Single pairs of time-bin-entangled photons

Time-bin-entangled photons are ideal for long-distance quantum communication via optical fibers. Here we present a source where, even at high creation rates, each excitation pulse generates, at most, one time-bin-entangled pair. This is important for the accuracy and security of quantum communication. Our site-controlled quantum dot generates single polarization-entangled photon pairs, which are then converted, without loss of entanglement strength, into single time-bin-entangled photon pairs.

Figure 1

Our setup, based on a quantum dot and a polarization–time-bin interface. Single polarization-entangled photon pairs from the quantum dot are converted into single time-bin-entangled photon pairs. A polarizer at ${45}^{\circ }$ erases all polarization entanglement. The time-bin measurement is performed via the orange path.

Figure 2

Pyramidal quantum-dot emitter. (a) Scanning electron microscopy image of the sample, containing an array of pyramids with a 7.5 $\mu \mathrm{m}$ pitch. Each pyramid contains a quantum dot. Scale bar: 30 $\mu \mathrm{m}$. Tilt angle: ${60}^{\circ }$. (b) Sketch of the internal epitaxial layer structure of a pyramid. The quantum dot is marked in red. (c) Emission spectrum of one quantum dot at two excitation powers. Indicated are the exciton ($X$), biexciton ($\mathit{XX}$), and trion ($X$*) emission lines. (d) The $\mathit{XX}$ (red) and $X$ (black) emission intensities vs excitation power. Fits to power functions show a quadratic dependence (power 2.0) for the $\mathit{XX}$ emission and a nearly linear dependence (power 1.1) for the $X$ emission. (e) Energy separation between $\mathit{XX}$ and $X$ vs polarization angle. This measurement determines the fine-structure splitting: the energy splitting between the two $X$ spin states is $0.6\pm 0.2\mu \mathrm{eV}$.

Figure 3

Single-photon time-resolved correlation measurements. (a) $X-X$ autocorrelation (start: $X$, stop: $X$), (b) $\mathit{XX}-\mathit{XX}$ autocorrelation (start: $\mathit{XX}$, stop: $\mathit{XX}$), and (c) $\mathit{XX}-X$ cross correlation (start: $\mathit{XX}$, stop: $X$) with 639 nm, 100 ps, 80 MHz excitation pulses at 150 nW. (d) $X-X$ autocorrelation, (e) $\mathit{XX}-\mathit{XX}$ autocorrelation, and (f) $\mathit{XX}-X$ cross correlation with 750 nm, 3 ps, 80 MHz excitation pulses at 150 nW.

Figure 4

Polarization entanglement. (a) Measurements of correlations between the polarization of the $\mathit{XX}$ photon and the polarization of the $X$ photons. The first letter represents the polarization of the $\mathit{XX}$ photon and the second letter represents the polarization of the $X$ photon, where $H, V, D, A, L$, and $R$ stand for horizontal, vertical, diagonal, antidiagonal, left-handed, and right-handed polarization, respectively. (b) Real and (c) imaginary parts of the measured density matrix of the polarization quantum state of the two photons from the $\mathit{XX}-X$ cascade. The positive matrix elements are orange and the negative matrix elements are green.

Figure 5

Time-bin entanglement. (a) Count rate vs arrival time at the detector with respect to the laser trigger pulse. Postselection on the photons in the blue time window. (b) Real and (c) imaginary part of the measured density matrix in the time-bin bases. The first and second letters stand for the time bin of the $\mathit{XX}$ photon and the $X$ photon, respectively.