The electron spin is now used as a quantum information carrier by a large number of research teams to develop quantum spintronics. This field of research aims to lay the foundations of a quantum electronics based on unique spins with the realization of a quantum computer using quantum bits (qubits) of spin as a spearhead. A Grenoble collaboration involving researchers from IRIG, CEA-Leti and the Néel Institute is currently working on this new electronics using CMOS silicon technology. This collaboration has just completed a new original reading of a spin qubit in silicon.

Before going into the details of this result, let's start with an analogy. Imagine a little girl swinging on a swing and let's focus on here two thumbs (Figure A).


A - Little girl on a swing. The frequency of the swing depends finely on the orientation of her thumbs.

Depending on her thumbs direction the mass repartition will vary a little which will induce a tinny change in the oscillating frequency of the swing. Then if you have an accurate frequency readout system, you can in principle know if the little girl has her thumbs parallel or anti-parallel just looking at the swing frequency. In a very simplified picture this is more or less what the Grenoble physicists have realized with the thumbs being two spins and the swing being an electrical LC oscillator.

The researchers have developed a spin qubit device (Figure B), in which they isolate two spins.


B - Schematic diagrams of the installation.
The sample used is a nanowire transistor with 2 silicon gates on insulator. Near absolute zero, it is possible to isolate two spins in the nanowire each under one gate. Gate 1 is connected to an LC resonator and gate 2 to a microwave generator capable of inducing spin precession. The LC resonator is read by a reflectometry setup.

The remarkable property of this device is that its electrical capacitance depends very slightly on the orientation of the spins. As the resonance frequency of an LC oscillator is extremely sensitive to capacitance variation, it is possible, by monitoring the oscillator frequency, to readout the spin orientation in the device.
To demonstrate the readout mechanism, the researchers have induce the precession of one spin with a microwave excitation. In this situation the two-spin system oscillates between a parallel state and an anti-parallel state. The oscillation then results in a slight variation in the resonance frequency of the LC resonator (Figure C). This detection method does not need local reservoirs of charges or embedded charge detectors. This could be used to scale spin qubit architecture where spin readout would be performed by any gate of a 2D quantum dot array by attaching a LC resonator to it.


C - The oscillation between a parallel state and an anti-parallel state results in a slight oscillation of the resonance frequency of the LC resonator. The frequency variation is shown in color according to the frequency and excitation time of the spin. Frequency oscillations result from the precession of on spin in the silicon transistor.