Scaling Up the Quantum Chip

MIT researchers have developed a process to manufacture and integrate artificial atoms, created by atomic-scale defects in micro­scopically thin slices of diamond, with photonic circuitry, producing the largest quantum chip of its type. The accomplish­ment marks a turning point in the field of scalable quantum processors, says Dirk Englund, an associate professor in MIT’s Department of Electrical Engi­neering and Computer Science. Millions of quantum processors will be needed to build quantum computers, and the new research demonstrates a viable way to scale up processor production, he and his colleagues note.

A stylized rendering of the quantum photonic chip and its assembly process: The bottom half of the image shows a functioning quantum micro-chiplet (QMC), which emits single-photon pulses that are routed and manipulated on a photonic integrated circuit. The top half of the image shows how this chip is made: Diamond QMCs are fabricated separately and then transferred. (Source: N. H Wan)

The qubits in the new chip are artificial atoms made from defects in diamond, which can be prodded with visible light and microwaves to emit photons that carry quantum infor­mation. The process is a hybrid approach, in which carefully selected quantum micro chiplets containing multiple diamond-based qubits are placed on an aluminum nitride photonic inte­grated circuit. “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,” Englund says. “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.”

Using their hybrid method, Englund and colleagues were able to build a 128-qubit system – the largest inte­grated arti­ficial atom-photonics chip yet. “It’s quite exciting in terms of the technology,” says Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at Harvard University, who was not involved in the study. “They were able to get stable emitters in a photonic platform while main­taining very nice quantum memories.”

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 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.

Diamond color centers make good solid-state qubits, but “the bottleneck with this platform is actually building a system and device archi­tecture that can scale to thousands and millions of qubits,” Wan explains. “Artificial atoms are in a solid crystal, and unwanted conta­mination can affect important quantum properties such as coherence times. Furthermore, variations within the crystal can cause the qubits to be different from one another, and that makes it difficult to scale these systems.”

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 Wan. “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 is analogous to an electronic integrated circuit but 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 the traditional silicon of some integrated circuits.

“The diamond color centers emit in the visible spectrum. 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,” Lu explains. “Further­more, aluminum nitride can support photonic switches that are func­tional at cryogenic tempera­tures, which we operate at for controlling our color centers.” Using this hybrid approach of photonic circuits and diamond chiplets, the researchers were able to connect 128 qubits on one platform. The qubits are stable and long-lived, and their emissions can be tuned within the circuit to produce spectrally indistin­guishable photons, according to Wan and colleagues.

While the platform offers a scalable process to produce arti­ficial atom-photonics chips, the next step will be to “turn it on,” so to speak, 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 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, 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,” says Lu.

Finding a way to automate the process and demons­trate further integration with opto­electronic components such as modulators and detectors will be necessary to build even bigger chips necessary for modular quantum computers and multi­channel quantum repeaters that transport qubits over long distances, the researchers say. (Source: MIT)

Reference: N. H. Wan et al.: Large-scale integration of artificial atoms in hybrid photonic circuits, Nature 583, 226 (2020); DOI: 10.1038/s41586-020-2441-3

Link: Quantum Photonics Laboratory, Massachusetts Institute of Technology MIT, Cambridge, USA

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