The new technique could help build the quantum computers of the future

Quantum computers have the potential to solve complex problems in human health, drug discovery and artificial intelligence millions of times faster than some of the world’s fastest supercomputers. A network of quantum computers could advance these discoveries even faster. But before that happens, the computer industry will need a reliable way to join billions of qubits — or quantum bits — with atomic precision.

However, qubit binding has been challenging for the research community. Some methods form the cube by placing an entire mass of silicon in a rapid baking oven at very high temperatures. With these methods, qubits are randomly formed from defects (also known as color centers or quantum emitters) in the silicon crystal lattice. And without knowing exactly where the qubits are located in a material, a quantum computer with bound qubits will be difficult to realize.

But now, getting qubits to connect may soon be possible. A research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) says they are the first to use a femtosecond laser to create and “annihilate” qubits on demand, and with precision, by doping silicon with hydrogen.

The breakthrough could enable quantum computers that use programmable optical qubits or “spin-photon qubits” to connect quantum nodes in a remote network. It could also advance a quantum internet that is not only more secure but can also transmit more data than current fiber-optic information technologies.

“This could create a potential new avenue for the industry to overcome challenges in cube fabrication and quality control.”

– Thomas Schenkel, Senior Scientist, Division of Accelerator Technology and Applied Physics

“To create a scalable quantum architecture or network, we need qubits that can be reliably formed on demand, in desired locations, so that we know where the qubit is in a material. And that’s why our approach is critical,” said Kaushalya Jhuria, a postdoctoral researcher in Berkeley Lab’s Division of Acceleration Technology and Applied Physics (ATAP). She is first author on a new study describing the technique in the journal Nature Communications. “Because once we know where a specific qubit is located, we can determine how to connect this qubit to other components in the system and create a quantum network.”

“This could create a potential new path for industry to overcome challenges in cube fabrication and quality control,” said principal investigator Thomas Schenkel, head of the Fusion Science & Ion Beam Technology Program in Berkeley Lab’s ATAP Division. His group will host the first cohort of students from the University of Hawaii in June as part of a DOE Fusion Energy Sciences-funded RENEW project on workforce development, where students will be immersed in the science and technology of the color center /qubit.

Forming qubits in silicon with programmable control

The new method uses a gas environment to form programmable defects called “color centers” in silicon. These color centers are candidates for special telecommunication qubits or “spinning photon qubits”. The method also uses an ultrafast femtosecond laser to anneal the silicon with precise precision where those qubits must be precisely formed. A femtosecond laser delivers very short pulses of energy within a quarter of a second to a focused target the size of a dust particle.

Spin photon qubits emit photons that can carry information encoded in the spin of electrons over long distances – ideal properties to support a secure quantum network. Qubits are the smallest components of a quantum information system that encode data in three different states: 1, 0, or a superposition that is everything between 1 and 0.

With the help of Boubacar Kanté, a faculty scientist in Berkeley Lab’s Division of Materials Science and professor of electrical engineering and computer science (EECS) at UC Berkeley, the team used a near-infrared detector to characterize the resulting color centers by probing their optics (photoluminescence) signals.

What they discovered surprised them: a quantum emitter called a Ci center. Because of its simple structure, room temperature stability, and promising spin properties, the Ci center is an interesting spin photon qubit candidate that emits photons in the telecom band. “We knew from the literature that Ci can be formed in silicon, but we didn’t expect to actually make this new spin qubit photon candidate with our approach,” Jhuria said.

The researchers learned that processing silicon with a low-intensity femtosecond laser in the presence of hydrogen helped create the Ci color centers. Further experiments showed that increasing the laser intensity can increase the mobility of hydrogen, which passivates unwanted dye centers without damaging the silicon lattice, Schenkel explained.

A theoretical analysis conducted by Liang Tan, staff scientist at Berkeley Lab’s Molecular Foundry, shows that the brightness of the Ci color center is increased by several orders of magnitude in the presence of hydrogen, confirming their observations from laboratory experiments.

“Femtosecond laser pulses can eject hydrogen atoms or turn them around, allowing the programmable formation of desired optical qubits at precise locations,” Jhuria said.

The team plans to use the technique to integrate optical qubits into quantum devices such as reflective cavities and waveguides, and to discover new spin photon qubit candidates with optimized properties for selected applications.

“Now that we can make color centers reliably, we want to get different qubits to talk to each other – which is the epitome of quantum entanglement – ​​and see which ones perform better. This is just the beginning,” said Jhuria.

“The ability to form qubits at programmable sites in a material like silicon that is available at scale is an exciting step toward practical quantum networks and computation,” said Cameron Geddes, Director of the ATAP Division.

Theoretical analysis for the study was performed at the Department of Energy’s Energy Research Scientific Computing Center (NERSC) at Berkeley Lab with support from the NERSC QIS@Perlmutter program.

The Molecular Foundry and NERSC are user facilities of the DOE Office of Science at Berkeley Lab.

This work was supported by the DOE Office of Fusion Energy Sciences.

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Lawrence Berkeley National Laboratory (Berkeley Lab) is committed to providing solutions for humanity through clean energy research, a healthy planet, and discovery science. Founded in 1931 on the belief that the biggest problems are best tackled by teams, Berkeley Lab and its scientists have been awarded 16 Nobel Prizes. Researchers from around the world rely on the Laboratory’s world-class scientific facilities for their pioneering research. Berkeley Lab is a multi-program national laboratory managed by the University of California for the US Department of Energy’s Office of Science.

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