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When it comes to quantum computation, superconducting qubits are currently leading the field. But there are other ways to represent a qubit. The spin of an electron is arguably a much simpler object than a superconducting circuit: In a straightforward way, spin up and spin down represent the two qubit states, 0 and 1. However, also for spin qubits, the stored quantum information does not persist forever. The electrons interact with their environment and thereby lose their spin polarization over time.
Principle of spin locking: A periodic laser pulse train locks the spin precession frequency to an integer multiple n of the laser repetition rate. In the example, the spins precess either exactly n = 6 or n = 7 times between two subsequent laser pulses.
Electron spins are typically captured on semiconductor “islands” (so-called quantum dots) that consist of more than 100,000 atoms. Depending on the material used, the atomic nuclei may have a spin themselves. Each nuclear spin slightly modifies the energy of the electron spin. This by itself would not be a problem. It however becomes an issue because the nuclear spins randomly change their direction over time and with this the energy of the qubit starts to fluctuate.
Different techniques have been developed to get such fluctuations under control and keep the nuclear spin polarization constant. A very intriguing possibility consists in 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.
The color in this image encodes the spin polarization along a fixed direction in space, measured on semiconductor quantum dots by the IBM team. The spins oscillate at a frequency that can be varied by a magnetic field. The sample is illuminated with periodic laser pulses. Time zero marks the moment when a laser pulse arrives at the dot. The spin oscillation is locked to the laser repetition rate irrespective of the applied magnetic field.
Together with collaborators at ETH Zurich, our team at IBM Research – Zurich has investigated this effect and has found that it is much more universal than previously thought: We could observe nuclear focusing also in islands that contain many electron spins and that are fabricated using standard techniques of the semiconductor industry. We found that the optical Stark effect is responsible for the nuclear focusing: Each laser pulse creates a short-lived and tiny magnetic field that slowly steers the nuclear spins into a well-defined polarization. This became clear when we changed the energy of the laser pulses. With a lower energy, the direction of the tiny magnetic field reverses and with it the electron spins become anti-locked with the laser repetition rate, which is well explained by the model.
With this technique, the lifetime of the spin qubit can be substantially prolonged beyond the limit given by the fluctuating nuclear spins. As we have shown, nuclear focusing also works in quantum dots carved out of a semiconductor material using lithography and etching techniques. With this, dots of well-controlled shape, size and position can be fabricated, which is essential to apply this technique to spin qubits in a scalable way.
Sergej Markmann, Christian Reichl, Werner Wegscheider and Gian Salis, Universal nuclear focusing of confined electron spins, Nature Communications 10, 1097 (2019).