The lab of Stanford University electrical engineer Jelena Vučković recently joined the microcomb community. “Many groups have demons­trated on-chip frequency combs in a variety of materials, including recently in silicon carbide by our team. However, until now, the quantum optical pro­perties of frequency combs have been elusive,” said Vučković. “We wanted to leverage the quantum optics background of our group to study the quantum properties of the soliton microcomb.”

While soliton microcombs have been made in other labs, the Stanford researchers are among the first to investigate the system’s quantum optical properties, using a special process. When created in pairs, microcomb solitons are thought to exhibit entanglement which is the basis of all proposed quantum techno­logies. Most of the classical light we encounter on a daily basis does not exhibit entanglement. “This is one of the first demonstrations that this minia­turized frequency comb can generate interesting quantum light – non-classical light – on a chip,” said Kiyoul Yang, a research scientist in Vučković’s Nanoscale and Quantum Photonics Lab. “That can open a new pathway toward broader explora­tions of quantum light using the frequency comb and photonic inte­grated circuits for large-scale experiments.”

Proving the utility of their tool, the researchers also provided convin­cing evidence of quantum ent­anglement within the soliton microcomb, which has been theorized and assumed but has yet to be proven by any existing studies. “I would really like to see solitons become useful for quantum computing because it’s a highly studied system,” said graduate student Melissa Guidry. We have a lot of technology at this point for genera­ting solitons on chips at low power, so it would be exciting to be able to take that and show that you have entanglement.”

Former Stanford physics professor Theodor W. Hänsch won the Nobel Prize in 2005 for his work on developing the first frequency comb. To create what Hänsch studied requires compli­cated, tabletop-sized equipment. Instead, these researchers chose to focus on the newer, micro version, where all of the parts of the system are integrated into a single device and designed to fit on a micro­chip. This design saves on cost, size and energy. To create their miniature comb, the researchers pump laser light through a micro­scopic ring of silicon carbide. Traveling around the ring, the laser builds up inten­sity and, if all goes well, a soliton is born.

“It’s fasci­nating that, instead of having this fancy, compli­cated machine, you can just take a laser pump and a really tiny circle and produce the same sort of specia­lized light,” said graduate student Daniil Lukin. He added that generating the microcomb on a chip enabled a wide spacing between the teeth, which was one step toward being able to look at the comb’s finer details. The next steps involved equipment capable of detecting single particles of the light and packing the micro-ring with several solitons, creating a soliton crystal. “With the soliton crystal, you can see there are actually smaller pulses of light in between the teeth, which is what we measure to infer the entangle­ment structure,” explained Guidry. “If you park your detectors there, you can get a good look at the interes­ting quantum behavior without drowning it out with the coherent light that makes up the teeth.”

Seeing as they were performing some of the first experi­mental studies of the quantum aspects of this system, the researchers decided to try to confirm a theoretical model, the linearized model, which is commonly used as a shortcut to describe complex quantum systems. When they ran the comparison, they were astonished to find that the experiment matched the theory very well. So, while they have not yet directly measured that their micro­comb has quantum entangle­ment, they have shown that its perfor­mance matches a theory that implies ent­anglement. “The take-home message is that this opens the door for theorists to do more theory because now, with this system, it’s possible to experi­mentally verify that work,” said Lukin.

Microcombs in data centers could boost the speed of data transfer; in satellites, they could provide more precise GPS or analyze the chemical compo­sition of far-away objects. The Vučković team is parti­cularly interested in the potential for solitons in certain types of quantum computing because solitons are predicted to be highly entangled as soon as they are generated. With their platform, and the ability to study it from a quantum perspective, the researchers are keeping an open mind about what they could do next. Near the top of their list of ideas is the possi­bility of performing measure­ments on their system that defini­tively prove quantum ent­anglement. (Source: Stanford U.)

Reference: M. A. Guidry et al.: Quantum optics of soliton microcombs, Nat. Phot. 16, 52 (2022); DOI: 10.1038/s41566-021-00901-z

Link: Nanoscale and Quantum Photonics Lab, Stanford University, Palo Alto, USA