Polarization-correlated photons from a positively charged quantum dot

Abstract

Polarized cross-correlation spectroscopy on a quantum dot charged with a single hole shows the sequential emission of photons with common circular polarization. This effect is visible without magnetic field, but becomes more pronounced as the field along the quantization axis is increased. We interpret the data in terms of electron dephasing in the X + state caused by the Overhauser field of nuclei in the dot. We predict the correlation timescale can be increased by accelerating the emission rate with cavity-QED. PACS numbers:Spins in quantum dots (QDs) provide a promising platform for manipulating and storing quantum information in the solid state. Optical measurements have demonstrated spin preparation [1, 2] coherent spin control [3] and electron-spin − photon entanglement [4,5]. There are also proposals for achieving photon entanglement [6] and non-destructive measurement of photons [7] using charged QDs. However, the time evolution of the carrier spin is unavoidably affected by the 10 4 − 10 5 nuclei in the dot, all with non-zero spin. One example of the utility of the electron-nuclear interaction is its use in spin pumping the hole into the spin down state in zero external field, by pumping on the spin up state of X + [2]. From a fundamental point of view then, the hyperfine interaction provides an interesting system for manipulating a mesoscopic nuclear ensemble and observing its dynamics.It has now been established that the electron-nuclear hyperfine interaction is dominated by the contact interaction and is isotropic in a QD [8]. The dynamics of this interaction manifests itself in studies of polarized photo-luminescence [9,10]. Contrastingly, the hole's plike wave-function has a node at each nucleus leaving the dipole-dipole interaction between the hole and nuclear spins to dominate [11]. This interaction has a strength one order of magnitude below that of the electron [12,13]. Thus, there has been interest in using the hole-spin as quantum bit with reduced decoherence. Direct measurements of the hole spin relaxation time in a vertical magnetic field, T h 1 , have shown it is hundreds of microseconds [2,14]. Without applied magnetic field some experiments suggest the hole spin T h 1 time is 13 ns [8]. Several studies have now estimated the hole dephasing time, T h 2 in magnetic field is approximately 1 µs [3,17].We study here the emission from the X + state in a dot deterministically charged with a hole using a diode * Electronic address: anthony.bennett@crl.toshiba.co.uk ( Figure 1a). We show photons from this transition display polarization correlation over a timescale one order of magnitude greater than the radiative lifetime when excited by a linearly polarized laser. This time is short relative to some reports of the hole spin polarization lifetime [14] and we show that this is a result of dephasing caused by the electron when the system is excited. We investigate the magnitude of the effect as a function of applied external magnetic field and radiative lifetime. The X + consists of two holes in a...

Ramsey interference in a multilevel quantum system

“…The forbidden transitions are allowed by heavy-light hole mixing, so we set the transition strength ratio between the allowed and forbidden transitions to be 5:1, in accordance with prior work [24]. However, this does not determine how strongly we are driving each of the transitions.…”