All optical quantum control of a spin-quantum state and ultrafast transduction into an electric current

Abstract

The ability to control and exploit quantum coherence and entanglement drives research across many fields ranging from ultra-cold quantum gases to spin systems in condensed matter. Transcending different physical systems, optical approaches have proven themselves to be particularly powerful, since they profit from the established toolbox of quantum optical techniques, are state-selective, contact-less and can be extremely fast. Here, we demonstrate how a precisely timed sequence of monochromatic ultrafast (~ 2-5 ps) optical pulses, with a well defined polarisation can be used to prepare arbitrary superpositions of exciton spin states in a semiconductor quantum dot, achieve ultrafast control of the spin-wavefunction without an applied magnetic field and make high fidelity read-out the quantum state in an arbitrary basis simply by detecting a strong (~ 2-10 pA) electric current flowing in an external circuit. The results obtained show that the combined quantum state preparation, control and read-out can be performed with a near-unity (≥97%) fidelity.

Figure 1

(a) Coherent photoresponse of the system upon driving the cgs − X transition. (b–e) Pump-probe spectra recorded by pumping the cgs − X transition for co- (cross-) linear configurations along the [110] and crystal axes. (d)Temporal evolution of the peaks/dips from (b). (c) and (e) Same experiment reported in (b) and (d) but for for co- (cross-) circular polarisations (In (a) a linear background is subtracted - see text).

Figure 2

(a) Temporal evolution of ΔI for pumping excitons with R polarisation and probing the different projections R, D, L and . (b) Schematic illustration of the exciton spin on the Bloch sphere. (c) Change of PC for a fixed time delay of 38 ps, R (H) readout projection in red (black) for varying the initialisation angle ϕ = 0 − 2π θ = π/2. (d) Same as (c) but for initialisation angles ϕ = 0 θ = 0 − π.

Figure 3

(a) Schematic illustration of the experiment. (b–c) Fully resonant coherent optical control for control and readout pulses tuned to cgs − X. The control angles are varied in (b) over the range ϕ′ = 0 − π θ′ = π/2 and in (c) from ϕ′ = 0 θ′ = 0 − π/2.

Figure 4

(a–b) Quantitative analysis of the data presented in figure 3(c–d): (a) Amplitudes and (b) phase change as a function of the control pulse angles ϕ′ (θ′) in black (red). (c–d) Effect of varying the control pulse detuning for the configuration illustrated in (c).

Figure 5

(a–b)Schematic illustration of the sample structure (a) and resulting band structure (b).(c) Scanning electron microscopy image of an aperture.

Figure 6. Schematic illustration of the setup for ultrafast pump-control-probe spectroscopy.