Entangled photons on-demand from a single quantum dot

Abstract

Polarisation-entangled photon pairs are generated from single InAs quantum dots. This is achieved by restoring the degeneracy of the exciton level with an in-plane magnetic field or by careful selection of the dot.At present the most robust sources of entangled photon pairs are based around the non-linear optical process of parametric down conversion [1,2]. These processes produce an unpredictable number of entangled photons which make them of limited use in many quantum optics applications requiring entangled photon pairs on-demand [3].The two photon biexciton cascade from a single quantum dot has been proposed as a source of triggered polarisation entangled photon pairs [4], if the two intermediate exciton states are degenerate in energy within the homogeneous linewidth of the transition. A device based upon this principle has not been realised as in practise the exciton level is usually split by 10-200µeV due to the anisotropic exchange interaction [5], which is much larger than the typical homogenous linewidth of ~1.5µeV. As a result photon pairs emitted by the biexciton cascade are found to be polarisation correlated in a linear basis aligned to the major and minor axes of elongation of the exciton [6-8].Recently we reported two methods for engineering InAs quantum dots whose intermediate exciton levels are degenerate within the homogeneous linewidth of the transition; the first involves careful selection of quantum dots whose exciton emits at ~1.4eV [9], and the second employs the use of a modest in-plane magnetic field to tune the splitting of a particular class of dot to zero [10]. In this paper we introduce results of time correlated experiments from such dots which strongly suggest they are emitting pairs of polarisation entangled photons. 0 10 20 30 0 10 20 30 Correlation Intensity S=50µeV HH HV (b) dot A S=~0µeV (c) dot B DD DD' -40 -20 0 20 40 RR RL -40 -20 0 20 40 Energy (meV) Delay (ns) Dot B Dot A Degree of Correlation (%) (d) HH DD VV 1402 1404 H V Photoluminescence X XX (a) Figure 1: (a) Polarised photoluminescence from a single quantum dot with a small fine structure splitting. The red (blue) line corresponds to horizontally (vertically) polarised photoluminescence, exciton (X) and biexciton (XX) states are labelled. (b) and (c) show correlation measurements taken from (b) a reference quantum dot with a typical fine structure splitting (S=50µeV) and (c) a quantum dot with a small fine structure splitting. Measurements of photons of the same basis are shown in red and from orthogonal bases in blue, a small delay was added to the red traces to aid comparison. The top, middle and bottom panels represent correlations measured in the rectilinear (Horizontal, Vertical), diagonal (Diagonal, D'-orthodiagonal) and circular (Right-hand, Left-hand) bases. Panel (d) plots the degree of linear correlation as a function of basis angle.a2141_1.pdf JTuC2.pdf 1-55752-813-6/06/$25.00 ©2006 IEEE

Photon-pair generation based on superconductivity

“…This drawback has triggered the research toward "on-demand operation" of regulated EPP sources. Semiconductor quantum dots (QD) have been extensively studied on the biexciton and exciton cascaded photon-pair generation process and EPP generations have been reported [13,14,15,16]. Photon pairs generated in this cascaded process have the time separation on the order of exciton lifetime, which is ∼ 1 ns in semiconductor QDs.…”

Polarization-entangled mid-infrared photon generation inp-doped semiconductor quantum wells

“…Raman scattering noise, which is hard to suppress in SFWM [9], has made the SPDC scheme more attractive. The SPDC and SWFM processes, producing entangled photon pairs, can be induced in different media, like bulk crystals (possibly with tailored inhomogeneous nonlinearity [10]), quantum well heterostructures, in quantum dots [11,12], or NV centres in diamond [13].…”