Demonstration of a Single-Photon Router in the Microwave Regime

We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons ($\sim 6\mathrm{GHz}$). When an input coherent state, with an average photon number $N\ll 1$ is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT). We then use EIT to make a single-photon router, where we can control to what output port an incoming signal is delivered. The maximum on-off ratio is around 99% with a rise and fall time on the order of nanoseconds, consistent with theoretical expectations. The router can easily be extended to have multiple output ports and it can be viewed as a rudimentary quantum node, an important step towards building quantum information networks.

Figure 1

Scattering from a single artificial atom. (a) A micrograph of our artificial atom, a superconducting transmon qubit embedded in a 1D open transmission line. The light regions are Al while the dark regions are the oxidized silicon substrate. We see the center conductor of the CPW in between the two ground planes and the two plates of the interdigitated capacitance of the transmon. The enlargement is a scanning-electron micrograph of the SQUID loop of the transmon, which allows us to tune its transition frequency with an external magnetic flux $\Phi$. The junctions are formed using the standard double-angle evaporation technique. (b) Schematic of the measurement setup. A strong control pulse at ${\omega }_c={\omega }_{12}$ is used to route a weak microwave signal at the probe frequency ${\omega }_{01}$. We measure the transmitted and reflected probe simultaneously in the time domain. The circulators, numbered 1–4, allow us to separate signals propagating in different directions in the lines. (c) Transmittance, $T=|t{|}^2$, on resonance as a function of incident power. For $N\ll 1$ we see extinction of the coherent probe of 90% and 99.6% for sample 1 and 2, respectively. $N=1$ for a power of $-125\mathrm{dBm}$ ($-123\mathrm{dBm}$) for sample 1 (2). The symbols are data while the lines are fits based on Eq. (1). (Inset) A weak, resonant coherent state is scattered by the atom.

Figure 2

(a) Two-tone spectroscopy of sample 1. A microwave pump is continuously applied at ${\omega }_{01}$ with increasing power while a weak probe tone is swept in frequency. As the population in the first state is increased, due to the drive at ${\omega }_{01}$, scattering at ${\omega }_{12}$ becomes possible, appearing as another dip in the transmittance. From this, we extract ${\omega }_{12}/2\pi =6.38\mathrm{GHz}$. (b) The microwave pump is now applied at ${\omega }_{12}$. As the power of the ${\omega }_{12}$ pump increases, we see electromagnetically induced transparency (EIT) at ${\omega }_{01}$ as the Autler-Townes doublet splits with a separation equal to the Rabi frequency ${\Omega }_c/2\pi$ (black dashed lines). (inset) Energy level diagram. (c) Cartoon of the router. With the control off, the input probe is reflected from the transmon, and is routed to port 1 through the circulator. When the control is on, the input is transmitted to port 2. Inset: the control pulse sequence. (d) Normalized on-off ratio (see text) of the transmittance (T) and reflectance (R) as a function of control pulse power, measured simultaneously on sample 1. The symbols are the data and the solid lines are fits. (e) Time dependence of T and R at ${\omega }_{01}$, measured simultaneously for sample 1, while a control pulse is applied. (f) Same for sample 2, although T and R are measured separately. We see that the input signal is routed with an on-off ratio of $\sim 90\%$ ($\sim 99\%$) for sample 1 (2). (g) The response of sample 1 to a 10 ns Gaussian control pulse (circles), along with a Gaussian fit (solid line). We see that the transmittance smoothly follows the control on the few ns time scale while maintaining the high on-off ratio.

Figure 3

Cartoon of a multiport router. The router demonstrated here can easily be cascaded to distribute photons to many channels. Here we show a four-port router using 3 atoms (A,B,C) in series, each separated by a circulator. The 0–1 transition frequencies of the atoms are the same, while the 1–2 transition frequencies, ${\omega }_A\ne {\omega }_B\ne {\omega }_C$, are different. This arrangement can be designed in a straightforward manner by controlling the ratio of $E_J/E_c$. By turning on and off control tones at the various 1–2 transition frequencies, we can determine the output channel of the probe field, according to the table. For instance, if we wish to send the probe field to channel 3, we apply two control tones at ${\omega }_A$ and ${\omega }_B$. We note that all the control tones can be input through the same port regardless of the number of output channels, reducing the complexity of the design.