Three-qubit register based on a T center in silicon photonics. Credit: Song et al.

Quantum technologies are highly promising devices that process, transfer or store information leveraging quantum mechanical effects. Instead of relying on bits, like classical computers, quantum devices rely on entangled qubits, units of information that can also exist in multiple states (0 and 1) at once.

A research team at the University of California Berkeley (UC Berkeley) supervised by Alp Sipahigil recently demonstrated the potential of leveraging atomic‑scale defects on silicon chips, known as T‑centers, to create small multi‑qubit memory units that store quantum information (i.e., quantum registers).

Their paper, published in Nature Nanotechnology, could open new possibilities for the development of quantum technologies that are based on silicon, which is the most widely used material within the electronics industry.

“The silicon T center is an emerging spin‑photon interface that combines telecom O‑band optical transitions with spin qubits exhibiting millisecond‑scale coherence times,” Hanbin Song, lead author of the paper, told Phys.org.

“As a point defect in silicon, the T center shows promise for scalability by leveraging the high‑performance electronic‑photonic integration available in silicon. In this work, we show on‑chip control of a three‑qubit register with coherence time up to ~100 ms using a single T center integrated into an optical waveguide.”

Leveraging quantum defects to create new devices

In recent years, several quantum engineers and physicists have been trying to develop quantum technologies that rely on memory “nodes” containing several qubits, which are connected to each other via optical components. Some studies have also highlighted the potential of leveraging defects in silicon to create these multi‑qubit units, particularly T centers.

As part of their study, Song and her colleagues set out to demonstrate the potential of T centers for hosting several qubits and creating multi‑qubit memory nodes. They specifically wanted to show that these defects in silicon can retain excellent quantum coherence between qubits, resulting in highly performing photonic quantum devices.

To do this, they first created T centers, tiny light‑emitting defects, in silicon employing a technique known as ion implantation. This technique essentially entails firing electrically charged atoms (i.e., ions) onto a solid so that they become embedded within it.

After implanting the ions into silicon, they heated the resulting chip using a method known as rapid thermal annealing. This heat‑based method allows atoms to move slightly within a material and ultimately settle into more stable positions to form T centers. Finally, the team created tiny optical and electrical structures on the chip using lithography, a widely used method that allows electronics manufacturers to produce desired patterns on chips.

“The photonic integration efficiently couples T‑center photoluminescence into an optical fiber for photon collection, while the metal traces enable on‑chip spin control,” explained Song. “We achieve coherent control of the three‑qubit register in a compact, device‑integrated platform.”

A promising path for realizing quantum memories

In initial tests, the device created by the researchers was found to perform very well, producing an entanglement between the qubits forming the register and retaining quantum information for over 100 milliseconds. The team showed that quantum states in the system could be read using optical techniques.

“We show that the T center can be used as a three‑qubit quantum register with an optical interface,” said Song. “We realized controlled‑NOT gates between multiple qubits, generated entanglement between nuclear spins, and measured up to 100 ms of nuclear spin coherence time. The work also suggests the possibility of extending the qubit register using a ^29Si nuclei bath and using the silicon T center as a scalable multi‑qubit memory node for quantum communication.”

Notably, this new multi‑qubit register is based on silicon and can be created using industry‑compatible processes. In the future, it could be scaled up to produce bigger systems for quantum communication and information processing comprised of an even larger number of qubits.

“Several advances are still needed to realize a quantum communication node based on the T center,” added Song. “We are now focusing on improving the qubit readout and reducing optical linewidths to enable coherent spin‑photon gates.”

Edited by Stephanie Baum; fact‑checked and reviewed by Robert Egan.