GHz bandwidth electro-optics of a single self-assembled quantum dot in a charge-tunable device

The response of a single InGaAs quantum dot, embedded in a miniaturized charge-tunable device, to an applied GHz bandwidth electrical pulse is investigated via its optical response. Quantum dot response times of 1.0 \pm 0.1 ns are characterized via several different measurement techniques, demonstrating GHz bandwidth electrical control. Furthermore a novel optical detection technique based on resonant electron-hole pair generation in the hybridization region is used to map fully the voltage pulse experienced by the quantum dot, showing in this case a simple exponential rise.Comment: 7 pages, 4 figure

Transform-limited single photons from a single quantum dot

A semiconductor quantum dot mimics a two-level atom. Performance as a single photon source is limited by decoherence and dephasing of the optical transition. Even with high quality material at low temperature, the optical linewidths are a factor of two larger than the transform-limit. A major contributor to the inhomogeneous linewdith is the nuclear spin noise. We show here that the nuclear spin noise depends on optical excitation, increasing (decreasing) with increasing resonant laser power for the neutral (charged) exciton. Based on this observation, we discover regimes where we demonstrate transform-limited linewidths on both neutral and charged excitons even when the measurement is performed very slowly

Electrically-tunable hole g-factor of an optically-active quantum dot for fast spin rotations

We report a large g-factor tunability of a single hole spin in an InGaAs quantum dot via an electric field. The magnetic field lies in the in-plane direction x, the direction required for a coherent hole spin. The electrical field lies along the growth direction z and is changed over a large range, 100 kV/cm. Both electron and hole g-factors are determined by high resolution laser spectroscopy with resonance fluorescence detection. This, along with the low electrical-noise environment, gives very high quality experimental results. The hole g-factor g_xh depends linearly on the electric field Fz, dg_xh/dFz = (8.3 +/- 1.2)* 10^-4 cm/kV, whereas the electron g-factor g_xe is independent of electric field, dg_xe/dFz = (0.1 +/- 0.3)* 10^-4 cm/kV (results averaged over a number of quantum dots). The dependence of g_xh on Fz is well reproduced by a 4x4 k.p model demonstrating that the electric field sensitivity arises from a combination of soft hole confining potential, an In concentration gradient and a strong dependence of material parameters on In concentration. The electric field sensitivity of the hole spin can be exploited for electrically-driven hole spin rotations via the g-tensor modulation technique and based on these results, a hole spin coupling as large as ~ 1 GHz is expected to be envisaged.Comment: 8 pages, 4 figure

High resolution coherent population trapping on a single hole spin in a semiconductor

We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physics-like dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify this process as charge noise by, first, demonstrating that the hole spin g-factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process

Decoupling a hole spin qubit from the nuclear spins

A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is <1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform

