Flying electron qubits and electron interferometry

Qubits that move

In a flying qubit the quantum information is carried by a particle while it propagates, and operations are applied in flight. Photons are the usual choice. Electrons are an interesting alternative: they travel more slowly, so they are easier to control in real time, and they interact through the Coulomb force, which allows two qubits to be entangled directly.

The challenge is to inject single-electron wavepackets on demand that are shorter than the device they travel through. For a device a few tens of micrometres long this means pulses lasting picoseconds, which is where our THz techniques come in.

Electron interferometry

How do we check the quality of the single-electron wavepackets we create? An electronic Mach-Zehnder interferometer splits an electron wavepacket along two paths and recombines it, so that the output depends on the quantum phase picked up along the way. We have injected ultrashort single-electron plasmonic pulses into a 14-micrometre-long Mach-Zehnder interferometer and observed quantum interference, a step towards flying electron qubits operated on picosecond time scales.

Controlling plasmon wavepackets

A short voltage pulse in a nanoelectronic circuit travels as a plasmon wavepacket shared between several conduction channels. Its eigenstate sets its speed, and so the number of operations possible during its flight. Using a cavity ‘hack’ we can select which channels take part, giving precise control of the eigenstate. Earlier work in which the group leader took part showed the bosonic nature of ultrashort few-electron pulses and the shortest ever measured electron wavepacket in a 2-dimensional system.