Sub-Doppler rubidium atom cooling using a programmable agile integrated PZT-on-SiN resonator

Abstract

Programmability and precise control of laser frequency are essential for quantum experiments and applications such as atomic clocks, quantum computers, and cold-atom sensors. Current systems use bulky, power-hungry modulators and frequency shifters which are difficult to integrate and limit portability and scalability. We report an electrically controllable, agile optical frequency source based on a semiconductor laser stabilized to a photonic-integrated, lead zirconate titanate (PZT)-actuated resonator cavity. We demonstrate this approach with precision programmable frequency control of a 780-nm laser that can periodically reference to rubidium spectroscopy followed by fast, programmable, arbitrary frequency tuning sequences for quantum control. We use this approach to demonstrate sub-Doppler cooling of rubidium-87 without any external modulators, achieving atom-cloud temperatures as low as 16 $μ$K. The device achieves a tuning strength up to 1 GHz/V with 11 MHz modulation bandwidth while consuming only 10 nW of electrical power. This work establishes a route toward compact, low-power, and chip-scale laser systems for next-generation quantum and atomic sensing technologies.

Figures (12)

• Figure 1: Working principle of rubidium sub-Doppler cooling using an agile integrated PZT-controlled silicon nitride cavity.a) A 780-nm laser is Pound-Drever-Hall (PDH) locked to an integrated resonator containing a PZT stress-optic actuator. The laser closely tracks the resonator (PZT PIC lock) and the laser drift is reduced at short timescales (i.e. drift holdover). The PZT is actuated such that the locked laser is stabilized to a rubidium hyperfine transition using saturation absorption spectroscopy (SAS), which provides Rb disciplining and long-term frequency stability. During the cooling cycle, we switch off the Rb lock (while PZT PIC lock remains active) and apply a fast control signal to the PZT to rapidly shift the laser frequency for the Rb MOT (magneto-optical trap) and for sub-Doppler atom cooling. b) The entire cold atom cycle consists of times when the laser and PZT PIC are Rb disciplined (1) and when the PZT PIC provides frequency agility and short-term holdover during the atom cooling experiment (2). c) Micrograph of the PZT-actuated SiN ring resonator cavity, ring radius 750 $\mu$m.
• Figure 2: PZT PIC resonator design and characterization.a) Cross-section of the silicon nitride ring resonator with PZT actuator and Pt thermal tuner, where the offset represents the lateral displacement between the PZT and the waveguide. b) Transmission spectrum quality factor (Q) measurement of the ring resonator results in an intrinsic quality factor ($Q_i$) of 2.8 million, a loaded quality factor ($Q_L$) of 2.3 M, loss $\alpha$=20 dB/m, and a total linewidth $\Delta \nu$ of 170 MHz. The frequency detuning is calibrated using an unbalanced Mach-Zehnder interferometer (MZI, blue trace). c) Frequency response of the PZT stress-optic small-signal modulation. The 6-dB modulation bandwidth is 11 MHz. d) Transmission spectra for a single resonance as a function of applied bias voltage to the PZT. The frequency tuning is calibrated with an unbalanced Mach-Zehnder interferometer (Fiber MZI, green trace) e) The resonance frequency shift as a function of the applied DC bias. The measured tuning strength is 1 GHz/V $\approx$ 2 pm/V. f) Map of the 4” wafer DUV stepper lithography exposure reticles. Each reticle contains many devices; we report the data (PZT tuning strength, Qi) for only a single add-thru resonator device in the reticle. Green outline: device with largest tuning strength, blue outline: device used for atom cooling demonstration, N/A: not applicable due to device damage.
• Figure 3: Cold atom experiment diagram.a) The 780-nm DBR cooling laser is stabilized to the PZT PIC resonator cavity for reduced laser drift (i.e. holdover) and locked laser frequency control applied with $V_{\rm PZT}$. The PZT control switches between fast frequency control ($V_{\rm agile}$) used to control the cavity for 1) Rb disciplining, by locking to the Rb saturation absorption spectroscopy (SAS) and 2) MOT formation and for sub-Doppler cooling. The cooling laser light is amplified with a semiconductor optical amplifier (SOA) which is voltage-controlled ($V_{\rm SOA}$) for power ramping and shuttering, used for sub-Doppler cooling temperature measurements. b) The $V_{\rm agile}$ frequency control tunes the cooling laser frequency enabling arbitrary laser frequency control within the tuning range of the PZT actuator and at speeds within the locking range of the laser lock. c) Timing diagram for one cold atom experiment cycle consisting of the Rb disciplining of the PZT PIC cavity and tuning the PIC to set the cooling laser frequency with respect to Rb hyperfine transitions. The Rb disciplining is done at the strong $F'=(2,3)$ cross-over transition for 25 ms (see Fig. \ref{['fig:sup_timing_diagram']}). The PZT is jumped by a set voltage to set the cooling laser at $\Delta_{\rm MOT}$ detuning from the cooling ($F'=3$) transition. After full MOT loading, the fast frequency control signal is applied to the PZT to rapidly ramp the laser by 15 MHz in 12 ms for polarization gradient cooling (PGC) to achieve sub-Doppler cooling, measured during the time-of-flight (TOF) stage.
• Figure 4: MOT temperature measurements for different cooling laser control configurations.a) Time-of-flight (TOF) atom cloud temperature measurements, with the squared cloud radius along the $z$ (open markers) and $y$ (filled markers) dimensions for different squared TOF times $t_{\rm TOF}^2$. The linear fits are for $\sigma^2_i = \sigma^2_{i,0} + \frac{k_B T_i}{m} t_{\rm TOF}^2$, where $\sigma_i$ is the Gaussian standard deviation of the cloud along an axis ($y,z$), $\sigma_{i,0}$ is the initial width of the cloud, $k_B$ is the Boltzmann constant, $m$ is the mass of a single $^{87}$Rb atom, and $T_i$ is the temperature along an axis. The schematic for the experiment configurations (I-IV) is shown in Figure \ref{['fig:sup_schematics_during_TOF']}. c) Fluorescence images of the MOT cloud after free-expansion for $t_{\rm TOF}$ = 6 ms. The color scale is identical for both images.
• Figure 5: Cooling laser frequency agility measurements.a) Schematic for monitoring the PZT-controlled cooling laser frequency during the MOT and sub-Doppler cooling. A reference 780-nm DBR laser is locked to another Rb spectroscopy system and the heterodyne beat-note between the reference and the cooling lasers is recorded on a photodiode and a frequency counter. During the sequence the beat-note is monitored on an electronic spectrum analyzer (ESA) and the MOT fluorescence signal is extracted from a camera. b) Laser beat-note recorded on a frequency counter during 20 minutes of continuous operation. When the PZT cavity is re-referenced to the Rb SAS (with Rb reference, top), the beat-note remains near 78 MHz (185 MHz) for the referencing (MOT formation) parts of the cycle. We record the MOT atom number the entire time. Without the Rb referencing (bottom) the drift results in the loss of trapped atom within the first minute of operation. c) Zoom-in of the beat-note in (b, top) for six cycles. (d) One period from of (c) overlaid with 100 consecutive cycles, showing the stability of the PZT response during this period. The first jump has a 25-ms Rb referencing time and the triangular ramp at 290 ms is the PGC ramp. (e) Pre-programmed reference image illustrating the intended laser beat-note trajectory shown in (f).