INTRODUCTION AND MOTIVATION
A group from the Centre for Cold Matter at Imperial College London have studied the quantum dynamics of single molecules in the presence of temperature-controlled dephasing, which was recently published in Phys. Rev. A . By collecting fluorescence from a single dibenzoterrylene (DBT) molecule in an anthracene crystal illuminated with a laser tuned on resonance with the singlet-singlet energy, they were able to measure autocorrelations at various temperatures and illumination intensities, proving that the molecule behaves as an ideal two-level system. Such molecules can be employed as single photon sources and can mediate a photon-photon interaction  which can be used to build novel quantum technologies.
DBT-doped anthracene crystals were grown using a co-sublimation technique developed at Imperial College London . These were then placed on a custom sample mount on a three-axis positioner setup in a Montana Instruments Cryostation which included the Cryo-Optic Microscope, a 100x objective with an NA of 0.9. A tunable diode laser with a wavelength of ~785 nm was used to illuminate the molecule through a confocal setup, shown in Fig. 1. The laser was reflected from a dichroic mirror and sent to the objective which focussed the light onto the molecule with a spot size of ~500 nm. By scanning the angle of the laser entering the objective the position of this spot was varied on the sample. By scanning the laser spot and the laser frequency they were able to locate single DBT molecules.
Figure 1 - A simplified schematic of the confocal microscope used in this work. Blue: excitation. Red: emission.
Figure 2 - A typical second-order correlation function g(2)(τ) measurement from a single DBT molecule.
By varying the laser frequency and intensity, as well as the temperature of the sample between 4 K and 10 K, the group were able to measure the saturation of the molecule fluorescence counts and the homogeneous linewidth of the zero-phonon line transition in the molecule. From this, they were able to measure the variation in dephasing time of the single DBT molecule, as well as the activation energy for the first available phonon that causes this dephasing. The group then looked at the temporal quantum dynamics of the molecule by separating the fluorescence light in a fiber beamsplitter and measuring the second-order correlation function of the emitted light. A typical curve is shown in Fig. 2. The dip at the centre is due to the molecule only emitting one photon at a time, the so called “anti-bunching” effect. The ripples in the wings of the curve are due to Rabi oscillations between the ground and excited states of the molecule. The solid line is derived from the optical Bloch equations for a two level system and agrees very well with the data.
According to Dr. Alex Clark, project leader for this work, the next step is to “couple these cold molecules to optical cavities and waveguides, with the aim of making an on-demand source of indistinguishable photons.” The paper was published in Phys. Rev. A and was highlighted as an editor’s suggestion .
 S. Grandi, K. Major, C. Polisseni, S. Boissier, A. S. Clark, and E. A. Hinds, “Quantum dynamics of a driven two-level molecule with variable dephasing,” Phys. Rev. A 94, 063839 (2016).
 J. Hwang and E. A. Hinds, “Dye molecules as single-photon sources and large optical nonlinearities on a chip,” New J. Phys. 13, 085009 (2011).
 K. D Major, Y.-H. Lien, C. Polisseni, S. Grandi, K. W. Kho, A. S Clark, J. Hwang, and E. A. Hinds, “Growth of optical-quality anthracene crystals doped with dibenzoterrylene for controlled single photon production,” Rev. Sci. Instrum. 86, 083106 (2015).