An Army-funded project marks a turning point in the field of scalable quantum processors, producing the largest quantum chip of its type using diamond-based qubits and quantum photonics.
Millions of quantum processors will be needed to build quantum computers, and new research at MIT and Sandia National Laboratories, funded and managed in part by the U.S. Army Combat Capability Development's Command's Army Research Laboratory's Center for Distributed Quantum Information, demonstrates a viable way to scale-up processor production.
"Building large scale quantum devices will entail both the assembly of large numbers of high-quality qubits and the creation of reliable circuits for transmitting and manipulating quantum information between them," said Dr. Fredrik Fatemi, Army researcher and CDQI co-manager. "Here, the research team has demonstrated exceptional progress toward reliably manufacturing complex quantum chips with both critical elements."
Unlike classical computers, which process and store information using bits represented by either 0s and 1s, quantum computers operate using quantum bits, or qubits, which can represent 0, 1, or both at the same time. This strange property allows quantum computers to simultaneously perform multiple calculations, solving problems that would be intractable for classical computers.
The qubits in the new chip are artificial atoms made from defects in the diamond, which can be prodded with visible light and microwaves to emit photons that carry quantum information.
The process, which the researchers describe in the peer-reviewed journal Nature, 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," said 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."
Using their hybrid method, the researchers were able to build a 128-qubit system -- the largest integrated artificial atom-photonics chip yet.
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," said Noel Wan, MIT research and the paper's coauthor.
"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," Wan said. "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.
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 the researchers.
While the platform offers a scalable process to produce artificial atom-photonics chips, the next step will be to test its processing skills.
"This is a proof of concept that solid-state qubit emitters are very scalable quantum technologies," Wan said. "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," said Tsung-Ju Lu, MIT researcher and the paper's co-author.
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 said.
"The team has made an incredible advance toward the large-scale integration of artificial atoms and photonics and, looking forward, we are very excited for increasingly complex testing of the devices," said Dr. Sara Gamble, program manager at the Army Research Office, an element of CCDC ARL, and CDQI co-manager. "The modular approach so far successfully demonstrated by the team has enormous promise for the future quantum computers and quantum networks of high interest to the Army."
*Adapted with permission from an article by Becky Ham, MIT News.