On-chip single photon generation – News in Quantum Physics and Computing

As buzz grows about the future of quantum, researchers around the world are working overtime to figure out the best way to unlock the promise of superimposed, entangled, tunneling, or otherwise primetime-ready quantum particles. , whose ability to occur in two states at once could dramatically increase power and efficiency in many applications.

From a development perspective, however, today’s quantum devices are “pretty much where the computer was in the 1950s,” that is, the very beginning. That’s according to Kamyar Parto, a sixth-year Ph.D. student at the UC Santa Barbara laboratory of Galan Moody, expert in quantum photonics and assistant professor of electrical and computer engineering. Parto is co-lead author of an article published in the journal Nano-lettersdescribing a key breakthrough: the development of a sort of ‘factory’ on a chip to produce a steady and rapid stream of single photons, essential for enabling photonic-based quantum technologies.

In the early stages of computer development, Parto explains, “Researchers had just made the transistor, and they had ideas of how to make a digital switch, but the platform was pretty weak. Different groups developed different platforms, and eventually everyone converged on CMOS (complementary metal-oxide semiconductor) Then we had the huge explosion around semiconductors.

“Quantum technology is in a similar place – we have the idea and an idea of ​​what we could do with it, and there are many competing platforms, but no clear winner yet,” he continued. . “You have superconducting qubits, silicon spin qubits, electrostatic spin qubits, and quantum computers based on ion traps. Microsoft is trying to create topologically protected qubits, and at 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 example, it’s very easy to transfer information using quantum photonics, because light likes to move,” he said. “A spin qubit, however, makes it easy to store information and do 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, then transforms it back into another format once it’s there? »

Qubits, those oddly-behaving drivers of quantum technologies, are of course different from classical bits, which can only exist in a single state of zero or one. Qubits can be both one and zero simultaneously. In the field of photonics, Parto said, a single photon can be made to exist (state one) and not to exist (state zero).

This is because a single photon constitutes what is called a two-level system, meaning it can exist in a zero state, one state, or any combination, such as 50% one and 50 % zero, or maybe 80% one and 20% zero. This can be done routinely in the Moody Group. The challenge is to generate and collect single photons with very high efficiency, for example by routing them onto a chip using waveguides. Waveguides do exactly what their name suggests, guiding light where it needs to go, much like wires guide electricity.

Parto explained: “If we put these single photons in many different waveguides – a thousand single photons on each waveguide – and we kind of choreograph how the photons travel along the waveguides. ‘waves on the chip, we can do a quantum calculation. »

While it is relatively simple to use waveguides to deliver photons to a chip, isolating a single photon is not easy, and setting up a system that produces billions of them quickly and efficiently is much more difficult. . The new paper describes a technique that uses a particular phenomenon to generate single photons with much higher efficiency than previously achieved.

“The job is to amplify the generation of these single photons so that they become useful for real applications,” Parto said. “The breakthrough described in this paper is that we can now reliably generate single photons at room temperature in a way that lends itself to CMOS (mass production process). »

There are different ways to generate single photons, but Parto and his colleagues do this by using defects in certain two-dimensional (2D) semiconductor materials, which are only one atom thick, essentially removing a little of material to create a defect.

“If you shine light (generated by a laser) on the right kind of defect, the material will respond by emitting single photons,” Parto said, adding, “The defect in the material acts like what’s called a rate-limiting state, allowing it to behave like a factory to produce single photons, one at a time. A photon can be produced as often as every three to five nanoseconds, but researchers aren’t sure of the rate yet, and Parto, who earned her Ph.D. on the topic of engineering such flaws, says the current pace could be much slower.

A great advantage of 2D materials is that they lend themselves to the integration of defects at specific locations. Additionally, Parto said, “The materials are so thin that you can pick them up and put them on any other material without being constrained by the lattice geometry of a 3D crystalline material. This makes 2D material very easy to integrate, a capability we show in this article. »

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

The researchers try to do this in several ways, such as placing the material on the waveguide and then looking for a single existing defect, but even if the defect is precisely aligned and in exactly the right position, the efficiency of extraction will be only 20 to 30%. This is because the single defect can only emit at a specific rate and some of the light is emitted at oblique angles, rather than straight along the path to the waveguide. The theoretical upper limit of this design is only 40%, but manufacturing a useful device for quantum information applications requires an extraction efficiency of 99.99%.

“The light from a defect inherently shines everywhere, but we prefer it to shine in these waveguides,” Parto explained. “We have two choices. If you put waveguides above the defect, maybe ten to fifteen percent of the light would go into the waveguides. It’s not sufficient. But there is a physical phenomenon, called the Purcell effect, that we can use to increase this efficiency and direct more light into the waveguide. You do this by placing the defect inside an optical cavity – in our case, it comes in the form of a micro-ring resonator, which is one of the only cavities that allows you to couple light in and out of a waveguide.

“If the cavity is small enough,” he added, “it will eliminate vacuum fluctuations from the electromagnetic field, and these fluctuations cause photons to spontaneously emit from the defect in a light mode. By reducing this quantum fluctuation in a cavity of finite volume, the fluctuation on 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 the experiments using the micro-ring resonator that were performed for this article, the team achieved an extraction efficiency of 46%, which is an order of magnitude increase over previous reports.

“We are really encouraged by these results, as single-photon emitters in 2D materials address some of the major challenges faced by other materials in terms of scalability and manufacturability,” Moody said. “In the short term we will explore their use 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. »

To do this, the group must improve its efficiency to over 99%, and achieving this will require higher quality nitride resonator rings. “To improve efficiency, you need to smooth the ring when you cut it out of the silicon nitride film,” Parto explained. “However, if the material itself is not fully crystalline, even if you try to smooth it out at the atomic level, surfaces can still appear rough and sponge-like, causing light to scatter. »

While some groups get the highest quality nitride by buying it from companies that grow it perfectly, Parto explained, “We have to grow it ourselves, because we have to put the defect under the material, and also, we use a special kind of silicon nitride system that minimizes background light for single photon applications, and the companies don’t. »

Parto can grow his nitrides in a plasma-enhanced chemical vapor deposition furnace in UCSB’s clean room, but because it’s a heavily used shared facility, he’s not able to customize certain parameters that would allow him to grow materials of sufficient quality. The plan, he says, is to use those results to apply for new grants that would “get our own tools and hire students to do that work.”

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On-chip single photon generation – News in Quantum Physics and Computing

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