Diamond Metalens for Qubits

By finding defects inside a block of diamond and fashioning a pattern of nanoscale pillars on the surface above it, the researchers can control the shape of individual photons emitted by the defect. Because those photons carry information about the spin state of an electron, such a system could be used as the basis for compact quantum technologies. (Source: A. S. Blevins)

Even a seemingly flawless diamond contains defects: spots in that lattice where a carbon atom is missing or has been replaced by something else. Some of these defects are highly desirable; they trap individual electrons that can absorb or emit light, causing the various colors found in diamond gemstones and, more importantly, creating a platform for diverse quantum tech­nologies for advanced computing, secure communi­cation and precision sensing. Qubits from diamonds are of particular interest to quantum scientists because their quantum-mechanical properties, including super­position, exist at room temperature, unlike many other potential quantum resources.

The practical challenge of collecting information from a single atom deep inside a crystal is a daunting one, however. Penn Engineers addressed this problem in a recent study in which they devised a way to pattern the surface of a diamond that makes it easier to collect light from the defects inside. A metalens, this surface structure contains nanoscale features that bend and focus the light emitted by the defects, despite being effec­tively flat.

The key to harnessing the potential power of quantum systems is being able to create or find structures that allow electron spin to be reliably mani­pulated and measured, a difficult task considering the fragility of quantum states. Bassett’s lab approaches this challenge from a number of directions. Recently, the lab developed a quantum platform based on the two-dimensional (2D) material hexagonal boron nitride which, due to its extremely thin dimensions, allows for easier access to electron spins. In the current study, the team returned to a 3D material that contains natural imper­fections with great potential for controlling electron spins: diamonds.

Small defects in diamonds, nitrogen-vacancy (NV) centers, are known to harbor electron spins that can be manipulated at room tempera­ture, unlike many other quantum systems that demand tempera­tures approaching absolute zero. Each NV center emits light that provides information about the spin’s quantum state. Bassett explains why it is important to consider both 2D and 3D avenues in quantum technology: “The different material platforms are at different levels of development, and they will ulti­mately be useful for different applications. Defects in 2D materials are ideally suited for proximity sensing on surfaces, and they might eventually be good for other appli­cations, such as integrated quantum photonic devices,” Bassett says. “Right now, however, the diamond NV center is simply the best platform around for room-tempera­ture quantum infor­mation processing. It is also a leading candidate for building large-scale quantum communi­cation networks.”

So far, it has only been possible to achieve the combi­nation of desirable quantum properties that are required for these demanding appli­cations using NV centers embedded deep within bulk 3D crystals of diamond. Unfortunately, those deeply embedded NV centers can be difficult to access since they are not right on the surface of the diamond. Collecting light from those hard-to-reach defects usually requires a bulky optical microscope in a highly controlled labora­tory environment. Bassett’s team wanted to find a better way to collect light from NV centers, a goal they were able to accomplish by designing a specialized metalens that circumvents the need for a large, expensive micro­scope.

“We used the concept of a meta­surface to design and fabricate a structure on the surface of diamond that acts like a lens to collect photons from a single qubit in diamond and direct them into an optical fiber, whereas pre­viously this required a large, free-space optical micro­scope,” Bassett says. “This is a first key step in our larger effort to realize compact quantum devices that do not require a room full of elec­tronics and free-space optical components.” Metas­urfaces consist of intricate, nanoscale patterns that can achieve physical phenomena otherwise impossible at the macroscale. The researchers’ metalens consists of a field of pillars, each 1 micrometer tall and 100-250 nanometers in diameter, arranged in such a way that they focus light like a tradi­tional curved lens. Etched onto the surface of the diamond and aligned with one of the NV centers inside, the metalens guides the light that represents the electron’s spin state directly into an optical fiber, stream­lining the data collection process.

“The actual metalens is about 30 microns across, which is about the diameter of a piece of hair. If you look at the piece of diamond that we fabricated it on, you can’t see it. At most, you could see a dark speckle,” says Tzu-Yung Huang. “We typically think of lenses as focusing or collimating, but, with a meta­structure, we have the freedom to design any kind of profile that we want. It affords us the freedom to tailor the emission pattern or the profile of a quantum emitter, like an NV center, which is not possible, or is very difficult, with free-space optics.” To design their metalens, Bassett, Huang and Richard Grote had to assemble a team with a diverse array of knowledge, from quantum mechanics to electrical engineering to nano­technology. Bassett credits the Singh Center for Nano­technology as playing a critical role in their ability to physically construct the metalens.

“Nano­fabrication was a key component of this project,” says Bassett. “We needed to achieve high-resolution lithography and precise etching to fabricate an array of diamond nanopillars on length scales smaller than the wavelength of light. Diamond is a challenging material to process, and it was Richard’s dedicated work in the Singh Center that enabled this capability. We were also lucky to benefit from the experienced cleanroom staff. Gerald helped us to develop the electron beam litho­graphy techniques. We also had help from Meredith Metzler, the Thin Film Area Manager at the Singh Center, in developing the diamond etch.”

Although nano­fabrication comes with its challenges, the flexi­bility afforded by meta­surface engin­eering provides important advantages for real-world appli­cations of quantum tech­nology: “We decided to collimate the light from NV centers to go to an optical fiber, as it readily interfaces with other techniques that have been developed for compact fiber-optic tech­nologies over the past decade,” Huang says. “The compa­tibility with other photonic structures is also important. There might be other structures that you want to put on the diamond, and our metalens doesn’t preclude those other optical enhance­ments.”

This study is just one of many steps towards the goal of compacting quantum technology into more efficient systems. Bassett’s lab plans to continue exploring how to best harness the quantum potential of 2D and 3D materials. “The field of quantum engi­neering is advancing quickly now in large part due to the conver­gence of ideas and expertise from many disciplines including physics, materials science, photonics and elec­tronics,” Bassett says. (Source: U. Pennsylvania)

Reference: T.-Y- Huang et al.: A monolithic immersion metalens for imaging solid-state quantum emitters, Nat. Commun. 10, 2392 (2019); DOI: 10.1038/s41467-019-10238-5

Link: Quantum Engineering Laboratory, Dept. of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, USA

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