Spins of electrons or their vacancies (holes) are promising candidates for encoding quantum information because they can be isolated in silicon quantum dots, using technology compatible with industrial microelectronics processes.
Unlike electron spins, hole spins can be manipulated by an electric field. They exhibit a strong 'spin-orbit interaction', so the displacement of a hole induced by an electric field couples to the spin state via the 'spin-orbit interaction'. For the first time, researchers at Irig have managed to use this effect to coherently couple a hole spin in silicon to a microwave photon.
How did they proceed? "In the channel of a silicon transistor fabricated at CEA-LETI, we have trapped a hole between two gates at very low temperature (T=10 mK), and connected the end of a microwave superconductor resonator to one of the two gates. When the resonator hosts a single photon, it induces electric field fluctuations delivered directly to the transistor gate, which will make the hole oscillating in the transistor channel. The magic happens when the frequency of these oscillations is exactly equal to the spin resonance frequency of the hole", explains Cécile Xinquing Yu, a PhD student at IRIG. Indeed, in this configuration, the photon is absorbed to change the spin from the ↓ state to the ↑ state and then re-emitted by changing the spin of the hole from the ↑ state to the ↓ state and so on. The rate of this 'absorption/emission' being directly related to the strength of the coupling between the spin and the photon.
By varying the orientation of the magnetic field, the researchers followed this absorption/emission rate. The results obtained, compared with a theoretical model, clearly establish that the spin and the photon are intricated thanks to the spin-orbit interaction. It should be noted that the strongest coupling observed transforms the photon into a spin excitation in less than three nanoseconds! "These results therefore indicate that hole spin qubits trapped in silicon transistors and microwave photons can talk to each other very quickly, much faster than their coherence time," says Romain Maurand, a physicist at IRIG. This makes it possible to exchange a photon between several spins in order to achieve long-distance spin-spin entanglement, which could be advantageous for the creation of quantum processors based on silicon spin qubits.