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High speed, on demand Single photon source or emitter (SPE) is critical technology for secure communications systems, quantum computers and quantum sensors

The possibility of totally secure communication through the application of quantum mechanical properties was introduced in 1984 by Bennett and Brassard in their paper, “Quantum Cryptography: Public Key Distribution and Coin Tossing”. As Bennett and Brassard outlined,
quantum communication makes it impossible for an eavesdropper to intercept a message without detection by the desired parties. This phenomenon paves the way for a future of indecipherably secure communication, even with the advent of more powerful and even quantum computers. Quantum Key Distribution, or QKD, enables two remote parties, “Alice” and “Bob”, who are connected by a passive optical link to securely generate secret key material. Single-photon sources (SPSs) and single-photon detectors (SPDs) are key devices for enabling practical quantum key distributions (QKDs).


Single photon sources are important for current research in quantum information, cryptography, and the production of truly random numbers. Single photon sources are used in quantum information and cryptography for encoding of information and communication. A
single photon can be detected at long distances away from the source and still contain its information. With the use of single photons, communication can be completely secured as any intercept would not be able to decrypt the message and the communicators would be able to see that the single photon had been intercepted


Other applications to exploit sources of single photons could include quantum computing, where photons would play the role of quantum bits. It has also been shown that the availability of a single-photon source enables implementation of quantum computation using only linear optical elements and photodetectors.


Quantum metrology, meanwhile, would benefit from a true single-photon source because its signal-to-noise ratio would not be restricted by lasers’ “shot-noise” limit (which is equal to the square root of the laser intensity).  They  would greatly support the emergence of applications of quantum technology such as entanglement assisted measurement techniques, i.e. sub-shot noise metrology, microscopy and spectroscopy.


“Without having a source of coherent single photons, you can’t use any of these quantum effects that are the foundation of optical quantum information manipulation,” says Bawendi, who is the Lester Wolfe Professor of Chemistry. Another important quantum effect that can be harnessed by having coherent photons, he says, is entanglement, in which two photons essentially behave as if they were one, sharing all their properties.


“The ability to generate single photons, which form the backbone of technology used in laptops and the internet, will drive the development of local secure communications systems – for safeguarding defence and intelligence networks, the financial security of corporations and governments and bolstering personal electronic privacy, like shopping online,” said Professor Eggleton.

Single photon source or emitter (SPE)  requirements

Single photon generation is necessary for secure quantum transmission; otherwise, an eavesdropping party might intercept one of the transmitted photons and thus get a copy of the message. Moreover, Single-photon sources (SPSs) at Telecom-band are of special interest because the existing telecom backbone networks exhibit a minimal transmission loss around 1.55 μm.


An ideal single-photon source can be described as: “One photon is emitted on demand at a time chosen by the user” with “The emitted photons being indistinguishable from one another in all relevant degrees of freedom: central frequency, bandwidth, spatial mode, and polarization” and having an adjustable repetition rate. Although significant progress has been currently available single-photon sources are still far from being completely predictable and deterministic.


The ability to produce individual photons with precisely known and persistent properties, including a wavelength, or color, that does not fluctuate at all, could be useful for many kinds of proposed quantum devices. Because each photon would be indistinguishable from the others in terms of its quantum-mechanical properties, it could be possible, for example, to delay one of them and then get the pair to interact with each other, in a phenomenon called interference.


“This quantum interference between different indistinguishable single photons is the basis of many optical quantum information technologies using single photons as information carriers,” Utzat explains. “But it only works if the photons are coherent, meaning they preserve their quantum states for a sufficiently long time.


One way to achieve this is to attenuate a laser beam to single-photon level through the use of optical filters.  This attenuated laser beam is a good approximation for single photon emission, but there is always a possibility that bunches of photons will be emitted at one time.


When the energy per pulse is much less than hw (the energy associated with a single photon) most pulses will contain zero photons and a small percentage will contain single photons. However we cannot predict which pulses will contain a photon and a very small percentage of pulses will always contain more than one photon. Despite the fact that most QKD implementations use weak attenuated lasers to simulate SPE, a true SPE is still preferred due to its longer secure distance in theory and therefore its potentially better performance.


Single Photon source (SPE)  generation technology

There are two categories of single photon sources: deterministic and probabilistic.


