A new diode nanocavity created by a team of researchers at Simon Fraser University (SFU) and quantum company Photonic Inc. in Canada enables optical and electrical control of silicon “color center” qubits. Until now, these qubits, a.k.a. T centers, were controlled optically with lasers. This dual control option of T centers hints at potential applications for future scalable quantum computers.
The microelectronics industry is rising to the challenge of developing silicon nanofabrication and ever-smaller transistors to meet the insatiable demand for computational power. It’s led to an enormous collection of knowledge about how to purify and process silicon to create extremely small devices—and silicon is now a versatile platform that can host both electronic and photonic devices.
“Mike Thewalt and Stephanie Simmons, the founders of the Silicon Quantum Technology lab at Simon Fraser University and Photonic Inc., were studying impurities within isotopically purified Si-28 (an isotope of silicon) and discovered these impurities have long spin coherence times that can make them useful for quantum applications,” says Michael Dobinson, a Ph.D. candidate in physics at SFU.
Then they searched for a luminescent defect, a “color center,” within silicon to act as a spin-photon interface—a quantum system that can store quantum information within a spin state and distribute quantum information through emitted photons. In 2020, they identified the T center, which has excellent spin properties, emits within the telecommunications O-band, and enables low-loss transmission over standard optical fibers.
During this time, development of other quantum computing platforms was underway across a range of modalities. A range of color centers were explored for solid-state platforms, including diamond and silicon carbide, and it turns out their capabilities can be extended by controlling the electronic environment—including its electrical excitation, charge depletion, and spin-charge readout.
“In 2022, our group developed silicon nanophotonic devices that allowed us to control single T centers optically with lasers,” says Dobinson, who has a background in electrical engineering and is interested in single-photon sources and optically active quantum defects. “Adding electrical control has the potential to unlock new capabilities, which made it intriguing for us to explore for the T center. We used industry-standard tools and processes to design and fabricate our devices.”
For his Ph.D., Dobinson wanted to work on quantum technology and he focused on whether it can scale. “Coming from engineering and seeing the requirements for quantum computing, I knew it was the most important metric,” he says. “I researched all modalities and eventually started in the Silicon Quantum Technology lab at SFU in 2022. After discussions with Stephanie Simmons and Daniel Higginbottom, now both assistant professors in physics at SFU, I saw the potential for integration of the T center with both optical and electrical controls as an exciting crossover between everything that interests me and as an important component for scalability.”
T centers
The team’s work involves several different optical and electrical components and a wide range of concepts that are helpful to understand.
For starters, the nanocavity device they used for their single-photon emission demonstration is based on a T center, a color center within silicon that’s a quantum emitter. It consists of two carbon atoms and a hydrogen atom, which take the place of a silicon atom.
“The T center hosts a ground state electron spin and a bound-exciton excited state that has an optical transition within the telecommunications O-band ~935 meV (1326 nm),” Dobinson explains. “This device’s T centers are within an optical cavity—a nanophotonic device that consists of a waveguide with a periodic array of holes. It creates a photonic crystal cavity to enhance the interaction between the T center and optical field by the Purcell effect, which causes an increased spontaneous emission rate into the cavity mode that results in brighter emission from the T center. The Purcell effect scales by Q/V, where Q is the quality factor of the cavity and V is the mode volume.”
T centers within the device can be excited electrically by the p–i–n diode, which is formed across the cavity. This diode is created by doping one side of the device with phosphorus to create an excess of electrons, and doping the other side with boron to create an excess of holes. “It creates a built-in potential that, when overcome by a forward-biased electrical voltage, allows charge carriers to be injected into the central region of the device,” Dobinson says. “T centers within the central undoped region can capture these free carriers to add energy to the system and take it to an excited state. Upon recombination, the T center emits light.”
The emission from the T centers coupled into the cavity is routed by a waveguide that confines the light to a mode that’s directed around the photonic chip. “It relies on the refractive indices of the waveguide and surrounding material. We use silicon-on-insulator, which has a high-index silicon layer with a low-index oxide layer on the bottom,” says Dobinson.
After routing the emission by the waveguide, it’s directed out of plane into an optical fiber by a grating coupler. “This consists of a periodic array of etches within the silicon to create a periodic variation in the effective refractive index, which causes reflection and refraction from each interface,” Dobinson says. “It directs the waveguide mode out of plane into the mode of the optical fiber positioned above, with some coupling loss.”
To determine whether T centers can be used as an electrically injected single-photon source requires collecting single photons. “To do it, we perform a Hanbury-Brown-Twiss measurement, in which we measure the second-order correlation function, g(2)(0). This takes the incoming stream of photons and directs it into two branches to record the photon arrival times,” Dobinson says. “For a true single-photon source, there should never be two photons at a time—so the detectors will not record simultaneous photon arrivals. By analyzing the data, we can look for correlations, instances where both detectors recorded photons, for different time delays. We can confirm that the emission is predominantly from a single-photon source if at zero delay g(2)(0) < 0.5.”
The team also explored spin initialization. “We consider the T center’s behavior as a spin-photon interface,” Dobinson adds. “Under a magnetic field, we see four optical transitions from the T center—each with different energy that corresponds to the initial and final spin state of the T center. To initialize the spin state of the T center, we filter for photons with the energy corresponding to an optical transition to that spin state. When we detect a photon that passes through our filter, we know it had a specific energy and that the T center is in a specific spin state, with some fidelity.”