Environment-assisted quantum control of a solid-state spin via coherent dark states

Abstract

* These authors contributed equally to this work.Understanding the interplay between a quantum system and its environment lies at the heart of quantum science and its applications. To-date most efforts have focused on circumventing decoherence induced by the environment by either protecting the system from the associated noise 1-5 or by manipulating the environment directly 6-9 . Recently, parallel efforts using the environment as a resource have emerged, which could enable dissipation-driven quantum computation 10,11 and coupling of distant quantum bits 12,13 .Here, we realize the optical control of a semiconductor quantum-dot spin by relying on its interaction with an adiabatically evolving spin environment. The emergence of hyperfineinduced, quasi-static optical selection rules enables the optical generation of coherent spin dark states without an external magnetic field. We show that the phase and amplitude of 1 the lasers implement multi-axis manipulation of the basis spanned by the dark and bright states, enabling control via projection into a spin-superposition state. Our approach can be extended, within the scope of quantum control and feedback 14,15 , to other systems interacting with an adiabatically evolving environment.Techniques for controlling spins often rely upon a well-defined Zeeman splitting due to a static external magnetic field. This field also controls the selection rules of the optical transitions enabling, for example, optical single-shot spin readout [16][17][18] and fast spin manipulation 2,19 in selfassembled quantum dots (QDs). Unfortunately, these two capabilities require different alignments of the external field with respect to the growth direction of the QD: a field along the growth direction of a QD (Faraday configuration) provides cycling transitions for spin readout, while a magnetic field applied perpendicular to the growth direction (Voigt configuration) enables optically-driven spin control. In an effort to achieve both readout and control on a QD spin qubit, a large, rapidly switchable, multi-axis magnetic field would be the ideal toolbox.While the quantisation axis due to a static field in Voigt configuration can in principle be converted to that in Faraday configuration via the optical Stark effect, the prohibitively large laser power required to achieve this renders the scheme impractical. We instead consider the smallest effective field we can devise: that due to the local fluctuating nuclear spins within the QD. They give rise to an effective magnetic field for an electron spin in the QD, known as the Overhauser (OH) field (Fig. 1a), with a dispersion of 10-30 mT and a many-microsecond correlation time [20][21][22] . The hyperfine interaction therefore lifts the Kramers degeneracy and provides a quantisation axis for the electron spin, which in turn leads to temporally quasi-stable optical selection rules due to the quasi-static nature of the nuclei (Fig. 1b). This OH field is small 2 enough to allow for Stark tilting the quantisation axis for optical read-out feasibl...

Decoupling a hole spin qubit from the nuclear spins

“…coherent population trapping (CPT) 8,37,[39][40][41] . The combination of a coherent hole spin, resonance fluorescence detection (RF) 42 and low-noise samples enables us to achieve a spectral resolution in Z h of just 10 neV (2.4 MHz).…”

Generating indistinguishable photons from a quantum dot in a noisy environment

“…4(a). Here, significant bunching around time zero is observed due to optical spin pumping, a consequence of the nonzero external magnetic field [12,[59][60][61]. We use a fitting procedure similar to Malein et al [12].…”

Spin-based single-photon transistor, dynamic random access memory, diodes, and routers in semiconductors

“…In the absence of external magnetic field, both the ground and excited states of charged QD are twofold degenerate due to the Kramers theorem. The electron spin degeneracy could be lifted by the nuclear spin magnetic fields [47,48] via the electron-nucleus hyperfine interactions in In(Ga)As QDs, however, the Zeeman splitting is too small to spoil the linear GCB [42]. The hole-spin degeneracy is not affected by the nuclear spin fields due to the lack of the hole-nucleus hyperfine interactions.…”