Quantum dots

Resonant excitation

Due to laser scattering, it is very hard to excite quantum dots resonantly. An alternative method is so-called above-band excitation. Here, the energy of the excitation laser is much higher than the energy needed to create an exciton. The excitation laser creates many carriers in the material surrounding the quantum dot, and these carriers can be randomly trapped in the quantum dot potential.

Nevertheless, in such an excitation scheme there is no excitation coherence perserved. A way to go around this problem is to perform resonant excitation. In particular, we possess expertise in performing resonant two-photon excitation of the biexciton

Here we exploit the biexciton binding energy in order to use a laser that is energy-wise half way between exciton and biexciton. This binding energy is a result of Coulomb interaction between two electron-hole pairs, which form the biexciton, and is present in most quantum dots.

Deterministic Photon Pairs and Coherent Optical Control of a Single Quantum Dot
H. Jayakumar, A. Predojević, T. Huber, T. Kauten, G. S. Solomon, and G. Weihs (Phys. Rev. Lett. 110, 135505, 2013) - arXiv

Time-bin entanglement

The idea to generate time-bin entanglement of photons emitted by a single quantum emitter can be traced back to the seminal paper by J.D. Franson (Phys. Rev. Lett. 62, 2205–2208 (1989).). He suggested that the interference between the probability amplitudes for a photon pair to be emitted by an excited atom at diverse times is a nonlocal effect that violates the Bell inequality.

In its simplest scheme, the time-bin entanglement is generated in a very similar manner for both parametric down-conversion and atom-like systems and it demands post-selection in order to be measured. Such a scheme is depicted in the figure below. The system is addressed by two excitation pulses, denoted early and the late pulse. These are derived from an unbalanced interferometer, so-called pump interferometer. The interferometric phase between the pulses determines the phase of the entangled state. The analysis of the generated state is performed using two construction-wise identical unbalanced interferometers, one for exciton and one for biexciton photons.

Time-bin entangled photons from a quantum dot
H. Jayakumar, A. Predojević, T. Kauten, T. Huber, G. S. Solomon, and G.Weihs, (Nature Communications 5, 4251, 2014) - arXiv

Photon statistics

Sending photons (entangled or not) through optical fibres is a process prone to losses and noise. What happens to the quantum state of light undergoing such an adventure? Are the quantum states we send is deteriorated in the process? If and when the state stops being quantum? Can we detect the non-classical characteristics of the state that has undergone significant attenuation? All this and many more questions answered. Just follow:

Efficiency vs. multi-photon contribution test for quantum dots
A. Predojević, M. Jezek, T. Huber, H. Jayakumar, T. Kauten, G. S. Solomon, R. Filip, and G. Weihs (Optics Express 22, 4789, 2014) - arXiv

Quantum non-Gaussian Depth of Single-Photon States
I. Straka, A. Predojević, T. Huber, L. Lachman, L. Butschek, M. Miková, M. Mičuda, G. S. Solomon, M. Ježek and G. Weihs (Phys. Rev. Lett. 113, 223603, 2014) - arXiv