Most telecommunications networks nowadays use optical fiber that can transport large amounts of data at high speeds. But these cables are incompatible with the wavelengths of even more powerful and much more secure quantum communications.
In a Science Daily article, Dutch researchers have found a way to leverage existing optical fiber networks for quantum communications. They created quantum bits (qubits) out of silicon carbide that contained molybdenum impurities.
The molybdenum act as color centers for the silicon carbide. The new qubit generated photons that came very close to the wavelengths of the optical fiber networks run by telecom providers.
The technique was developed by researchers from the University of Groningen (UG) and their Swedish partners Linköping University and Norstel AB. They reported that their qubit can transmit data at a wavelength of 1,100 nanometers.
For comparison, telecom providers use wavelengths of 1,300 or 1,500 nanometers for sending data. The European research team claims that they can adjust the wavelength of the photons from their silicon carbide-molybdenum qubit to match those commercial standards. (Related: Quantum physics puzzle SOLVED: Researchers say that totally secure data transfer now possible.)
Inducing impurities in semiconductors to turn them into qubits
UG researcher and primary author Tom Bosma explained that his team took advantage of impurities that form in silicon carbide. Normally, these defects are not welcome in the semiconductor since they affect the conductive properties of the crystal.
However, the impurities are also capable of forming color centers. These tiny structures will react to light of certain wavelengths and eventually give off photons of their own.
In the UG qubit, molybdenum atoms serve as the defects in silicon carbide crystals. Upon getting hit by laser-fired photons of the appropriate wavelength, the electrons in the outer shell of molybdenum transfer to a higher energy level.
The excited atoms will eventually resume their ground state. When they do, they expel their extra energy as photons. In the case of molybdenum, its atoms emit infrared light. The wavelengths of these photons are close to the ones used for data communication.
Bosma's fellow UG researcher, Carmen Gilardoni, also worked on the silicon carbide qubit. She employed coherent population trapping to impart superposition on color centers.
Electrons possess a quantum mechanical property called spin. This property imparts a magnetic moment on an electron that points either up or down. These opposite spin states can be used to represent 0 and 1, thereby turning the electron into a qubit.
"If you apply a magnetic field, the spins align either parallel or anti-parallel to the magnetic field," Gilardoni explained. "The interesting thing is that as a result the ground state for electrons with spin up or spin down is slightly different."
The path towards quantum internet
A laser-excited electron will return to one of two different ground states. The wavelength of the light can determine the ground state of the electron.
The UG researchers used two individual lasers. One laser would excite electrons in a ground state, while the other would do the same for the electrons that were in the other ground state. This led to the superposition of the two spin states 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," Bosma reported. He added that the qubit generated infrared photons that provided information on the quantum state.
Bosma, Gilardoni, and their teammates believe that changing the impurity from molybdenum to another element will alter the wavelength of the photons. They also hope to increase the stability of the superposition. If they succeed on both counts, they could help make quantum internet a reality.
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