Spin-based quantum computing using electrons on liquid helium

Numerous physical systems have been proposed for constructing quantum computers, but formidable obstacles stand in the way of making even modest systems with a few hundred quantum bits (qubits). Several approaches utilize the spin of an electron as the qubit. Here it is suggested that the spin of electrons floating on the surface of liquid helium will make excellent qubits. These electrons can be electrostatically held and manipulated much like electrons in semiconductor heterostructures, but being in a vacuum the spins on helium suffer much less decoherence. In particular, the spin-orbit interaction is reduced so that moving the qubits with voltages applied to gates has little effect on their coherence. Remaining sources of decoherence are considered, and it is found that coherence times for electron spins on helium can be expected to exceed $100\mathrm{s}$. It is shown how to obtain a controlled-NOT operation between two qubits using the magnetic dipole-dipole interaction.

Figure 1

A schematic diagram of a possible gate electrode arrangement for a quantum computer. The gray regions are conducting gates held at different potentials $({\Phi }_1-{\Phi }_{\mathrm{J}})$ arranged as a three-phase CCD. By adjusting the gate voltages, the electrons (dots with arrows) can be moved around the device. The light-colored region on the left represents a conductor below the gates through which a current, $I$, can be run, producing a local lateral magnetic field that combines with the large constant field, $B_0$, to bring the Zeeman transition of the electrons above the wire into resonance with applied microwaves.

Figure 2

A schematic illustration of a structure which can be used to hold two electrons (dots with arrows) in close proximity, allowing their spins to interact. The lower part of the figure shows the structure: metallic layers with two hemispherical bosses covered by a layer of LHe. The upper part of the figure shows the calculated one-electron potential energy created by this arrangement of metallic gates with $V_J=V_0$ and chosen to make the electron potential energy approach 0 asymptotically.