New technology could help build 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 can happen, the computer industry needs a reliable way to string together billions of qubits (or quantum bits) with atomic precision.

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

With these methods, qubits are formed randomly from defects (also known as color centers or quantum emitters) in the crystal lattice of silicon. And without knowing exactly where qubits are located in a material, a quantum computer of connected qubits will be difficult to realize.

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

The advances could enable quantum computers that use programmable optical qubits or “spin-photon qubits” to connect quantum nodes via an external network. It could also promote a quantum internet that is not only more secure but also capable of transmitting more data than current fiber optic information technologies.

“To create a scalable quantum architecture or network, we need qubits that can be reliably formed on-demand and in desired locations, so that we know where the qubit is in a material. And that’s why our approach is crucial importance,” says Kaushalya Jhuria. , a postdoctoral researcher in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) department. She is the first author of a new study describing the technique in the journal Nature communication.

“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 thus create a quantum network.”

“This could create a potential new path for industry to overcome challenges in qubit manufacturing and quality control,” said lead researcher Thomas Schenkel, head of the Fusion Science & Ion Beam Technology Program in Berkeley’s ATAP Division Lab. His group will host the first cohort of students from the University of Hawaii in June, where students will be immersed in color center/qubit science and technology.

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 telecommunications qubits or ‘spin photon qubits’. The method also uses an ultra-fast femtosecond laser to anneal silicon with pinpoint precision where exactly those qubits should form. A femtosecond laser delivers very short energy pulses within a billionth of a second to a sharp target the size of a dust particle.

Spin photon qubits emit photons that can transport information encoded in electron spin 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 help from Boubacar Kanté, a faculty scientist in Berkeley Lab’s Materials Sciences Division 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 measuring their optical (photoluminescence) to research. ) signals.

What they discovered surprised them: a quantum transmitter called the C_i Centre. Thanks to its simple structure, stability at room temperature and promising spinning properties, the C_i center is an interesting spin-photon qubit candidate that emits photons in the telecom band. “We knew from the literature that C_i can be formed in silicon, but we did not expect that with our approach we would actually create this new spin-photon qubit candidate,” Jhuria said.

The researchers found that processing silicon with a low femtosecond laser intensity in the presence of hydrogen helped create the C_i color centers. Further experiments showed that increasing the laser intensity can increase the mobility of hydrogen, passivating unwanted color centers without damaging the silicon lattice, Schenkel explains.

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

“The femtosecond laser pulses can kick out or bring back hydrogen atoms, enabling 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 properties optimized for selected applications.

“Now that we can reliably create color centers, we want to get different qubits talking to each other – which is an embodiment of quantum entanglement – ​​and see which ones perform best. This is just the beginning,” says Jhuria.

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