Nuclei unplug a spin-valve

A team of scientists shows how to electrically control the polarization of nuclear spins


In any spin-based memory device—or qubit—a major challenge is preserving the spin state of an electron over reasonable times. The reason is that the electron interacts with nearby spins that cause it to ‘flip’ and lose information.

The greatest contribution to this electron ‘dephasing’ in semiconductor spin qubits is from random fluctuations of the surrounding nuclear spins. This is why a group of scientists, including Keiji Ono from the RIKEN Discovery Research Institute, Wako, and colleagues at the University of Waterloo in Canada and the University of Tokyo, are exploring a way to electrically suppress nuclear spin fluctuations. Their results are reported in the journal Physical Review Letters1.

The team built an electronic device called a vertical double quantum dot (Fig. 1) that consists of two 10 nm-thick layers of the semiconductor GaAs, separated from one another and from metallic contacts by thin layers of the semiconductor Al0.3Ga0.7As. The Al0.3Ga0.7As layers act as ‘tunnel barriers’ that confine electrons to one of the thin GaAs layers, which are therefore called ‘quantum wells’.

The team has shown previously that it can control the transport of electrons, one at a time, through the quantum wells and that this transport depends on the spin state (‘up’ or ‘down’) of the electron. In fact, under certain conditions, an electron with one particular spin state (say, ‘up’) cannot tunnel across the double-dot structure without first flipping its spin.

In their new work, the team uses this ‘spin blockade’ effect to polarize the nuclear spins in the GaAs. This superconductor contains three isotopes—71Ga, 69Ga and 75As—each of which has a large nuclear spin that interacts with an electron moving through the double dot. When the device is set to the spin-blockade mode that prevents spin ‘up’ electrons from tunneling across, the nuclear spins open the door: the interaction between the electron and nuclear spins allows the electron spin to flip from ‘up’ to ‘down’ and tunnel through the device, while the nuclear spin flips from ‘down’ to ‘up’.

The process polarizes the surrounding nuclear spins by as much as 40%. Ono notes that these measurements are the first to show that nuclear spin fluctuations can be controlled electrically and could be useful in designing spin-memory devices with longer electron spin lifetimes or based entirely on storing information in the nuclear spins themselves.
Reference

1. Baugh, J., Kitamura, Y., Ono, K. & Tarucha, S. Large nuclear Overhauser fields detected in vertically coupled quantum dots. Physical Review Letters 99, 096804 (2007).