The hole spin in a semiconductor quantum dot

Extensive research on semiconductor quantum dots (QDs) has been a hot topic in the semiconductor community over the past 20 to 30 years and is still ongoing. In the late 1980s the term "quantum dot" was introduced to describe a semiconductor nano-structure. Some of the motivating prospects driving the research are low-threshold QD lasers, single dots for medical markers, lighting technologies for TVs or single spins for spintronic applications, e.g. quantum information processing. The size and the structure of a QD can vary from a few nanometres in colloidal dots (also known as nanocrystals) to a few hundred nanometres in lithographically defined electrostatic devices. The material components and the fabrication methods can differ a lot between the individual types of QDs. One feature all different kinds of QDs have in common is the restriction of the carrier motion in all three dimensions, which is induced by confinement. That property is the origin of the name zero-dimensional ("0D") structures. A second term often used describes the QD as an "artificial atom". The strong confinement establishes discrete energy states for the localized single carriers inside the QD, which resembles the properties of carriers in atoms. The QDs investigated in this thesis are self-assembled InAs QDs in a semiconductor heterostructure, laying the focus on the confined positive charged carriers, the holes. The spin properties of the individual quantum states are characterized with advanced optical spectroscopy techniques. The following thesis is split into four parts. The first part motivates the search for coherent single hole spins and explains how to get from a bulk semiconductor to a single spin. After a short introduction of semiconductor self assembled quantum dots, their optical properties and bandstructure, the requirements to perform single spin physics are described. The advantage to choose the hole spin for a spin qubit instead of the electron spin, regarding their decoherence properties is discussed. The second section of the introduction covers the experimental techniques and improvements to current systems paving the way to a highly coherent spin qubit via the hole spin and high quality data. The new device structure as well as the sophisticated technique of resonance fluorescence detection are explained here. A description of the laser frequency locking mechanism and a power stabilization concludes the chapter. In the second part the first experiments of this thesis on coherent hole spins are presented. With the spectroscopic measurement method of coherent population trapping (CPT) long decoherence times are achieved. Charge noise is determined as a hole spin dephasing mechanism. Despite the very promising results the experiment suffers from two disadvantages. First the measurement method via resonant absorption spectroscopy in combination with the unstable measurements conditions offers a very poor signal to noise ratio. Secondly the low frequency charge fluctuations, inherent in the sample, promote dephasing and induce shifts in the CPT resonance position from scan to scan. The third part covers different approaches to address the noise issue of part two. The optical linewidth and the noise are closely related in solid state emitters: The linewidth broadening is caused by spin and charge noise in the quantum device. First, low frequency charge fluctuations are reduced by a feedback scheme, which stabilizes the emission frequency of the quantum dot to a stabilized reference. The feedback loop minimizes the fluctuations in the emission frequency, even over several hours, and eliminates the charge noise in the quantum dot to a large extent. This method realises a frequency stabilized source of single photons in the solid-state. The next chapter introduces a new sample design in order to reduce spectral fluctuations. The n-i-p device growth sequence is inverted, which prevents the usual contamination of the QDs by the C-doping. The characteristics of the ultra clean p-doped samples are narrow linewidths in combination with high count rates. The "transform-limit" is reached with a fast scanning method. In the sample a voltage dependent blinking behaviour of the positively charged exciton is discovered. The story of low-noise samples and noise control continues in the next chapter. Transform-limited linewidth of the neutral and the negatively charged exciton are presented. For the neutral exciton this is even true for slow measurements lasting several seconds. For already low-noise structures the residual linewidth broadening is only caused by the nuclear spin noise. A two colour experiment provides control over the nuclear spins, which dominate the exciton dephasing. In the last part the interaction of the hole spin with its environment is investigated. The hole spin states interact in an in-plane magnetic field with an external electric field. The interactions result in a tunable hole g-factor, showing a linear dependency over a large electric field range. In contrast the electron g-factor is not influenced by the electric field at all. Theory reproduces the hole g-factor dependence, which arises from a soft hole confining potential, an In concentration gradient and a strong dependence of the material parameters on the In concentration. The last chapter demonstrates the anisotropic behaviour of the hyperfine interaction between nuclear spins and the hole spin. In the experiment, again with the measurement method of coherent population trapping, a low-noise sample and resonance fluorescence spectroscopy are combined. The resulting high signal to noise ratio and the ultra narrow CPT dip enable the measurement of very precise values for the energy splitting of the hole spin states. This is leading to the main result: a minimal hole hyperfine interaction in an in-plane magnetic field, proofing a decoupling from the hole spin and the nuclear spins

Frequency-Stabilized Source of Single Photons from a Solid-State Qubit

Single quantum dots are solid-state emitters which mimic two-level atoms but with a highly enhanced spontaneous emission rate. A single quantum dot is the basis for a potentially excellent single photon source. One outstanding problem is that there is considerable noise in the emission frequency, making it very difficult to couple the quantum dot to another quantum system. We solve this problem here with a dynamic feedback technique that locks the quantum dot emission frequency to a reference. The incoherent scattering (resonance fluorescence) represents the single photon output whereas the coherent scattering (Rayleigh scattering) is used for the feedback control. The fluctuations in emission frequency are reduced to 20 MHz, just ~ 5% of the quantum dot optical linewidth, even over several hours. By eliminating the 1/f-like noise, the relative fluctuations in resonance fluorescence intensity are reduced to ~ 10E-5 at low frequency. Under these conditions, the antibunching dip in the resonance fluorescence is described extremely well by the two-level atom result. The technique represents a way of removing charge noise from a quantum device.Comment: 14 pages, 4 figure