Since the ground breaking proposal by D. Loss and D. P. DiVincenzo , a significant amount of progress has been made towards the implementation of a universal quantum computer using spins in semiconductors. Despite much progress the achievement of efficient single spin rotations has proven to be a significant challenge. The search for an effective mechanism for driving single spin rotations has motivated the study of strong spin-orbit (SO) materials. Since the SO interaction couples the spin and orbital degrees of freedom of the electron it can be used for efficient all electrical control of spin dynamics .
Our device architecture is illustrated in panel (a). We place a single InAs nanowire, a material with a strong SO interaction, over an array of fine gate electrodes. Applying negative voltages to the gates allows us to selectively deplete regions of the nanowire, thereby establishing a double well potential (a double quantum dot). In the two-electron regime the current through the double dot is highly sensitive to the relative spin orientation of the two electrons due to a phenomenon called spin-blockade .
The effects of spin dynamics on the current through the structure can be understood by studying the transport cycle shown in (b). When a source-drain bias is applied, electrons tunnel from the left to right. When the two spins are in a singlet configuration this process is energetically allowed and the electron will sequentially tunnel through the device. However, in the case of a triplet configuration, the electron in the left dot will no longer be able to tunnel to the right dot due to the Pauli exclusion principle. As such, transport becomes blocked and highly sensitive to any processes that rotate the spins.
With the device configured in Pauli blockade, we apply microwaves to one of the gates, which periodically displaces the electron wavefunction. Through the SO coupling this becomes an effective oscillating magnetic field that then rotates the spin. We plot the leakage current as a function of magnetic field and microwave frequency in (c). A line tracing out the electron spin resonance condition is clearly visible.
One of the significant drawbacks of implementing a qubit in a nanowire is the difficulty of incorporating a sensitive charge detector. We have solved this problem by utilizing a radio frequency measurement technique . We connect the nanowire drain electrode to a lumped element resonator. The radio frequency admittance of the double dot is spin state dependent due to the Pauli exclusion principle, thereby giving the resonator a spin dependent damping rate. We detect changes in the device admittance by looking at the phase of the reflected signal in a homodyne measurement. The phase is plotted in (d) as a function of the device plunger gate voltages, allowing a simple measurement of the charge stability diagram.
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