Impurities in Semiconductor Enable Qubits That Emit Photons in IR

An international research team has discovered a way to create qubits that emit photons at wavelengths close to those used by telecom providers. Scientists at the University of Groningen, together with colleagues from Linköping University and Norstel AB, constructed a qubit that transmits information on its status at a wavelength of 1100 nm. The researchers said it is likely that the approach they used could be tuned to wavelengths of 1300 to 1500 nm.

The qubits were based on silicon carbide crystals in which molybdenum impurities created color centers that could respond to light of specific wavelengths.

Illustration of optical polarization of defect spin in silicon carbide. Courtesy of Tom Bosma, the University of Groningen.

When the researchers shined light at a certain wavelength onto the color centers, they found that electrons in the outer shell of the molybdenum atoms in the silicon carbide were kicked to a higher energy level. When the atoms returned to their ground state, they emitted their excess energy as IR photons, with wavelengths near the ones used in data communication. This discovery was the starting point for the team’s approach to constructing qubits.

The researchers used coherent population trapping, a technique that involves the use of spin, to create superposition in the color centers. Spin gave the electrons a magnetic moment that could point up or down, creating a qubit in which the spin states represented 0 or 1 (i.e., superposition). When laser light was used to excite the electrons, they subsequently fell back to one of the two ground states.

Tom Bosma and Carmem Gilardoni in their optical lab at the University of Groningen. Courtesy of the University of Groningen.

“If you apply a magnetic field, the spins align either parallel or antiparallel to the magnetic field,” said researcher Carmem Gilardoni. “The interesting thing is that as a result, the ground state for electrons with spin up or spin down is slightly different.”

The team, which was led by professor Caspar van der Wal, used two lasers, each tuned to move electrons from one of the ground states to the same level of excitation, to create a situation in which a superposition of both spin states evolved in the color center.

“After some fine-tuning, we managed to produce a qubit in which we had a long-lasting superposition combined with fast switching,” said researcher Tom Bosma. The team’s qubit emitted photons with information on the quantum state at IR wavelengths, showing that it is a promising platform for interfacing quantum and telecommunications technologies.

Although several transmissions of qubits through optical fibers have been reported, the transmission has typically occurred at wavelengths incompatible with the standard fibers currently used in worldwide data transmission.

“Silicon carbide is a semiconductor, and much work has been done to prevent impurities that affect the properties of the crystals,” said Bosma. “As a result, there is a huge library of impurities and their impact on the crystal.”

Given the large library of impurities that can create color centers in the silicon carbide crystals, the researchers expressed confidence that they can bring this wavelength up to the levels used in standard optical fibers. If they can manage this and produce an even more stable (and thus longer-lasting) superposition, the “quantum internet” could be closer to becoming reality.

The research was published in npj Quantum Information (https://doi.org/10.1038/s41534-018-0097-8).