A 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 connect billions of qubits — or quantum bits — with atomic precision.

However, connecting the qubits has been challenging for the research community. Some methods form qubits by placing an entire silicon wafer in a furnace for rapid annealing at very high temperatures.

Using 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 in the material, a quantum computer of interconnected qubits will be difficult to implement.

But now it may soon be possible to get qubits to connect. 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 advance could enable quantum computers that use programmable optical qubits or “spin-photon qubits” to connect quantum nodes over a remote network. It could also usher in a quantum internet that would not only be more secure, but could also carry more data than current fiber-optic information technologies.

“To create a scalable quantum architecture or network, we need qubits that can be reliably created on demand in the desired locations so that we know where the qubit is in the material. And that’s why our approach is critical,” said Kaushalya Jhuria. , a postdoctoral fellow in the Accelerator Technology & Applied Physics (ATAP) division at Berkeley Lab. She is the first author of the new study that describes the technique in the journal The nature of communication.

“Because once we know where a particular qubit sits, we can determine how to connect that qubit to other components in the system to create a quantum network.”

“This could create a potential new path for industry to overcome challenges in qubit 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, where students will immerse themselves in the science and technology of color centers/qubits.

Shaping qubits in silicon with programmable control

The new method uses a gaseous environment to create programmable defects called “color centers” in silicon. These colored centers are candidates for special telecommunication qubits or “spin photon qubits”. The method also uses an ultrafast femtosecond laser to anneal the silicon with pinpoint accuracy where these qubits should form exactly. A femtosecond laser delivers very short pulses of energy within a quadrillionth of a second to a focused target the size of a grain of dust.

Spin photon qubits emit photons that can transmit information encoded in electron spin over long distances—ideal properties for supporting a secure quantum network. Qubits are the smallest parts 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 Department of Materials Science and a 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 examining their optical (photoluminescence) signals.

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

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

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

“Femtosecond laser pulses can knock out hydrogen atoms or put them back, allowing for the programmable creation of desired optical qubits in precise locations,” Jhuria said.

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

“Now that we can reliably create color centers, we want to get different qubits to talk to each other — which is the epitome of quantum entanglement — and see which ones work best. This is just the beginning,” Jhuria said.

“The ability to create qubits at programmable locations in a material like silicon that is available at scale is an exciting step toward practical quantum networking and computing,” said Cameron Geddes, director of the ATAP division.