Quantum Interference of Tunably Indistinguishable Photons from Remote Organic Molecules

We demonstrate two-photon interference using two remote single molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we tune and explore the effect of photon distinguishability. We discuss future improvements on the brightness of single-photon beams, their integration by large numbers on chips, and the extension of our experimental scheme to coupling and entanglement of distant molecules.

Figure 1

(a) Schematics of the setup. DM, dichroic mirror; BS, beam splitter; BP, bandpass filter; HWP, half-wave plate; APD, avalanche photodiode; SIL, solid-immersion lens; $\mu \mathrm{Ed}$, gold microelectrodes. (b) Energy level diagram of a DBATT molecule. (c) Fluorescence excitation spectra of two selected molecules as a function of the voltage on the microelectrodes of one sample. This Stark effect was well described by the relation $\Delta \nu =50\times (V-63)$ (with $\Delta \nu$ in MHz and $V$ in Volts). (d, e) Intensity autocorrelation functions of the two molecules. The integration times corresponded to 3 min and 14 min at count rates of 34 and 16 kHz per APD, respectively. See text for the details of the theoretical fit.

Figure 2

(a) Intensity cross correlation function of photons with the same ZPL frequency exiting the output ports of the beam splitter. Integration time 9 min; count rate 27 kHz per APD. (b) Same as (a) if the ZPL of one molecule is frequency detuned by about 5 GHz (far-off resonant). Integration time 20 min; count rate 33 kHz per APD. (c),(d) Same as (a) if the two emitters are frequency detuned by 200 MHz and 300 MHz, respectively. The red curves are calculations based on Eqn. (2) with $\eta =0.5$ (solid) and $\eta =1$ (dashed), respectively. Integration times were 52 and 16 min, corresponding to count rate of 31 and 42 kHz per APD, respectively.

Figure 3

(a) Theoretical prediction of the $g_{34}^{(2)}$ function when one molecule is broadened by about 120 times the natural linewidth ${\gamma }_0$. (b) Fluorescence excitation spectra of the ZPL for different noise amplitudes applied to the electrode. $\mathrm{FWHM}=30\mathrm{MHz}$ (green), 120 (red), and 300 (blue). (c) Experimental measurement of the two-photon interference where the ZPL of one molecule was artificially broadened. The red solid curve is the prediction of Eq. (2) for $\eta =0.5$ and the green dashed curve displays the data from Fig. 2a. Integration time 7 min; count rate 28 kHz per APD.