Quantum Communication Through Optical Fiber

Illustration of optical polarization of defect spin in silicon carbide. (Source: T. Bosma, Univ. Groningen)

An inter­national team of scientists led by the Univer­sity of Groningen’s Zernike Institute for Advanced Materials has identi­fied a way to create quantum bits that emit photons that describe their state at wave­lengths close to those used by telecom providers. These qubits are based on silicon carbide in which molybdenum impurities create color centers.

By using phenomena like super­position and ent­anglement, quantum computing and quantum communi­cation promise superior computing powers and unbreakable crypto­graphy. Several successes in trans­mitting these quantum phenomena through optical fibers have been reported, but this is typically at wave­lengths that are incom­patible with the standard fibers currently used in worldwide data trans­mission. Physicists from the Univer­sity of Groningen in the Netherlands together with colleagues from Linköping Univer­sity and semi­conductor company Norstel AB, both in Sweden, have now reported about the construc­tion of a qubit that transmits infor­mation on its status at a wave­length of 1,100 nano­meters. Further­more, the mechanism involved can likely be tuned to wave­lengths near those used in data trans­mission around 1,300 or 1,500 nano­meters.

The work started with defects in silicon carbon crystals, explains Tom Bosma. “Silicon carbide is a semi­conductor, and much work has been done to prevent impurities that affect the properties of the crystals. As a result, there is a huge library of impu­rities and their impact on the crystal.” But these impu­rities are exactly what Bosma and his colleagues need: they can form what are known as color centers, and these respond to light of specific wave­lengths.

When lasers are used to shine light at the right energy onto these color centers, electrons in the outer shell of the molyb­denum atoms in the silicon carbide crystals are kicked to a higher energy level. When they return to the ground state, they emit their excess energy as a photon. “For molyb­denum impu­rities, these will be infrared photons, with wave­lengths near the ones used in data communi­cation”, explains Bosma. This material was the starting point for construc­ting qubits, says PhD student Carmem Gilardoni, who did a lot of the theo­retical work. “We used coherent popu­lation trapping to create super­position in the color centers.” This involved using the spin of electrons to represent 0 or 1.

Gilardoni: “If you apply a magnetic field, the spins align either parallel or anti-parallel to the magnetic field. The interes­ting thing is that as a result the ground state for electrons with spin up or spin down is slightly different.” When laser light is used to excite the electrons, they subse­quently fall back to one of the two ground states. The team, led by 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.

Bosma: “After some fine tuning, we managed to produce a qubit in which we had a long-lasting super­position combined with fast switching.” Further­more, the qubit emitted photons with infor­mation on the quantum state at infrared wave­lengths. Given the large library of impu­rities that can create color centers in the silicon carbide crystals, the team is confident 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 super­position, the quantum internet will be a whole lot closer to becoming reality. (Source: U. Groningen)

Reference: T. Bosma et al.: Identification and tunable optical coherent control of transition-metal spins in silicon carbide, npj Quan. Inform. 4, 48 (2018); DOI: 10.1038/s41534-018-0097-8

Link: Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands

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