Research

Efficient room-temperature source
of single photons with definite
polarization for quantum information

Supported by: ARO, NSF and Photonic Technology Access Program (PTAP).

A single-photon source (SPS) that efficiently produces photons with antibunching characteristics is one of the pivotal hardware elements for quantum communication technology. For single photons, the second order correlation function g⁽²⁾(t) is equal to 0 at an interphoton time t = 0. Using a SPS, secure quantum communication will prevent any potential eavesdropper from intercepting a message without the receiver noticing. In another implementation, a SPS becomes the key hardware element for quantum computers with linear optical elements and photodetectors.

In difference with photons exhibiting antibunching, attenuated light contains not only single (separated in time) photons but also pairs and triplets which destroy the security of quantum cryptography system. Because of the difficulties in developing robust and efficient sources of single photons, many of the quantum communication systems which are currently on the market, are using the ordinary, low-efficiency photon sources attenuated to the single-photon level contaminated with photon pairs and triplets.

The critical issue in producing single photons by a method other than by trivial attenuation of a beam is the very low concentration of photon emitters dispersed in a host, such that, within an excitation-laser focal spot, only one emitter becomes excited and which will emit only one photon at a time, because of its finite fluorescence lifetime (see Figure).

In an ideal, efficient SPS, each exciting laser pulse will provide a single photon. A desirable feature for a SPS is definite photon polarization, since in the case of single photons with definite polarization, the quantum cryptography system's efficiency will be twice that of an unpolarized SPS.

Our current, room-temperature solution is based on a new material concept using single-emitter excitation in specially prepared hosts based on liquid crystal photonic bandgap materials. Planar-aligned cholesteric liquid-crystal photonic bandgap microcavities possess unique properties:

• tunability;
• possibility of definite polarization (both linear and circular) of single photons;
• possibility to protect emitter from fast bleaching;
• there are no losses into the waveguide modes because the refractive index varies gradually rather than abruptly in chiral structures.

We received a US Patent for such a source of single photons. View pdf

As single emitters in a liquid crystal host we are using colloidal semiconductor quantum dots and single dye molecules. We recently started to work with NV-single color centers in nanodiamonds dispersed in a liquid crystal host. Many other single emitters can be dispersed/dissolved in liquid crystal hosts.

For sample preparation and details on cholesteric liquid crystal photonic bandgap structures and oriented nematic layers both in monomeric (fluid-like) and glassy liquid crystal oligomers see the Chiral and other photonic crystals page.

We have two single-photon source setups based on two confocal fluorescence microscopes. The first one is based on Witec Alpha SNOM device with cw excitation (532 nm and 635 nm). The second is a home-built confocal microscope with several output ports. We excite our samples with 76 MHz repetition-rate, 6 ps pulse duration, 532-nm light from a Lynx mode-locked laser (Time-Bandwidth Products Inc.).

The following diagnostics are placed in the separate output ports of a home-built micro-scope:

• A Hanbury Brown Twiss interferometer consisting of a 50/50 beamsplitter and two cooled, Si single photon counting avalanche photodiode modules (APDs) SPCM AQR-14 (Perkin Elmer). The time interval between two consecutively detected photons in separate arms is measured by a TimeHarp 200 time correlated single-photon counting card using a conventional start-stop protocol.
• Electron multiplying, cooled CCD-camera iXon DV 887 ECS-BV (Andor Technologies).
• Spectrometer (Princeton Instruments Acton SP2150 with EM-CCD camera or Ocean Optics).
• Fiber coupled GaAs single-photon counting detector for 1.55 μm wavelength (Princeton Lightwave).

Here are some of our recent results:

• First demonstration of emitter fluorescence antibunching in liquid crystal hosts [1, 2]. Figure below, left shows CdSeTe quantum dot fluorescence antibunching doped in a photonic bandgap liquid crystal host with the selective transmission curve (a) of right Figure below [3].

