The CEA-Leti and the CEA-Irig are developing quantum bits on silicon using the spin of gaps of electrons (holes) in a classical CMOS microelectronics environment. They chose to do this due to the possibility of coupling the spin of holes in silicon with a radiofrequency electric field – something that is impossible with electron spins. Applying an electric voltage on the transistor gate is indeed much simpler and less energy consuming than applying a magnetic field!

The quantum bit (qubit) is formed from a hole trapped in a "quantum box", inside a silicon nanowire (constituting the channel of a CMOS transistor), under the gate that blocks the flow of charges in the channel.

To use this qubit, another qubit must be added to the same nanowire as a reference. By applying an external magnetic field, the spins of the two qubits align themselves in the same direction. It is then possible to selectively flip the spin of the first qubit by coupling it with a radiofrequency electric field.

How is the state then read? The principle consists in trying to bring together the two holes in one of the two boxes by applying a properly selected voltage on the gate. Because of the Pauli exclusion principle, if the two spins point in the same direction, each hole will remain in its box and if not, the two holes can be brought together.

The Irig researchers have successfully extracted all the parameters describing this procedure for reading two-spin qubits in two quantum boxes using pump-probe spectrometry.

This characterization is essential before any manipulation and reading of qubits of this type. In fact, all the qubit control values (gate voltage, frequency and electric field intensity, etc.) must be finely adjusted in order to optimize the lifetime of the associated quantum states, as well as the accuracy of their reading. The good news is that it can be used on large-scale integrated qubits!