Quantum dot molecule devices with optical control of charge status and electronic control of coupling

Abstract

Tunnel-coupled pairs of optically active quantum dots - quantum dot molecules (QDMs) - offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet-triplet ($S-T_0$) qubits having extended coherence times. The $S-T_0$ basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter-dot orbital couplings, operation of the $S-T_0$ qubits under optimal conditions remains challenging. Here, we present an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand. We perform a four-phase optical and electric field control sequence that facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter-dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. We achieve one- and two-hole charging efficiencies of 93.5 $\pm$ 0.8 % and 80.5 $\pm$ 1.3 %, respectively. Combining efficient charge state preparation and precise setting of inter-dot coupling allows control of few-spin qubits, as would be required for on-demand generation of two-dimensional photonic cluster states or quantum transduction between microwaves and photons.