Quantum computing, which is deemed highly desirable for artificial intelligence and other computing-intensive applications, presently requires several bulky components such as a microwave generator, optical filter, and photodetector. MIT researchers have, reportedly for the first time, fabricated a diamond-based quantum sensor on a silicon chip they believe could pave the way toward low-cost, scalable hardware for quantum computing, sensing and communication.
In a paper published in Nature Electronics, the researchers found a way to integrate all those bulky components onto a millimeter-scale package, using traditional semiconductor fabrication techniques. The sensor operates at room temperature with the ability to sense the direction and magnitude of magnetic fields.
The researchers demonstrated the sensor’s use for magnetometry, meaning they were able to measure atomic-scale shifts in the frequency due to surrounding magnetic fields, which could contain information about the environment. The sensor could potentially have applications ranging from mapping electrical impulses in the brain to detecting objects, even without a line of sight.
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“It’s very difficult to block magnetic fields, so that’s a huge advantage for quantum sensors,” said co-author Christopher Foy, a graduate student in the Department of Electrical Engineering and Computer Science (EECS). “If there’s a vehicle traveling in, say, an underground tunnel below you, you’d be able to detect it even if you don’t see it there.”
The other researchers included: Mohamed Ibrahim, a graduate student in EECS; Donggyu Kim PhD ’19; Matthew E. Trusheim, a postdoc in EECS; Ruonan Han, an associate professor in EECS and head of the Terahertz Integrated Electronics Group, which is part of MIT's Microsystems Technology Laboratories (MTL); and Dirk Englund, an MIT associate professor of electrical engineering and computer science, a researcher in Research Laboratory of Electronics (RLE), and head of the Quantum Photonics Laboratory.
The researchers developed a chip architecture that positions and stacks tiny, inexpensive components in a certain way using standard complementary metal-oxide-semiconductor (CMOS) technology, so they function like those components. “CMOS technologies enable very complex 3-D structures on a chip,” said Ibrahim. “We can have a complete system on the chip, and we only need a piece of diamond and green light source on top. But that can be a regular chip-scale LED.”
Nitrogen-vacancy (NV) centers within a diamond slab are positioned in a “sensing area” of the chip. A small green pump laser excites the NV centers, while a nanowire placed close to the NV centers generates sweeping microwaves in response to current. Basically, the light and microwave work together to make the NV centers emit a different amount of red photons—with the difference being the target signal for readout in the researchers’ experiments.
Below the NV centers is a photodiode, designed to eliminate noise and measure the photons. In between the diamond and photodiode is a metal grating that acts as a filter that absorbs the green laser photons while allowing the red photons to reach the photodiode. This enables an on-chip ODMR device, which measures resonance frequency shifts with the red photons that carry information about the surrounding magnetic field.
“I am enthusiastic about this quantum sensor technology and foresee major impact in several fields,” says Ron Walsworth, a senior lecturer at Harvard University whose group develops high-resolution magnetometry tools using NV centers.
“They have taken a key step in the integration of quantum-diamond sensors with CMOS technology, including on-chip microwave generation and delivery, as well as on-chip filtering and detection of the information-carrying fluorescent light from the quantum defects in diamond. The resulting unit is compact and relatively low power. Next steps will be to further enhance the sensitivity and bandwidth of the quantum diamond sensor [and] integrate the CMOS-diamond sensor with wide-ranging applications, including chemical analysis, NMR spectroscopy and materials characterization.”