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-temperature source
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
 I (t ) I (t  t' ) 
g(2)(t’) =
(1)
 I (t )  2
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-downconversion, 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
2
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], singlemolecule [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, threedimensional 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).
Fig. 1. Schematic diagram of singlephoton device (top-left), scanningelectron microscope image of pillar
structures (bottom), and optical excitation
scheme (top-right) - from Ref. 32.
Fig. 2. Dependence of singlequantum dot photo-luminescence on
excitation power : lower curve – for
bulk semiconductor; two other curves
– for pillar cavities (from Ref. 33).
Fig. 3. Parcell factor versus
detuning for pillar microcavity
(from Ref. 32 ).
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].
3
The cavity fluorescence lifetime of heterostructured quantum dot was sharply reduced to
210 ps, roughly eight times below the average bulk value (Figure 5).
Fig. 4. 2D-photonic crystal cavity for
heterostructured semiconductor quantum dot:
left – electric field intensity ; right – scanning
electron microscopy image of a fabricated
structure [ 29].
Fig. 5. Resonant line of Fig. 4 cavity shows lifetime
shortening to 210 ps [29].
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.
Fig. 6. Scanning electron micrograph showing the
photonic crystal cavity for PbS semiconductor
nanocrystals [47].
Fig. 7. Cavity resonance mapped out by PbS
semiconductor nanocrystal in PMMA [47].
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
4
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).
Fig. 8. Single-molecule fluorescence spectra (grey
shaded area) observed for dye molecules enclosed
between the mirrors of a microcavity with Q-factors of
15 and 45 and in free space as reference [48].
Fig. 9. Measured cavity-controlled fluorescence decay
rates for 57 molecules at different cavity lengths. The
solid curves show the theoretical values calculated for
parallel (θ = 0o), tilted (θ = 40o) and perpendicular
orientation (θ = 90o) of the transition dipole moment
with respect to the cavity mirrors at position z=0 [48].
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 singlephoton 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 pterphenyl 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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