In QLUSTER, we are developing beyond state of the art quantum dot systems with optical micro cavities for single photon sources, entangled photon sources, and develop and implement protocols for generation of multi-photon entanglement. In particular:
* We have improved single photon sources based on a semiconductor quantum dot in optical microcavities, in fully monolithic integrated systems and using the open cavity approach. With this, we have achieved a single photon source that simultaneously has very high brightness (57% in fiber), a photon purity of 98% and 97.5% indistinguishability. We also developed novel two-photon excitation schemes and demonstrated purities beyond 99.9%, while improving photon indistinguishability for this scheme from 60% to over 90%.
* We have done the most comprehensive investigation of the dynamics of the nuclear spin bath and their interaction with single quantum dot spins to date. We have developed protocols for controlling the nuclear spin bath, which resulted in experimental demonstration of extending the electron spin coherence time by nearly two orders of magnitude to more than 0.1 ms.
* We investigate alternative spin systems and developed hole-spin systems in single quantum dot and quantum dot molecule devices. This allowed us to probe hole spin dynamics for timescales up to 40µs, which is promising as alternative spin systems for the production of cluster and graph states.
* We have developed singlet-triplet qubits in quantum dot molecules, as we have shown before these might enable the direct production of 2D cluster states. We have conceived and optimized new device designs, including annular Bragg reflectors for enhanced out-coupling of light, and achieved reproducible production of such devices.
* We introduced an ultra-fast single-shot readout scheme of the quantum dot spin - and demonstrated experimentally spin read-out within 3 ns with a fidelity of 97%. This is a crucial ingredient for spin-based production of cluster and graph states.
* Based on our single photon sources and using novel linear-optical setups, we have achieved the quasi-deterministic productioon of linear (1D) cluster states of up to 4 photons at a detection rate of 10 Hz.
* We have developed cavity-compatible spin control techniques that are compatible with polarization encoding of the cluster states. With this, we were able to show spin-photon and spin-photon-photon entanglement at a rate which is 3 and 2 orders of magnitude higher than the previous state of the art, respectively.
* With linear optics, Bell-state measurements are inefficient, we have developed a novel protocol based on encoding of Bell states in graph states that strongly improves efficiency.
* We have designed and optimized protocols for the generation of cluster and graph states with simple and coupled quantum emitters.
* We have theoretically analyzed the performance of all-photonic quantum repeater schemes based on graph states, and identified parameter regions where these schemes show a clear advantage over quantum memory based approaches.
* We have achieved the fastest spin-photon and spin-photon-photon entanglement generation rate exceeding by 2-3 orders of magnitude previous results
* We have also achieved nuclear-spin control in low-strain quantum dots
* We have developed novel excitation schemes for quantum dots and molecules
* We have achieved the fastest ever single-shot readout of a qubit by orders of magnitude exceeded other systems
* We have developed the most stable open-cavity devices in wet and dry cryostats