• Observation, for the first time, of circular polarized single photons with definite handedness from single fluorescence emitters [3]. Photonic bandgap microcavity chiral structure provides right-handed circular polarization (RHP) of single photons from CdSe quantum dots (Figure at the right shows a fluorescence spectrum for two polarization components with different handedness).
• Observation, for the first time at room temperature, of single photons with definite linear polarization [4,5]. Definite orientation of single molecules of fluorescent DiI dye doped in glassy nematic liquid crystal has been made by planar alignment of nematic liquid crystal molecules (left Figure below). Center Figure shows single molecule fluorescence images (10 ?m x 10 ?m raster scan in confocal microscope) for a polarization component perpendicular to the liquid crystal alignment direction, right Figure shows the same image for a "parallel" polarization component. Molecular dipoles of DiI dye orient perpendicular to the direction of liquid crystal molecules.

• Significant reducing of terrylene dye bleaching (several single molecules) in a monomeric liquid crystal host by oxygen depletion using saturation of liquid crystal by helium (red curve) [1, 2]. Black curve shows fluorescence intensity drop of several single molecules in the same material and cw laser intensity without oxygen depletion. (Some emitters are bleached by laser radiation in oxygen environment).
• Fluorescence antibunching at 850 nm of PbSe colloidal quantum dots (left Figure). We also imaged PbSe colloidal quantum dots (850 nm) in a photonic bandgap cholesteric liquid crystal host (center Figure) [3]. We are working on fluorescence imaging of the same emitters at 1.55 ?m wavelength in photonic bandgap hosts using a Princeton Lightwave GaAs single-photon counting detector (right Figure shows a photograph of this setup).

• Fluorescence antibunching of NV-color centers in nanodiamonds (left Figure). Confocal fluorescence image of several single NV color centers in agglomerate free ~ 25 nm size nanodiamonds is shown in right Figure.

References:

1. S.G. Lukishova, A.W. Schmid, A. J. McNamara, R.W. Boyd, and C.R. Stroud, "Room temperature single photon source: single dye molecule fluorescence in liquid crystal host", IEEE J. of Selected Topics in Quantum Electronics, Special issue on Quantum Internet Technologies, Vol. 9, No 6, pp.1512-1518, 2003. View pdf
2. S.G. Lukishova, A.W. Schmid, Ch. M. Supranowitz, N. Lippa, A. J. McNamara, R.W. Boyd, C.R. Stroud, Jr., "Dye-doped cholesteric-liquid-crystal room-temperature single photon source, J. of Modern Optics, Special Issue on Single Photon: Detectors, Applications and Measurements Methods, Vol. 51, No 9-10, pp.1535-1547, 2004. View pdf
3. S.G. Lukishova, L. J. Bissell, V.M. Menon, N. Valappil, M.A. Hahn, C.M. Evans, B, Zimmerman, T.D. Krauss, C. R. Stroud, Jr., R.W. Boyd, "Organic photonic bandgap microcavities doped with semiconductor nanocrystals for room-temperature single photon sources on demand", J. Modern Optics, Special Issue on Single Photon, will be published in Feb-March 2009. View pdf
4. S.G. Lukishova, A.W. Schmid, R. Knox, P. Freivald, L. Bissell, R.W. Boyd, C.R. Stroud, Jr, K.L. Marshall, "Room-temperature source of single photons of definite polarization", J. Modern Optics, Special Issue on Single Photon: Sources, Detectors, Applications and Measurement Methods, Vol. 54, iss. 2 & 3, pp. 417-429, 2007. View pdf
5. S.G. Lukishova, A.W. Schmid, R.P. Knox, P. Freivald, A. McNamara, R.W. Boyd, C.R. Stroud, Jr., K.L. Marshall, "Single-photon source for quantum information based on single dye molecule fluorescence in liquid crystal host", Molec. Cryst. Liq. Cryst., Vol. 454, pp. 403-416, 2006. View pdf
6. S.G. Lukishova, L.J. Bissell, J. Winkler, C.R. Stroud, "Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host", Optics Letters, Vol. 37(7), 1259-1261 (2012).