Quantum chip scales up with photonics, ‘artificial atoms’

Technology News | July 9, 2020

By Rich Pell

The process, say the researchers, manufactures and integrates “artificial atoms” – created by atomic-scale defects in microscopically thin slices of diamond – with photonic circuitry. The artificial atoms, which can be prodded with visible light and microwaves to emit photons that carry quantum information, are the qubits in the new chip.

To create the chip, the researchers used carefully selected “quantum micro chiplets” containing multiple diamond-based qubits that are placed on an aluminum nitride photonic integrated circuit. The approach, say the researchers, demonstrates a viable way to scale up quantum processor production.

Using their hybrid method, the researchers were able to build a 128-qubit system – said to be the largest integrated artificial atom-photonics chip yet. The qubits are stable and long-lived, and their emissions can be tuned within the circuit to produce spectrally indistinguishable photons. The accomplishment, say the researchers, “marks a turning point” in the field of scalable quantum processors.

“In the past 20 years of quantum engineering, it has been the ultimate vision to manufacture such artificial qubit systems at volumes comparable to integrated electronics,” says Dirk Englund, an associate professor in MIT’s Department of Electrical Engineering and Computer Science. “Although there has been remarkable progress in this very active area of research, fabrication and materials complications have thus far yielded just two to three emitters per photonic system.”

The artificial atoms in the chiplets consist of color centers in diamonds – defects in diamond’s carbon lattice where adjacent carbon atoms are missing, with their spaces either filled by a different element or left vacant. In the MIT chiplets, the replacement elements are germanium and silicon.

Each center functions as an atom-like emitter whose spin states can form a qubit. The artificial atoms emit colored particles of light, or photons, that carry the quantum information represented by the qubit.

While diamond color centers make good solid-state qubits, say the researchers, the bottleneck with this platform is actually building a system and device architecture that can scale to thousands and millions of qubits. Instead of trying to build a large quantum chip entirely in diamond, the researchers decided to take a modular and hybrid approach.

“We use semiconductor fabrication techniques to make these small chiplets of diamond, from which we select only the highest quality qubit modules,” says Noel H. Wan, co-author of a paper on the research. “Then we integrate those chiplets piece-by-piece into another chip that ‘wires’ the chiplets together into a larger device.”

The integration takes place on a photonic integrated circuit, which uses photons rather than electrons to carry information. Photonics provides the underlying architecture to route and switch photons between modules in the circuit with low loss. The circuit platform is aluminum nitride, rather than traditional silicon.

“The diamond color centers emit in the visible spectrum,” says Tsung-Ju Lu, a co-author of the paper. “Traditional silicon, however, absorbs visible light, which is why we turn to aluminum nitride for our photonics platform, as it is transparent in that regime. Furthermore, aluminum nitride can support photonic switches that are functional at cryogenic temperatures, which we operate at for controlling our color centers.”

While the platform offers a scalable process to produce artificial atom-photonics chips, say the researchers, the next step will be to “turn it on” to test its processing skills.

“This is a proof of concept that solid-state qubit emitters are very scalable quantum technologies,” says Wan. “In order to process quantum information, the next step would be to control these large numbers of qubits and also induce interactions between them.”

The qubits in this type of chip design, say the researchers, wouldn’t necessarily have to be these particular diamond color centers. Other chip designers might choose other types of diamond color centers, atomic defects in other semiconductor crystals like silicon carbide, certain semiconductor quantum dots, or rare-earth ions in crystals.

“Because the integration technique is hybrid and modular,” says Lu, “we can choose the best material suitable for each component rather than relying on natural properties of only one material, thus allowing us to combine the best properties of each disparate material into one system.”

Looking ahead, say the researchers, finding a way to automate the process and demonstrate further integration with optoelectronic components, such as modulators and detectors, will be necessary to build the even bigger chips necessary for modular quantum computers and multichannel quantum repeaters that transport qubits over long distances. For more, see “Large-scale integration of artificial atoms in hybrid photonic circuits.”