We aim to develop single-photon sources, quantum gates and quantum memory, all based on single organic dye molecules.

Molecular Photon Source

With standard down-conversion and four-wave mixing sources, the detection of one photon  heralds the presence of another, but one does not know when that will occur. The single quantum emitter, by contrast, emits a photon on demand, i.e., soon after being excited. Quantum dots are promising single-photon emitters, but it is difficult to fabricate many dots with identical properties, as required for scalability. N-vacancy and Si-vacancy centres in diamond also show promise, but the former has only a 4% branching ratio to the zero-phonon line (ZPL), while the latter has undesirable substructure in the line and low brightness. Instead, we will use single DBT molecules embedded in anthracene. They will be closely coupled to single-mode photonic waveguides to ensure high photon collection efficiency. These sources will be tuned electrically to bring them into resonance with cavities and other molecules, and to prove the indistinguishability of the generated photons – a requirement for scalable quantum photonic technology.

Schematic of an array of waveguide-coupled molecules, pumped through a common grating coupler input.

Molecular Quantum Operations

Single photon production relies on the dissipative part of the molecule-photon coupling. There are also many applications of the dispersive coupling, i.e., the phase shifts of the light and the optical dipole. To develop devices based on this phase shift, we will embed a molecule in an interferometer, as shown in the figure below. A photon introduced into input 1 splits into a superposition of upper and lower guided modes. The upper guide containing the molecule is also coupled to a bus mode. With no photon in the bus, the interferometer photon arrives at output 1, but when there is a photon in the bus, the two experience a mutual phase shift mediated by the molecule, shifting probability towards output 2. We will monitor these output ports with single-photon detectors to measure the differential phase shift achieved. With the addition of a second bus line, this becomes a controlled-NOT gate.

The use of a molecule as a phase shifter in an interferometer. (a) Scheme to measure differential phase shifts. (b) Realisation of a ontrolled-NOT gate.

Molecular Quantum Memory

The molecular photon sources and quantum operations discussed above rely on coupling light to the strongly allowed singlet transition of the DBT molecule, S0S1. However, these molecules also have a triplet state T1, which lies between the singlets. The triplet lives for 40 μs and has the multiple sub-levels needed for convenient storage of quantum information. We will develop the tools needed to exploit this in the DBT molecule.