A variety of new colour centres (luminescing crystal defects that can emit individual photons) have been found in light-emitting diodes made from silicon carbide (SiC). The result confirms once again that it is a promising single-photon source and a good material out of which to make quantum bits (qubits).

Single-photon emitters operating at room temperature could be used in on-chip quantum communication applications, and as a source of “flying” qubits for quantum computers. Such computers exploit the ability of quantum particles to be in a “superposition” of two or more states at the same time unlike classical computers that store and process information as “bits” that can have one of two logic states – “0” or “1”

Quantum computers could, in principle, outperform classical computers on certain tasks, like code decryption for example, because their processing speed should increase exponentially with the number of qubits of information involved. In reality, it is difficult to create even the simplest quantum computer, however, because the fragile nature of these quantum states means that they are easily destroyed and are difficult to control.

Colour centres in SiC

In recent years, there have many studies on point defects (or colour centres) in SiC, a material that is already widely used in high-power electronics thanks to its high thermal conductivity and high maximum current density to name but two good properties. These defects possess electron spin states that can be coherently controlled and manipulated as qubits using light.

Researchers led by Jorg Wrachtrup of the University of Stuttgart in Germany have now confirmed these previous findings.  They have discovered a variety of new colour centres in lateral p-i-n diodes made from a polytype (a crystal structure) of silicon carbide called 4H-SiC that contains naturally occurring defects (or “divacancies”). These defects, which correspond to a missing silicon atom next to a missing carbon atom in the crystal, are very much like the defects in diamond known as “nitrogen-vacancy centres” – that form when a nitrogen impurity finds itself next to a missing carbon atom in the diamond lattice.

Both types of defect form a muti-electron system that has a net angular momentum (or spin) that can be aligned either parallel (“1”) or antiparallel (“0”) to an applied magnetic field, and can so be exploited as a qubit. SiC has an advantage, however, in that it is CMOS-compatible and so could more easily be scaled up to larger systems than hard diamond can.

Photoluminescence experiments

The newly-discovered centres in 4H-SiC emit non-classical light in the visible and near-infrared range. One type of defect can even be excited using electrical means. This means that it might be integrated into compact electronics devices as there would be no need for an additional bulky laser system to optically excite it.

As in previous experiments on diamond nitrogen vacancy centres and SiC point defects, Wrachtrup and co-workers measured the spin of the divacancies in 4H-SiC using photoluminescence. This involved shining laser light onto the sample and collecting the fluorescence light subsequently emitted by it. And, as for diamond nitrogen vacancies, the fluorescence of the silicon carbide divacancies depends on their spin state, so it is possible to “read out” the state of the qubits in this way.

“In this work, a key concept is the generation and manipulation of individual particles of light – photons,” says Marina Radulaski of Stanford University, who was not involved in this study.  “The researchers have not only discovered new colour centres in silicon carbide that can generate single photons at high rates, they have also succeeded in integrating these with electronic elements that can turn the light emission on and off. In a way, they have developed an early prototype of a ‘quantum telegraph’.”

Sophia Economou of Virginia Tech, who was not involved in the study either, agrees: “Electrically operated single photon sources are of interest for miniaturized quantum devices and silicon carbide is an especially promising material for such devices thanks to its industrial maturity, compatibility with CMOS fabrication techniques and low cost. Despite its maturity, however, new colour centres are still being discovered in SiC and these will provide a range of properties in terms of emission frequency, spin structure and photon polarization.

“Wrachtrup and colleagues’ work opens new directions both for device engineering and for basic physics studies,” she adds. “It will be good to further understand the nature of the newly discovered colour centres — their composition, their spin structure, their dynamics under optical versus electrical excitation, and whether they can provide useful transitions for spin-photon entanglement interfaces.”

Full details of the research are reported in Applied Physics Letters 10.1063/1.5032291.