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Sources of single photons for quantum information

Single photon sources (SPSs) can be used for quantum communication and photonic based quantum computing. The long-term goal is to provide a room-temperaturesource of deterministically polarized single photons at optical communication wavelengths 1.3 and 1.55 µm, triggered on demand with a one-GHz repetition rate (single-photon “gun”).

Need for Single-Photon Sources

Photon-based quantum cryptography, communication and computation schemes are critically dependent on the development of light sources that produce individual photons on demand [1-6]. For single photons, the second order correlation function of an optical field

g(2)(t’) = (1)

that characterizes the difference between a single-photon source and an ordinary laser source should have a minimum at time t’ =0 (in an ideal case g(2)(0) = 0), indicating the absence of pairs, i.e., antibunching [7]. Here I is intensity, t is time.

In quantum communication, using SPS prevents an eavesdropper from being allowed to intercept, without the sender/receiver’s knowledge, a message with secret encryption key. Any e-mail message, telephone call, credit card information and other financial transaction will be safe. They will be protected by the Heisenberg uncertainty principle: if you try to measure the behavior of a quantum particle, you alter it in such a way that your measurements isn’t completely accurate. This means if you send the encryption key using a sequence of single photons, no one can intercept them without your knowledge. In another implementation, a SPS becomes the key hardware element for quantum computers with linear optical elements and photodetectors [8]. Again, its practical realization is held back in part by difficulties in developing robust and efficient sources of antibunched photons on demand. In spite of several solutions for SPSs presented in the literature, significant drawbacks remain. They are the reason for current quantum communication systems being baud-rate bottlenecked so that photon numbers from ordinary photon sources may be attenuated to the single-photon level (~0.1 photon per pulse on average) [1, 9]. For instance, such a highly attenuated laser source is currently being applied to a cryptography system using 67-km long, Swisscom telecommunication fiber link under Lake Geneva in Switzerland [9]. In addition to the low efficiency, the drawback of such faint-pulse quantum cryptography is pollution by multiple photons. The pollution restriction does not vanish in quantum cryptography based on parametric-down-conversion, entangled-photon pairs. A parametric-down-conversion photon source may contain a coherent superposition of multiple pairs.

An efficient (with an order of magnitude higher photon number per pulse) and reliable light source that delivers a train of pulses containing one and only one photon is a very timely challenge. To meet this challenge, several issues need addressing, from achieving full control of the quantum properties of the source to easy handling and integrability of these properties in a practical quantum computer and/or communication setup. In addition, in quantum information systems it is desirable to deal with single photons synchronized to an external clock, namely, triggerable single photons. Polarization states of single photons are also important as they enable polarization-qubit encoding of information.

Single Photon Sources/Spontaneous Emission Enhancement - State of the Art

Various methods are known for the production of single photons at definite time intervals, for instance, based on a single atom [10,11], a single trapped ion [12], single-molecule [13-15], single color center in diamond [16], Coulomb blockage effect in a micro-pin junction with a quantum well as the active layer [17-19]. Tremendous progress has been made in realization of single-photon sources based on excitonic emission from single heterostructured semiconductor quantum dots excited by pulsed laser light [20-33]. In this SPS, microcavities have been used for spontaneous emission enhancement, from whispering-gallery-mode resonator (turnstile device), 1-D photonic band-gap, three-dimensional pillar-microcavity and 2-D photonic crystals. For instance, using pillar microcavity (see Figure 1 [32]), the photoluminescence intensity of quantum dots yielded an enhancement factor of forty in comparison with photoluminescence intensity of quantum dots in bulk semiconductors (Fig. 2 from Ref. 33).

A Purcell factor F equals γ/γo ~ Q/V. Here γ and γo are the spontaneous emission rates (γ = 1/τ, where τ is the spontaneous emission lifetime) at the cavity resonant wavelength,

and in free space respectively, Q is a cavity quality factor, V is the mode volume. Near six-fold fluorescence-lifetime diminishing in a resonant cavity was obtained in Ref. [32] (Figure 3).

Further improvement in efficiency of the heterostructured quantum-dot SPS was achieved by employing better microcavities, with larger Q/V ratios, and consequently, stronger emitter-cavity coupling [29]. For example, photonic crystal microcavities shown in Figure 4 have an order of magnitude higher Q/V ratios than the best microposts [29]. The cavity fluorescence lifetime of heterostructured quantum dot was sharply reduced to 210 ps, roughly eight times below the average bulk value (Figure 5).

A weakness of heterostructured-quantum-dot SPSs is their operation only at liquid He temperatures. In addition, they are not readily tunable. To date, three approaches have been suggested for room-temperature single-photon sources: single molecules [13-15, 34-40], colloidal semiconductor quantum dots (nanocrystals) [41] and color centers in diamond [16, 42-46]. The color center source suffers from the problem that it is not easy to couple out the photons, and that the spectral spread of the light is typically quite large (~120 nm). Both single molecules and colloidal quantum dots dissolved in a proper solvent can be embedded in photonic bandgap materials to circumvent the deficiencies that plague the other system.

For, instance, in Reference 47, PbS colloidal quantum dots (nanocrystals) dissolved in PMMA were placed inside a 2-D photonic crystal cavity (Fig. 6). The dot emission at room temperature mapped out the cavity resonances and was enhanced relative to the bulk emission by a maximal Parcel factor of 30 (Fig. 7).

Planar cavity was used recently to control single-dye molecule fluorescence spectra (Fig. 8) and decay rate of single-molecules interacting at room temperature with the first longitudinal mode of a Fabry-Perot microcavity [48]. The spacing between two silver mirrors was ~ λ/2. The spontaneous emission rate of individual dye molecules was found to be enhanced by the Purcell effect by up to three times the free space value (Fig. 9).

The main problems of using fluorescence emitters in cavities as in Refs. 47, 48 are emitters’ bleaching and blinking, nontunability of the source and nondeterministic polarization of photons.

In Section 2 we will describe our results of avoiding emitter bleaching during more than one hour of cw-excitation using special host treatment [38-40], deterministic polarization state of emitted single photons [49-50] and suggestions on preparation of tunable single-photon source [49].

As to avoiding dye bleaching, the first impressive experiments in this field have been reported in Refs 14 and 35 as well. In Ref. 14, single terrylene-dye molecules in a p-terphenyl molecular crystal host did not bleach during several hours of pulsed, several MHz pulse repetition rate excitation. After sublimation procedure, p-terphenyl host protected dye molecules from oxygen. Similar experiments of reducing dye bleaching were also performed in Ref. 49, using a nitrogen stream during the excitation.

Single atoms coupled to high-finesse cavities have achieved very impressive couplings of 40-70%, but they have other formidable problems [11]. The main disadvantage is their complexity of operation, because of isolation, manipulation and trapping of single atoms requires sophisticated and expensive setups, including high-resolution stabilized lasers at several frequencies, and ultra-high vacuum.

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