Semiconductor spin qubits
Coupling spin qubits to microwave photons, controlling spins with an electric field.
Realizing a scalable, easy-to-handle device platform based on polariton quantum fluids.
Using nano- and micromechanical oscillators to enable navigation, timing, motional sensing and wireless communication.
Controlling nuclear noise in semiconductor spin qubits
A semiconductor quantum dot typically consists of more than 100,000 atoms. The nuclei of these atoms have a spin that interacts with the electron spin via the hyperfine interaction. This slightly changes the energy of the spin-up and spin-down states of the electron, depending on the size and direction of the nuclear spin polarization. Because the nuclear spins randomly change their direction over time, the electron spin dephases.
Different techniques have been developed to get such nuclear fluctuations under control and focus the nuclear spin polarization on a well-defined value. A very intriguing possibility consists of illuminating the semiconductor island with periodic laser pulses. It has been found that such illumination brings the qubit energy to a well-defined value that is directly related to the laser repetition rate. Such a locking of the qubit energy has been observed in quantum dots that contain a single electron, but the exact reason why the energy locks to the laser repetition rate has remained unclear. We have investigated this effect and have found that it is surprisingly much more universal than previously thought: We were able to observe nuclear focusing also in islands that contain many electron spins and that are fabricated using standard techniques of the semiconductor industry. This allows us to fabricate dots of well-controlled shape, size and position, which is essential in order to apply the technique to spin qubits in a scalable way.
Principle and measurement of spin mode-locking in lithographically defined quantum dots, see image.
S. Markmann, C. Reichl, W. Wegscheider and G. Salis, “Universal nuclear focusing of confined electron spins,” Nature Communications 10, 1097 (2019).
The polariton condensate appears as a nonlinearly growing peak in the momentum distribution, corresponding to the macroscopic population of the state. Interference fringes in a Michelson interferometer are a signature of the long-range spatial coherence. Pinned vortices result in fork-like dislocations.
Wavelength-scale Gaussian-shaped deformation
The pattern on the left is achieved by focused ion beam milling into the substrate. Atomic force microscopy (center) reveals the smooth Gaussian structure on the top mirror substrate. By mounting both halves of the optical microcavity on xyz-nanopositioners, we create tunable polaritons in the external potential given by the nanostructure.