UCSB's Moody Lab develops new method for chip generation of single photons

19 Jan 2023

Described in Nano Letters journal as “on-chip factory”, necessary for photonic-based quantum technologies.

The development of quantum devices today are “about where the computer was in the 1950s,” according to Kamyar Parto, a PhD student in the UC Santa Barbara lab of Galan Moody, an expert in quantum photonics and an assistant professor of electrical and computer engineering.

Parto is co-lead author of a paper in Nano Letters, describing the development of an “on-chip factory” for producing a steady, fast stream of single photons – essential to enabling photonic-based quantum technologies.

‘Competing platforms’

“Considering quantum technology, we have the idea and a sense of what we could do with it, and there are many competing platforms, but no clear winner yet,” said Parto. “You have superconducting qubits, spin qubits in silicon, electrostatic spin qubits and ion-trap-based quantum computers. Microsoft is trying to do topologically protected qubits, and in the Moody Lab, we’re working on quantum photonics.”

Parto predicts that the winning platform will be a combination of different platforms, given that each is powerful but also has limitations. “For instance, it’s very easy to transfer information using quantum photonics, because light likes to move,” he said. “A spin qubit, however, makes it easier to store information and do some local ‘stuff’ on it, but you can’t move that data around. So, why don’t we try to use photonics to transfer the data from the platform that stores it better, and then transform it again to another format once it’s there?”

A single photon constitutes what is called a two-level system, meaning that it can exist in a zero state, a one state, or any combination, such as 50% one and 50% zero. This can be done routinely in the Moody group. The challenge, said Parto, is to generate and collect single photons at high efficiency, such as by routing them on a chip using waveguides.

Parto explained,“If we put these single photons into many different waveguides — a thousand single photons on each waveguide — and we sort of choreograph how the photons travel along the waveguides on the chip, we can do a quantum computation.”

He added, “The breakthrough described in the Nano Letters paper is that we can now generate the single photons reliably at room temperature in a way that lends itself to the mass-production process of CMOS.” There are various approaches to generating single photons, but Parto and his colleagues are doing it by using defects in certain two-dimensional (2D) semiconductor materials, which are only one atom thick, essentially removing a piece of the material to create a defect.

A factory for single photons

“If you shine laser light onto the right kind of defect, the material will respond by emitting single photons,” Parto said. “The defect in the material acts as a rate-limiting state, which allows it to behave like a factory for single photons.” One photon might be produced as often as every three to five nanoseconds, but the researchers aren’t yet sure of the rate, and Parto, who earned his Ph.D. on the topic of engineering such defects, says that the current rate could be much slower.

An advantage of 2D materials is that they lend themselves to having defects engineered into them at specific locations. “The materials are so thin that you can pick them up and put them on any other material. That makes the 2D material very easy to integrate, a capability we show in this paper,” said Parto.

To make a useful device, the defect on the 2D material must be placed in the waveguides with extreme precision. “There is one point on the material that produces light from a defect,” Parto noted, “and we need to get that single photon into a waveguide.”

“The light from a defect inherently shines everywhere, but we prefer that it shine into these waveguides,” Parto explained. “We have two choices. If you put waveguides on top of the defect, maybe ten to fifteen percent of the light would go into the waveguides. That’s not enough. But there is a physics phenomenon, called the Purcell effect, which we can utilize to boost this efficiency and direct more of the light into the waveguide.

‘Encouraging results’

“If the cavity is small enough,” he added, “it will squeeze out the vacuum fluctuations of the electromagnetic field, and those fluctuations are what cause the spontaneous emission of photons from the defect into a mode of light. By squeezing that quantum fluctuation into a cavity of finite volume, the fluctuation over the defect is increased, causing it to emit light preferentially into the ring, where it accelerates and becomes brighter, thus increasing the extraction efficiency.”

In experiments using the micro-ring resonator that were done for this paper, the team achieved extraction efficiency of 46%, which is an order-of-magnitude increase over prior reports.

Galan Moody commented, “We’re really encouraged by these results, because single-photon emitters in 2D materials address some of the outstanding challenges facing other materials in terms of scalability and manufacturability. In the near term, we’ll explore using them for a few different applications in quantum communications, but in the long term, our goal is to continue to develop this platform for quantum computing and networking.”