Abstract
1. Introduction
With the development of quantum computing[
The first single-photon source was created by using an emission cascade in mercury atoms[
Figure 1.(Color online) (a) Solar radiation spectrum from UV to near-infrared light. (b) Photon attenuation in optical fiber as a function of wavelength. Wavelengths at 1300 and 1550 nm are called telecom O-band and C-band, respectively, which are commonly used in fiber-based applications. (a) Adapted with permission from Ref. [
2. Single photon emission
2.1. Concept
An ideal single-photon emission consists of exactly one photon at a time, which can be triggered on-demand, meaning that the user can emit a single photon at will. Furthermore, the emitted photons should be indistinguishable[
Figure 2.(Color online) Scheme of single photon excitation and emission in a two-level system.
The single photon emission should obey sub-Poissonian statistics:
where ∆n is the standard deviation of the photon numbers in a certain time interval, and
However, it is experimentally challenging to meet these properties. Often, there is only a certain probability to get single photon per trigger. In many cases, we get no photon or multi-photons. Therefore, the emission properties need to be precisely characterized with regard to the single-photon character.
2.2. Characterization of single-photon emission
In order to measure whether the light source emits single or multiple photons, the Hanbury Brown and Twiss experiment may be performed[
Figure 3.(Color online) Scheme of Hanbury Brown and Twiss setup for autocorrelation measurement. The beam of light is sent to a beam splitter with single-photon detectors at the two outputs. An electronic correlator then determines the time delay between the two detector signals. Single photon emission results in the absence of simultaneous detection events on D1 and D2, in contrast to the case of multi-photon emission. Reprinted with permission from Ref. [
where
Figure 4.(Color online) Second order correlation function measured under two types of optical excitations. (a) Continuous wave excitation. (b) Pulsed excitation.
3. Telecom wavelength single-photon sources
Compared with single photon sources emitting at the visible wavelength range, few research has been conducted on telecom sources till now. But due to their potentials in application, telecom wavelength single-photon sources are becoming more and more attractive. Table 1 lists different types of such single photon sources.
3.1. Atomic sources
Atomic cascade transitions were the first experimentally realized single photon sources[
By using two lasers to pump the hot rubidium vapour cell, a cascade transition will occur and two single photons will be generated, one with short wavelength at 780 nm and the other within the telecom O-band[
Figure 5.(Color online) (a) Scheme of single photon emission from Rb atoms. Two pumping lasers are needed for Rb excitation (795 and 1324 nm), and two wavelength of single photons will be emitted (780 and 1367 nm). (b) The configuration of Rb energy levels. Reprinted with permission from Ref. [
3.2. Spontaneous parametric down conversion
Till now, the most widely studied and used single-photon emitters are based on spontaneous parametric down conversion (SPDC). It was first experimentally achieved in 1970[
Figure 6.(Color online) (a) Scheme of SPDC process and phase matching. (b) High purity of single photon emission. The dots are experiment data, the blue and red curves are theoretical fitting with and without considering detector noise. The inset is
However, SPDC is not an ‘on-demand’ single-photon source. After the pump laser passes the nonlinear crystal, single photon emission can only be obtained with a certain probability. For different pump powers, either multiple photons will be emitted, or no photons are generated at most of the time. Although post-selection can ensure high single-photon emission
Recently, an ultra-fast single photon source at telecom wavelength has been realized[
Figure 7.(Color online) (a) Scheme of experimental setup of ultra-fast heralded single photon source. (b) Selected signal photons (ITU 50) and idler photons (ITU 43) from the SPDC spectrum (black line). Reprinted with permission from Ref. [
3.3. Semiconductor quantum dots
Another type of well-studied single photon source is epitaxially grown semiconductor QDs. Comparing with SPDC source, QDs can achieve on-demand and therefore bright single photon emission. When a QD is excited, it will emit one single photon at a time, which is very attractive for practical applications. Furthermore, QDs can be electrically triggered[
The general structure of QDs systems is shown in Fig. 8. On the substrate, the QDs layer is sandwiched between two barrier layers. The QDs have smaller band gap so that the excited electron-hole pair can be confined in the QDs. One method to achieve telecom wavelength in the InAs/GaAs system is to grow a thick InGaAs metamorphic buffer layer between GaAs substrate and the InAs QDs[
Figure 8.(Color online) Three types of QDs grown epitaxially on GaAs and InP substrate, respectively. Reprinted with permission from Refs. [
