Lasers are vital for experiments that require extremely precise measurements and atom control, like two-photon atomic clocks, cold-atom interferometer sensors, and quantum gates. The purity of the laser light (emitting a single frequency) directly influences the effectiveness of these experiments, and currently, traditional lab-scale laser setups achieve this reliable, low-noise light through large, expensive systems designed to emit photons in a very limited spectral range.
Imagine if these atomic technologies could be moved out of laboratories and onto portable devices. This is the goal of UC Santa Barbara engineering professor Daniel Blumenthal’s team, which is working to replicate the capabilities of these lasers in much smaller, hand-held devices.
Andrei Isichenko, a graduate student in Blumenthal’s lab, explained, “These compact lasers will create scalable solutions for genuine quantum systems, as well as portable, field-deployable, and space-based quantum sensors. This advancement will significantly affect technology areas like quantum computing with neutral atoms and trapped ions, as well as cold atom quantum sensors such as atomic clocks and gravimeters.”
In a publication in the journal Scientific Reports, Blumenthal and his team reveal their progress with a chip-scale ultra-low-linewidth self-injection locked laser operating at 780 nm. This device, comparable in size to a matchbox, is claimed to outperform existing narrow-linewidth 780 nm lasers, while being much cheaper to produce and requiring less space.
Mastering the Laser
Rubidium is the atom that drives this laser innovation, selected for its properties that make it perfect for various high-precision tasks. The stability of its D2 optical transition is crucial for atomic clocks, and its sensitivity makes it widely used in sensors and cold atom research. By directing a laser through a rubidium vapor, researchers can align the laser’s characteristics with the stable atomic transition.
“You can utilize the atomic transition lines to stabilize the laser,” Blumenthal, the paper’s senior author, pointed out. “Essentially, locking the laser to these atomic transition lines allows the laser to mirror the stability of that atomic transition.”
However, simply having a red light isn’t sufficient for a precise laser. It is crucial to eliminate “noise.” Blumenthal compared this to the difference between a tuning fork and guitar strings.
“Striking a C note on a tuning fork yields a perfect tone, while hitting a C on guitar creates multiple tones,” he said. “Similarly, lasers may emit various frequencies (colors) that introduce unwanted ‘tones.’ To achieve a single frequency—pure deep-red light in this instance—traditional systems include extra components to filter the laser light. The researchers faced the challenge of integrating all such functions onto a chip.”
The team combined a commercially available Fabry-Perot laser diode with some of the world’s lowest-loss waveguides and high-quality factor resonators, all made on a silicon nitride platform. This approach enabled them to replicate the performance of large, desktop systems. Their tests indicate that their laser surpasses some tabletop lasers and previously reported integrated lasers in key areas such as frequency noise and linewidth by a factor of 10,000.
“The low linewidth values signify that we can construct a compact laser without compromising performance,” Isichenko elaborated. “In fact, the performance is sometimes better than that of traditional lasers due to the complete chip-scale integration. These linewidths enhance our interaction with atomic systems, allowing us to erase contributions from laser noise and fully detect the atomic response to environmental changes, for instance.” The low linewidths achieved—a record sub-Hz fundamental and sub-KHz integral—demonstrate the technology’s stability and capacity to minimize noise from outside and internal sources.
Additional advantages of this technology include its affordability; it relies on a $50 diode and utilizes a cost-effective fabrication method derived from CMOS-compatible wafer scale processes, typical in electronic chip manufacturing.
The success of this technology paves the way for deploying these high-performance, precise, and low-cost integrated lasers in various environments, both in and out of laboratories. Potential applications include quantum experiments, atomic timekeeping, and detecting faint signals, like variations in gravitational acceleration across Earth.
Blumenthal stated, “These devices could be mounted on satellites to create detailed gravitational maps of Earth, allowing for measurements relating to sea level rise, shifts in sea ice, and earthquake detection by tracking gravitational fluctuations.” The compactness, low energy requirements, and lightweight nature of these lasers make them an ideal choice for deployment in space.
