Observation of strongly entangled photon pairs from a nanowire quantum dot

Abstract

A bright photon source that combines high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nanoscale is required for implementing ambitious schemes in quantum information processing, such as that of a quantum repeater. Still, all of these properties have not yet been achieved in a single device. Semiconductor quantum dots embedded in nanowire waveguides potentially satisfy all of these requirements; however, although theoretically predicted, entanglement has not yet been demonstrated for a nanowire quantum dot. Here, we demonstrate a bright and coherent source of strongly entangled photon pairs from a position-controlled nanowire quantum dot with a fidelity as high as 0.859±0.006 and concurrence of 0.80±0.02. The two-photon quantum state is modified via the nanowire shape. Our new nanoscale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes.

Figure 1. Nanowire quantum dot sample.

(a) Scanning electron microscopy image of a tapered nanowire waveguide with embedded quantum dot. (b) Photoluminescence spectrum of a single InAsP quantum dot embedded in an InP nanowire. The spectrum was taken at the excitation power used for the cross-correlation measurements needed to reconstruct the density matrix (100 nW), which is close to saturation of both XX and X_B transitions. Note that the excitonic transition X_A saturates the CCD camera. (c) Polarization-dependent measurement to determine the excitonic fine-structure splitting. To increase the accuracy of the polarization measurement we plot the relative difference between biexciton XX and exciton X_B emission energy. The amplitude of the sine-function fit indicates a fine-structure splitting of 1.2 μeV.

Figure 2. Cross-correlation measurements for the three different bases:

(a) Rectilinear, (b) diagonal and (c) circular. The plotted data are normalized to the Poisson level of the side peaks. Start: biexciton; stop: exciton X_B. The first letter stands for the measured polarization of the biexciton photon, whereas the second letter stands for the measured polarization of the exciton photon.

Figure 3. Quantum-state tomography.

Real (a) and imaginary part (b) of the density matrix for the full time window of 6 ns, in the rotated basis. The positive matrix elements are blue, and the negative matrix elements are red. (c) Illustration of the effect of birefringence in the nanowire. The orthogonal waves inside the nanowire experience different refractive indices, and therefore their wavelengths inside the waveguide are unequal. As a result, the polarization of the light emission by the quantum dot (red) is modified leading to a different quantum state. The tapered section of the nanowire is more symmetric and is free of birefringence.

Figure 4. Nanowire birefringence.

SEM images of: (a) symmetric nanowire waveguide, and (b) asymmetric nanowire waveguide. Top panel: side-view SEM images of nanowires with a tilt angle of 45 degrees. Bottom panel: SEM images of the nanowires viewed from the top at a small tilt angle. The blue-shaded circle represents the opening in the SiO2 mask. The example of the nanowire elongation in b is an extreme example that leads to geometric birefringence and corresponding rotation of the quantum state.