Deterministic Single Photon Sources

Deterministic single photon sources emit one photon at any arbitrary time determined by the experimenter. These quantum systems include color centers, quantum dots, single atoms, and atomic ensembles. Color centers are formed with a nitrogen atom in an adjacent lattice position in diamond. Quantum dots are created with molecular beam epitaxi where tiny islands of smaller bandgap semiconductors are embedded in a larger bandgap semiconductor. Single atoms are designed for cavity quantum electrodynamics where the emitted single photon enhances the dynamics of the atom-cavity system and the cavity aids the emission of single photons.


Probabilistic Photon Sources

Probabilistic photon sources generate correlated photon pairs via spontaneous nonlinear optical processes, where the first photon is used to herald the creation of the second single photon. In practice, these sources involve a laser excitation of a nonlinear optical material such as parametric down conversion in bulk crystals and four-wave mixing in optical fibers.


Parametric Down conversion

In parametric down conversion, a pump laser is aimed at a material with optical nonlinearity, creating two photons where the conservation of momentum and energy determine the wave vector relation between the two photons. A crystal is pumped by a suitable short wavelength laser, pairs of long wavelength photons are created simultaneously, travelling in correlated directions, and correlated in energy. Detection of one of the photons can be used to gate its partner thus producing a source rich in time tagged single photons. A problem with this source is the randomness in the emission times of the pairs. The process occurs with small probability (10−12 or less) inside a handful of optical materials with nonlinear optical susceptibilities, such as KDP, LiIO3, and BBO.


“However the photon pair generation events are unpredictable (being associated with vacuum fluctuations) and contain contributions from multi-pair events. Indeed the probabilities of single- (P1) and multi-pair (P>1) events are both related to the mean number of pairs created per pump pulse μ. They both increase with μ, and P>1 increases more rapidly (to leading order it grows quadratically rather than linearly). Therefore, these sources are usually operated in the μ<<1 (and thus P1<<1) regime to minimize the multi-photon noise.”


Four Wave Mixing

As a possible alternative, integrated single-photon sources in silicon-on-insulator (SOI) photonic circuits have been shown, based on enhanced four-wave mixing induced by the silicon χ(3)susceptibility and non-deterministic heralding. Four-wave mixing is the process where two uncorrelated photons are converted into two correlated photons in centrosymmetric materials such as glass. This type of single photon emission is not reliable as some false herald due to dark current and stray light will yield the absence of a heralded photon. Since this source is not as dependable as the deterministic photon source, research has neglected this type of single photon emitters.


Four-wave mixing  is a major step toward large-scale quantum technologies

Integrated quantum photonics is a promising platform for developing quantum technologies due to its capacity to generate and control photons—single particles of light—in miniaturized complex optical circuits. Leveraging the mature CMOS Silicon industry for the fabrication of integrated devices enables circuits with the equivalent of thousands of optical fibres and components to be integrated on a single millimetre-scale chip.


“An important challenge that has limited the scaling of integrated quantum photonics has been the lack of on-chip sources able to generate high-quality single photons. Without low-noise photon sources, errors in a quantum computation accumulate rapidly when increasing the circuit complexity, resulting in the computation being no longer reliable. Moreover, optical losses in the sources limit the number of photons the quantum computer can produce and process.


“In this work we found a way to resolve this and in doing so we developed the first integrated photon source compatible with large-scale quantum photonics. To achieve high-quality photons, we developed a novel technique—”inter-modal spontaneous four-wave mixing”—where the multiple modes of light propagating through a Silicon waveguide are non-linearly interfered, creating ideal conditions for generating single photons.”


Together with colleagues at the University of Trento in Italy, the team based at Prof Anthony Laing’s group in Bristol’s Quantum Engineering Technology Labs (QETLabs) benchmarked the use of such sources for photonic quantum computing in a heralded Hong-Ou-Mandel experiment, a building block of optical quantum information processing, and obtained the highest quality on-chip photonic quantum interference ever observed (96% visibility).


Dr. Paesani said: “The device demonstrated by far the best performances for any integrated photon source: spectral purity and indistinguishability of 99% and > 90% photon heralding efficiency.” Importantly, the Silicon photonic device was fabricated via CMOS-compatible processes in a commercial foundry, which means thousands of sources can easily be integrated on a single device. The research, funded by the Engineering and Physical Sciences Research Council (EPSRC) Hub in Quantum Computing and Simulation and the European Research Council (ERC), represents a major step toward building quantum circuits at scale and paves the way for several applications.


