Stretching and Breaking the Persistent Spin Helix

We introduce the concept of the stretchable persistent spin helix (PSH), i.e. a PSH with gate-adjustable pitch [1]. This opens the door for electrical spin manipulation while spins are protected from the usual Dyakonov-Perel spin decay when propagating through the material. To implement this, a top and back gate is employed to independently tune both the Rashba coefficient α and the effective Dresselhaus coefficient β in-situ. Tunability of β was predicted ca 1990 based a density dependence in the projected 2D term. Here, we demonstrate this in an experiment [1], and we employ the suppression of weak antilocalization as a sensitive detector for matched SO fields. Varying both α and β controllably and continuously with gate voltages, we can demonstrate robust continuous locking at α=β over a wide range, thus stretching the PSH. When combined with numerics, this yields th spin-orbit parameters of the system. Stretchable PSHs could provide the platform for long-distance communication ∼8–25 μm between solid-state spin qubits.

Further, we exploit the high spin-symmetry PSH state to derive a closed-form expression for the weak localization magnetoconductivity -- the paradigmatic signature of spin-orbit coupling in quantum transport. The small parameter of the theory is the deviation from the symmetry state introduced by the mismatch of the linear terms and by the cubic Dresselhaus term. In this regime, we perform quantum transport experiments in GaAs quantum wells. Top and back gates allow independent tuning of the Rashba and Dresselhaus terms in order to explore the broken-symmetry regime where the formula applies. We present a reliable two-step method to extract all parameters from fits to the new expression, obtaining excellent agreement with recent experiments. This provides experimental confirmation of the new theory, and advances spin-orbit coupling towards a powerful resource in emerging quantum technologies.

[1] Dettwiler et al., Phys. Rev. X7, 031010 (2017).