this setup where we infer that $95\,\%$ of the photons at the input of the measurement TED are detected. Reset and photon emission/detection each require about $2\,μ$s, for a minimum protocol duration of $4\,μ$s, for our choice of TED parameters. Transmon-waveguide links like the DTC serve an important role in quantum information processors: they provide a mechanism for unconditional fast reset, metrology, and as nascent quantum communication interfaces for quantum networking.

Figures (10)

• Figure 1: (a) CAD file illustrating the transmon emitter/detector (TED) layout and (b) equivalent circuit for the TED where we index between two TEDs: $x\in\{s,m\}$. Transmon $Q_{dx}$ (red) is designed for long term storage while $Q_{wx}$ (green) strongly couples to a waveguide. $Q_{cx}$ (purple) and $Q_{wx}$ comprise a double transmon coupler (DTC). Flux line $\phi_{px}$ biases and sinusoidally drives the DTC to mediate interactions between $Q_{dx}$ and $Q_{wx}$. (c) Flux tunability with $\phi_{pm}$ of the measurement TED's (mTED's) $Q_{cm}$ and $Q_{wm}$ transitions. $Q_{wm}$ was measured using backscatter while $Q_{cm}$ was measured using two-tone spectroscopy. The red dash-dot line is a guide to the eye showing $Q_{dm}$'s transition, which only varies on the $\sim 10$ MHz scale. (d) Depiction of emission-detection setup. A source TED (sTED) is connected to the mTED via 1 meter of coaxial cable, a 10 dB directional coupler, and a circulator. The coupler and circulator allow backscatter measurements of $Q_{wx}$, which does not have an independent read-out resonator. $Q_{dx}$ and $Q_{cx}$ are capacitively coupled with amplitude $g_{Cx}$ while $Q_{cx}$ and $Q_{wx}$ are inductively coupled with amplitude $g_{Lx}$. $\gamma_x$ is the rate $Q_{wx}$ relaxes into its waveguide.
• Figure 2: (a) Layout of mTED, where a coherent drive is applied to $Q_{wm}$. (b-c) Calibration of coherent drive power using a vector network analyzer measurement of $Q_{wm}$ illuminated when $g_{pm}=0$. $|r|$ is the normalized amplitude of the returning coherent state. The plots vary coherent drive frequency and power.
• Figure 3: (a) Pulse timing diagram for parametric coupling and qubit gates. A flux drive is initially applied to the sTED resonant with the $|10\cdot\cdot\rangle \leftrightarrow |01\cdot\cdot\rangle$ transition to reset the residual thermal population from $Q_{ds}$. $Q_{ds}$ is then excited to $(\mathinner{|10\cdot\cdot\rangle}+\mathinner{|00\cdot\cdot\rangle})/\sqrt{2}$ by a $\pi/2$-pulse and subsequently driven by a shaped flux drive on the reset transition to release a photon state. (b) (left) Layout of sTED, where a photon emits into a $50\,\Omega$ waveguide. (right) Level diagram with $Q_{ds}$ prepared in a superposition state, depicted using half filled circles. $g_{ps}$ is the amplitude of a drive on a transition that releases the state on $Q_{ds}$. $\gamma_s$ is the relaxation rate of $Q_{ws}$ into the waveguide. (c) Markers show the average of ten thousand repetitions of the photon superposition state after amplification. Error bars on the data are smaller than the marker size. The amplitude of the output was rescaled to match the theory fit (lines). The complex dynamics of the photon wavefunction result from a frequency shift of $Q_{ws}$ with $g_{ps}/\gamma_s$. These shifts can be passively eliminated (see Appendix \ref{['sec:acStark']}) (d) Measured frequency shift in the $\omega_{ws}$ as a function of $g_{ps}/\gamma_s$ where the marker size is roughly equivalent to the error and the line is a fit to a quadratic function.
• Figure 4: (a) Pulse timing diagram for the mTED and an externally applied coherent drive (which is always on). The mTED is initialized using a reset pulse on the $|\cdot\cdot10\rangle\leftrightarrow|\cdot\cdot01\rangle$ transition followed by exciting $Q_{dm}$ to $\mathinner{|\cdot\cdot10\rangle}$ with a $\pi$-pulse. A parametric drive on the $|\cdot\cdot11\rangle\leftrightarrow|\cdot\cdot02\rangle$ transition turns on detection. (b) Layout of mTED, where a coherent drive is applied to $Q_{wm}$. (c) Level diagram indicating the driven transitions and relaxation pathways. $g_p$ is the amplitude of a driven coupling while $\gamma_m$ is the mTED's relaxation of $Q_{wm}$ into the waveguide. (d-e) Population of $Q_{dm}$ as a function of duration $g_p$ is applied and coherent drive power.
• Figure 5: (a) Protocol of (top) $g_{ps}$ and (bottom) $g_{pm}$ to perform a pitch and detect demonstration. The red traces correspond to operations on (top) $Q_{ds}$ and (bottom) $Q_{dm}$. (b) (left) Turning on $g_{ps}$ drives an excitation from $Q_{ds}$ into $Q_{ws}$, which rapidly decays, emitting a photon into the waveguide. A circulator eliminates resonant modes in the cables while also allowing photons to be collected and measured. When the photon arrives at the mTED (right) the population of $Q_{dm}$ goes to zero. (c) Normalized population of $Q_{dm}$, inverse from the probability of detecting a photon, is plotted as a function of $\delta\omega_{wm}$ and $\delta\omega_{pm}$. $\delta\omega_{wm}$ and $\delta\omega_{pm}$ correspond to excursions from $\omega_w$ and $\omega_w-\omega_d-\nu_w$ by varying $\bar{\phi}_p$ and $\omega_p$, respectively. The star indicates where detection probability is maximized. (d) Population of $Q_{dm}$ as a function of $g_{pm}$ measured at the star in panel c. The line is a prediction from the model for which $\eta$ is a free parameter. The size of the markers is similar to the error. (e) Timing of $g_{pm}$, corresponding to the detection window (blue), and $g_{ps}$, where the pulse time ($3~\mu s$ in this schematic) denotes the middle of the pulse relative to the beginning of the detection window. (f) Detection fidelity is largely unchanged by the arrival time of the photon. The size of the markers is similar to the error.