“We have solved a critical set of noises that had previously limited the scaling of photonic quantum information processing. For example, arrays of hundreds of these sources can be used to build near-term noisy intermediate-scale quantum (NISQ) photonic machines, where tens of photons can be processed to solve specialised tasks, such as the simulation of molecular dynamics or certain optimisation problems related to graph theory.”


Now researchers have devised how to build near-perfect photon sources, over the next few months the scalability of the Silicon platform will allow them to integrate tens to hundreds on a single chip. Developing circuits at such a scale will make it possible for NISQ photonic quantum machines to solve industrially-relevant problems beyond the capability of current supercomputers. “Furthermore, with advanced optimisation and miniaturisation of the photon source, our technology could lead to fault-tolerant quantum operations in the integrated photonics platform, unleashing the full potential of quantum computers,” said Dr. Paesani.


Spatial Multiplexing

A promising solution is to actively multiplex non-deterministic photons in different spatial or temporal modes to enhance the probability of single-photon output.

Spatial multiplexing has been implemented in a few architectures, but scaling quickly becomes infeasible as the number of photon sources and heralding detectors increases rapidly with the number of modes to be multiplexed. Temporal multiplexing, proposed in refs, reuses the same detectors and photon-generation components, and thus is significantly more resource efficient and scalable. Even if the efficiency of such integrated sources can be improved by spatial multiplexing, compactness and scalability remain open issues.


Photon Blockade mechanism

An alternative route to single-photon generation relies on the photon blockade mechanism, where a strong third-order nonlinearity in an optical resonator enables a shift of the resonant frequency by more than its linewidth when a single photon is already present. As a consequence, the device can absorb a photon only after the previous one has been emitted. However, this mechanism requires a stronger optical nonlinearity than what is achieved in state-of-the-art SOI devices.


Single atom or molecule

The third  single photon source is a single atom or molecule.  Researchers have exploited the naturally quantum mechanical nature of the emission process. An atom excited by an optical pulse much shorter than its lifetime can only emit a single photon. Single-photon sources on-demand can be realized with artificial two-level emitters, such as semiconductor quantum dots or colour centres in diamond, which have increasingly improved their radiative efficiency over the last few years. These systems typically emit single photons nearly on-demand, with a recent demonstration showing that the emitted photons from a single quantum dot can be highly indistinguishable.


The ideal source of single photons is an individual atom. Being a two-level quantum-mechanical system, it emits a single photon when a laser pulse excites a single electron and that electron relaxes down to the ground state. Among the technologies developed to imitate this process is the quantum dot, a tiny piece of semiconductor that can emit single photons by virtue of its atom-like structure of discrete electronic states.

Quantum Dots

Specific advantages of Quantum Dots based Single Photon Sources are Stability, Compatible with chip-technology, wide spectral range, Electrical Pumping, High repetition rate and Strong interactions “available”. However, producing highly indistinguishable photons from distinct emitters remains challenging because of the difficulty of fabricating identical emitters at the nanoscale. The Specific disadvantages of single quantum dots are Low temperature operation, Non-uniformity, Device production yield, Decoherence and low Efficiency.


In July 2019, it was reported that Researchers at the U.S. Naval Research Laboratory (NRL) developed a new technique that squeezes quantum dots, tiny particles made of thousands of atoms, to emit single photons (individual particles of light) with precisely the same color and with positions that can be less than a millionth of a meter apart.


In order for quantum dots to “communicate” (interact), they have to emit light at the same wavelength. The size of a quantum dot determines this emission wavelength. However, just as no two snowflakes are alike, no two quantum dots have exactly the same size and shape—at least when they’re initially created. This natural variability makes it impossible for researchers to create quantum dots that emit light at precisely the same wavelength [color], said NRL physicist Joel Grim, the lead researcher on the project.


“Instead of making quantum dots perfectly identical to begin with, we change their wavelength afterwards by shrink-wrapping them with laser-crystallized hafnium oxide,” Grim said. “The shrink wrap squeezes the quantum dots, which shifts their wavelength in a very controllable way.” While other scientists have demonstrated “tuning” of quantum dot wavelengths in the past, this is the first time researchers have achieved it precisely in both wavelength and position.


