Reviewed by Lexie CornerDec 13 2024
Researchers at UC Santa Barbara have developed a chip-scale ultra-low-linewidth laser that matches or exceeds the performance of large, high-cost tabletop systems. By using rubidium atoms as a reference, the laser offers exceptional stability and low noise, making it well-suited for applications like quantum computing, atomic clocks, and portable quantum sensors. The study was published in Scientific Reports.
Andrei Isichenko holds the ultra-high-quality ring resonator (left), which can help turn the “coarse” light from a commercially available Fabry-Perot laser diode (right) into a low linewidth laser. Image Credit: Sonia Fernandez
Lasers are the preferred technology for experiments that require extremely precise measurements and control over atoms, such as two-photon atomic clocks, cold-atom interferometer sensors, and quantum gates.
The more spectrally pure the laser (emitting a single color or frequency), the better. Currently, this ultra-low-noise, stable light is produced by traditional lab-scale laser technology, which relies on large, expensive tabletop systems designed to produce, capture, and emit photons within a specific spectral range.
But what if these atomic applications could move beyond the confines of laboratories and benchtops? This is the primary goal of Daniel Blumenthal's lab at UC Santa Barbara, where his team is working to replicate the performance of these lasers in portable, lightweight devices small enough to fit in the hand.
These smaller lasers will enable scalable laser solutions for actual quantum systems, as well as lasers for portable, field-deployable, and space-based quantum sensors. This will impact technology spaces such as quantum computing with neutral atoms and trapped ions and also cold atom quantum sensors such as atomic clocks and gravimeters.
Andrei Isichenko, Graduate Student Researcher, University of California Santa Barbara
Blumenthal, Isichenko, and their colleagues have developed a chip-scale ultra-low-linewidth self-injection locked 780 nm laser as part of their work in this direction. The researchers suggest that this compact device, about the size of a matchbox, can outperform existing narrow-linewidth 780 nm lasers at a fraction of the cost and space required to manufacture them.
Lassoing the Laser
The key to the laser’s performance lies in rubidium, the atom driving its development. Rubidium was chosen for its well-established properties that make it ideal for a variety of high-precision applications. The atom’s sensitivity makes it particularly useful in cold atom physics and sensors, while its stable D2 optical transition makes it an excellent candidate for atomic clocks.
By passing a near-infrared laser through a vapor of rubidium atoms, which serve as the atomic reference, the laser can acquire the characteristics of the stable atomic transition.
You can use the atomic transition lines to lasso the laser. In other words, by locking the laser to the atomic transition line, the laser more or less takes on the characteristics of that atomic transition in terms of stability.
Daniel Blumenthal, Study Senior Author, University of California Santa Barbara
However, a precision laser is more than just a simple light source. To achieve the desired quality of light, "noise" must be minimized. Blumenthal compares this to the difference between a guitar string and a tuning fork.
If you have a tuning fork and hit a C note, it is probably a pretty perfect C. But if you strum a C on a guitar, you can hear other tones in there. These smaller lasers will enable scalable laser solutions for actual quantum systems, as well as lasers for portable, field-deployable, and space-based quantum sensors.
Daniel Blumenthal, Study Senior Author, University of California Santa Barbara
Lasers can use different frequencies (or "colors") to produce additional “tones.” In tabletop systems, extra components are added to further fine-tune the laser light to produce the desired single frequency—in this case, pure deep-red light. The challenge for the researchers was to incorporate all of this functionality and performance onto a chip.
To achieve this, the team used a silicon nitride platform to create some of the world’s lowest-loss waveguides (developed in Blumenthal's lab), a commercially available Fabry-Perot laser diode, and high-quality resonators with excellent performance.
Their device outperforms some tabletop lasers and previously reported integrated lasers by four orders of magnitude in key metrics like frequency noise and linewidth. This breakthrough enabled them to replicate the performance of large, bulky systems in a much smaller form factor.
Isichenko explained, “The significance of the low linewidth values is that we can achieve a compact laser without sacrificing laser performance. In some ways, the performance is improved compared to conventional lasers because of full chip-scale integration. These linewidths help us better interact with atomic systems, eliminating contributions from the laser noise to fully resolve the atomic signal in response to, for example, the environment they are sensing.”
The low linewidths—record-low sub-Hz fundamental and sub-KHz integral for this project—demonstrate the stability and capability of the laser to filter out both internal and external noise.
Other advantages of this technology include its affordability (it uses a $50 diode) and its scalable, cost-effective fabrication process. The method, developed using a wafer-scale approach, is compatible with CMOS technology and draws inspiration from the field of electronic chip fabrication.
If successful, this technology could make high-performance, precise, and inexpensive photonics-integrated lasers available for a range of applications, both in and out of the lab. These include quantum experiments, atomic timekeeping, and detecting very small signals, such as changes in gravitational acceleration around the Earth.
Blumenthal said, “You can put these on satellites to make a gravitational map of the Earth and around the Earth with a certain amount of precision. You could measure sea level rise, changes in sea ice, and earthquakes by sensing the gravitational fields around the Earth.”
He also emphasized that the technology’s lightweight design, low power consumption, and compact size make it a “perfect fit” for space deployment.
Additional authors on the study include Andrew S. Hunter, Dabapam Bose, Nitesh Chauhan, Maiting Song, Kaikai Lu, and Mark W. Harrington.
Journal Reference:
Isichenko, A., et al. (2024) Sub-Hz fundamental, sub-kHz integral linewidth self-injection locked 780 nm hybrid integrated laser. Scientific Reports. doi.org/10.1038/s41598-024-76699-x.