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Oct 28, 2023

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Quantum technologies promise to revolutionize society by enabling radically new

Quantum technologies promise to revolutionize society by enabling radically new methods for communication, sensing and computation. It's a world of possibilities that science, in many ways, has only begun to outline.

Quantum cryptography, if it could be achieved, for example, would provide unparalleled levels of data security against nefarious hackers. That's because quantum information can be encoded in photons — single particles of light — that can't be copied or measured. Interlopers would be exposed immediately.

One of the high hurdles for quantum cryptography that scientists must first overcome, however, is the ability to create photons in ways that would reliably feed quantum networks, or a quantum internet.

Now, a team of researchers led by Boubacar Kanté, Chenming Hu Associate Professor in UC Berkeley's Department of Electrical Engineering and Computer Sciences and faculty scientist at the Materials Sciences Division of Lawrence Berkeley National Laboratory (Berkeley Lab), has demonstrated the first on-demand quantum light source using silicon. Kanté says silicon — the material upon which millions of tiny electronic devices are manufactured each day — is the most "scalable" optoelectronic material known.

Their research was published today in Nature Communications.

"The possibility to use silicon as a source of quantum light signifies that current large-scale Complementary Metal-Oxide Semiconductor (CMOS) chip manufacturing processes at the core of today's optoelectronics and artificial intelligence (AI) devices may be directly used for future quantum systems," Kanté said.

Since the late 1970s, many promising single-photon-emitting quantum devices for quantum cryptography have been demonstrated. They include such materials science exotica as quantum dots, color centers in wide band gap materials, nonlinear crystals and atomic vapor cells.

Despite decades of investigation, however, there's no clear winner for a quantum light source that would feed a quantum internet.

A quantum internet at scale, Kanté explained, would require not only a bright and efficient quantum light source but also photons that can propagate in existing optical fibers without being absorbed. No light source available today can meet that high bar. All require energy conversion for integration with CMOS compatible platforms, like what happens today with integrated "classical" light sources.

But the challenge for integrating quantum devices with CMOS compatible platforms is even more significant than for classical systems, Kanté said. That's because each interface allows losses of quantum light that need to be minimized.

The on-demand silicon quantum light source developed by the UC Berkeley/Berkeley Lab team is the first experimental work demonstrating integration of a single silicon atomic emissive center, known as the G center, directly in a silicon nanophotonic cavity, Kanté explained.

"In this work, we successfully embedded for the first time an atomic defect in silicon the size of atoms (1 angstrom) in a silicon photonic cavity (1 micron) with the size of less than one-tenth of a human hair. The cavity forces the atom to be brighter, and it emits photons at a faster rate. Those are necessary ingredients for scalable quantum light sources for the future [quantum] internet," he said.

Successful manufacturing of the single-photon emitters involves a controlled fabrication sequence, starting with a commercial-grade silicon wafer that is carbon implanted. The implantation is followed by lithography, etching and thermal annealing — all standard processes available in today's semiconductor foundries.

The challenge, Kanté said, resided in creating atomic emissive centers and controlling their density and distribution for successful overlap with optical cavities. The team has overcome some of the key challenges, but improvements are needed, and many questions are yet to be answered.

Left to right: Boubacar Kanté, Christos Papapanos, Kaushalya Jhuria, Walid Redjem, Thomas Schenkel, Wayesh Qarony, Vsevolod Ivanov, Yertay Zhiyenbayev, Wei Liu, Liang Tan, Prabin Parajuli and Scott Dhuey at the Molecular Foundry of the Lawrence Berkeley National Laboratory. (Berkeley Lab photo)

"We found that during the creation of the single emissive centers, the annealing process creates fluctuations in quantum properties, and we now understand critical parameters that control these properties," said Berkeley Lab researcher Thomas Schenkel.

Using silicon has been somewhat counterintuitive, said Walid Redjem, a postdoctoral research fellow in Kante's group. "Silicon is what you call an indirect bandgap semiconductor. That means it is not favorable for light emission. For example, there is no efficient laser using silicon."

But it turns out that reality only applies to classical light sources. "It's not a problem for quantum light sources," Kanté said. He and his team are already hard at work further refining their all-silicon quantum light source.

The study was led by Redjem, postdoctoral researcher Wayesh Qarony and Yertay Zhiyenbayev, a third-year Ph.D. student in Kanté's group. Other co-authors include Schenkel, Vsevolod Ivanov, Christos Papapanos, Wei Liu, Kaushalya Jhuria, Zakaria Al Balushi, Scott Dhuey, Adam Schwartzberg and Liang Tan.

The National Science Foundation and the Department of Energy provided the primary support for the study. Additional funding came from the Office of Naval Research, the Moore Inventor Fellows program and UC Berkeley's Bakar Fellowship.