One challenge for Rydberg atom-based quantum sensors is to rapidly change the radio frequency signal detection frequency. To change the sensor detection frequency, it is necessary to shift the coupling laser wavelength over a broad wavelength range that can be on the order of a several nanometers. Laser relocking and wavelength shifting is slow and takes at least hundreds of milli-seconds. Wavelength shifting also has limited range given conventional laser technologies. Fast laser wavelength switching is essential for applications like efficient test and measurement, multi-frequency communications and signal analysis.

A laser feeds into several components in sequence, resulting in an amplified output of the desired frequency
Fig. 1: A schematic of the method we used for fast switching of the 1455 nm laser source. An electro-optic modulator (EOM) is used to produce a frequency comb, which is split using an arrayed waveguide grating (AWG). The desired frequency is selected by an optical switch, then shifted to the correct frequency using another EOM.

We designed a stable laser system capable of swiftly switching among various target wavelengths. The switching times can be less than 1 microsecond, limited by electro-optic switching times. The system consists of several key components: a continuous-wave, stable laser locked to a frequency reference; a flat frequency comb; a frequency filter; and a high-speed frequency shifter. In this approach, the laser is never required to change its frequency and acts as a stable frequency reference for the system. A comb line generated from the stable frequency reference near to the target wavelength is selected, amplified and fine-tuned to the target Rydberg state.

In order to demonstrate the system’s versatility, we utilized the system for Rydberg atom-based radio frequency sensing in vapour cells.  The observed spectral profile of the frequency comb is shown in Fig. 2 and is remarkably flat. All 61 lines are within a 3dB bandwidth across a wavelength range of 8 nm.

In Fig. 3(a), we show the system switching between two different Rydberg states. The switching action starts at t = 10 ms, as marked by the dashed vertical line in Fig. 3(a). The falling edge on the graph shows the delay time in the switching process. The switching time was 400 μs in the case shown in the figure. By utilizing the frequency sweeping capability of the frequency shifter, the laser system’s output can be scanned across the Rydberg spectral features, Fig. 3(b).

A jagged line graph plotting normalized power in decibels on the Y axis, and wavelength in nanometres on the X axis.
Fig. 2: Observed spectral profile of the frequency comb used for fast wavelength switching. 61 lines are within a 3dB range, allowing switching over a wavelength range of 8 nm.
The left line graph plots RF field strength in arbitrary units on the Y axis, and time in milliseconds on the X axis. The right line graph plots RF field strength in arbitrary units on the Y axis, and scan frequency in megahertz on the X axis.
Fig. 3: Demonstration of the switching time of the laser. In (a), the laser frequency offset is held constant while the laser is switched from the 65D Rydberg level to 62D. The switching time is 400 us. In (b), the laser frequency offset is swept, allowing the lineshapes of the two Rydberg levels to be observed.

We improved the system’s switching speed by constructing a ping-pong scheme for changing the sensing frequency, Fig. 4. The RF detection process alternates between two RF sensing frequencies: one at 9.6 GHz with 100 kHz modulation and one at 8.3 GHz with 65 kHz modulation, Fig. 5. Ultimately, the switching speed was < 50 microseconds, limited by the re-locking time of the frequency shifter.

Several optical components are connected in a complex circuit.
Fig. 4: Illustration of the ping-pong scheme used for changing the sensing frequency. Two RF synthesizers are fed into an RF switch. While one frequency is active, the other RF synthesizer is tuned to the next frequency.
A graph with modulation frequency in kilohertz on the Y axis, and time in milliseconds on the X axis. A coloured key indicates the power/ frequency in arbitrary units.
Fig. 5: Measured observation of the switching time from 65D to 62D using the improved frequency switching method.

Representative Publications:

  1. Tuning the output of a laser,” C. Liu, K.A. Nickerson, M. Hajialamdari, and J.P. Shaffer, US patent 11,658,461 B1 (2023).

Funding agencies:

DARPA logo

Interested in Collaborating or Joining Our Team?

If you are interested in collaborating with us or becoming a technical staff member, including student internships and postdoctoral training, please contact James Shaffer at [email protected].