Epitaxial QDs are usually grown by metal-organic vapour phase epitaxy or molecular beam epitaxy. The size of individual QDs is not perfectly identical, resulting in a distribution of emission wavelengths. This is detrimental for experiments requiring multiple sources of single photons, e.g. for quantum repeaters. Identical wavelengths are required to ensure the indistinguishability of photons from separate emitters. There are many ways to tune the wavelength, such as using electric field[
One challenge for single photon sources based on QDs is the extraction efficiency. The employed semiconductors have high refractive indices so that most of the photons are trapped in the material due to total internal reflection. Many structures are studied to enhance the extraction efficiency, such as micro-pillars[
3.4. Defects in silicon carbide
Silicon Carbide (SiC) is a wide band gap semiconductor. There are three major polytypes: 3C-, 4H- and 6H-, with band gaps 2.36, 3.23, 3.05 eV, respectively. For studying the optical and magnetic properties, the defects in SiC are more attractive. The divacancy in 4H-SiC and carbon vacancy-antisite pair in 6H-SiC showed near infrared photon emission[
Recently, the single photon emission in 3C-SiC at telecom wavelength range has been reported[
Figure 9.(Color online) (a) Confocal map of four single photon emitters in 3C SiC epitaxy layer. (b) Room temperature photoluminescence spectra of three representative single photon emitters. (c) Second order autocorrelation measurement of the single photon emission. Reprinted with permission from Ref. [
3.5. Carbon nanotubes
Carbon nanotubes have been widely studied since more than two decades ago due to their extraordinary electrical, mechanical and thermal properties[
In addition, by introducing ether-d and eposide-I groups as the oxygen dopants to SWCNT, a deep trap state will be created below the band-edge excitons (E11 excitons), as shown in Fig. 10[
Figure 10.(Color online) (a) Upper panel: Scheme of single wall carbon nanotube with oxygen-doping (Ether-d and Epoxide groups). Lower panel: Trap energy levels at doped areas. Oxygen-doping creates deep trap states below the
In order to further shift the single-photon emission to telecom C-band, the aryl
Figure 11.(Color online) Photoluminescence spectra of two types of aryl-functionalized carbon nanotubes with different chiralities ((a) (6,5), (b) (7,5), (c) (10,3)) and their corresponding second-order correlation function. Reprinted with permission from Ref. [
4. Applications of single photon sources
Single photon sources have many applications. The most important and practical application for telecom wavelength single photon sources is quantum key distribution thanks for the low transmission loss in optical fiber and free-space. Quantum teleportation is also an important part among quantum technologies. Till now, the main workhorse of single photon source is the SPDC source.
4.1. Quantum key distribution (QKD)
After Bennett and Brassard’s proposal in 1984[
Figure 12.(Color online) Illustration of satellite based QKD among three ground stations (Xinglong, Nanshan and Graz). Reprinted with permission from Ref. [
4.2. Quantum teleportation
Entanglement is a very important mechanism in quantum mechanics. If two photons are entangled, the state of one photon cannot be treated independently from the state of the other photon. Entangled photon pairs can be generated directly by SPDC or cascade transitions in QDs or atoms. They can also be created by performing Bell state measurements on single photons. With entangled photons and single photons, quantum teleportation has been realized.
Quantum teleportation was first experimentally verified by teleporting the polarization of one photon to another photon in the lab[
Quantum teleportation was first experimentally verified by teleporting the polarization of one photon to another photon in the lab[
Figure 13.(Color online) (a) Bird’s-eye view of experiment site in China. (b–d) Illustration of the entangled photon pair generation and distribution from Charlie, single photon state preparation and Bell state measurement from Alice and single photon state reconstruction from Bob. Reprinted with permission from Ref. [
5. Conclusion and outlook
Single photon sources, especially at telecom wavelengths, are important constituents for quantum communication networks. Although SPDC sources have matured and are commonly used, there is a fundamental limit to its brightness. In contrast, semiconductor QDs already show great potential as bright single and entangled photon sources. Although some challenges remain to be solved, such as low extraction efficiency and relatively low photon indistinguishability, it has become the major competitor of SPDC sources. Other telecom wavelength single photon sources are being explored and yet requiring a substantial amount of research to reveal their potential for future applications.
In order to implement telecom wavelength single photon sources in real-world scenarios, electrically driven devices with small footprints are necessary. Promising QD-based photon sources have been already realized[
Acknowledgement
The work was financially supported by the ERC Starting Grant No. 715770 (QD-NOMS) and the National Natural Science Foundation of China (No. 61728501).
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