Waveguide-coupled quantum dot-photonic crystal cavity system Single-Photon Source

In a waveguide-coupled quantum dot-photonic crystal cavity system, researchers at the University of Sheffield placed a nanocrystal (i.e., a quantum dot) inside a cavity within a larger crystal (i.e., a semiconductor chip). When researchers shined a laser on the quantum dot it absorbed energy, which was emitted in the form of a photon. The laser light bounced around inside the cavity that held the quantum dot, speeding up photon production.


To separate the photons carrying data information from the laser light, researchers funneled the photons away from the cavity and into the semiconductor chip. The Sheffield team’s technique is based on a phenomenon known as the Purcell effect. Researchers demonstrated a photon emission rate about 50 times faster than would be possible without using their technique. Researchers say that, although their approach does not achieve the fastest photon light pulse yet developed, it has an advantage because the photons produced are all identical — an essential quality for many quantum computing applications.


“This technology could be used within secure fiber optic telecoms systems, although it would be most useful initially in environments where security is paramount, including governments and national security headquarters,” Fox said. The difficulty of QKD using true SPE lies on the fact that it is challenging to find a bright, room temperature (RT) SPE working in the telecom range, which is required to minimize the transmission loss in optical fibers. Quantum dots made from indium arsenide could emit single photons at telecom wavelengths (1.3 to 1.5 µm). Unfortunately, these devices need to operate at cryogenic temperatures (4 K), making them too expensive for practical use. Nitrogen-vacancy defects in diamond can also generate single photons at room temperature, but unfortunately only at visible wavelengths.


Quantum dot based High Performance Single Photon Source in China

XSingle photon source is the core resource of optical quantum information technology. Recently, Pan Jianwei , a member of the Chinese Academy of Sciences and a professor at the University of Science and Technology of China, led by Lu Chaoyang and Huo Yongheng, and several domestic and German and Danish scholars, proposed a new theoretical scheme for the first time in the world, in narrowband and broadband,  as reported by Xinhua News Agency, Hefei, August 13 (Reporter Xu Haitao). The single photon source with deterministic polarization, high purity, high isotacticity and high efficiency has been successfully realized on the cavity, which lays an important scientific foundation for the optical quantum computer to surpass the classical computer. The internationally authoritative academic journal “Nature·Photonics” recently published the results and evaluated that it “solved a long-standing challenge.”


The perfect single photon source required by optical quantum information technology must meet the four contradictory and severe conditions of deterministic polarization, high purity, high isotacticity and high efficiency. Since 2000, the University of California, etc. has made progress in the research of single photon source, but its quality can not meet the needs of practical use.


Since 2013, China’s Pan Jianwei, Lu Chaoyang and others have pioneered the quantum dot pulse resonance excitation technology in the world and began to lead the development of high performance single photon sources. But to achieve a perfect single photon source, there are two major technical problems that need to be overcome: one is that the quantum dots will randomly emit two polarized photons, and the other is that the resonant excitation needs to eliminate the background laser.


Recently, the Pan Jianwei team of the Chinese University of Science and Technology has proposed the theoretical solution of deterministic polarization single photons for elliptical microcavity coupling for the first time in the world. They collaborated with the research team of Yu Siyuan of Sun Yat-Sen University, the Daiqing Research Group of the National Nanoscience Center, the Hoflin Research Group of the University of Würzburg, Germany, and the Gregson Research Group of the Danish University of Science and Technology to develop vertical polarization lossless extinction technology. Solved the above two major problems. On this basis, they separately prepared a single photon source that satisfies both deterministic polarization, high purity, high isotacticity and high efficiency in narrow-band microcolumns and broadband target microcavities, again refreshing the single photon source comprehensive performance. The international record is an important step towards achieving the scientific goal of “quantum hegemony” beyond traditional classical computers.


According to reports, this achievement marks China’s continued international leadership in the research of scalable optical quantum information technology. The reviewer of Nature·Photonics commented that this achievement “solves a long-standing challenge” is “a huge step”.


New tunable  single-photon microwave source developed

Circuits which produce single photons are a vital component in quantum computers. They usually consist of a quantum bit or ‘qubit’, coupled to a resonance circuit. The resonant circuit limits the photon output to specific frequencies depending on the design of the circuit. This limitation means that researchers have to rebuild them each time a different frequency is required, which is time and labour intensive.


A team of researchers at NPL, in collaboration with RIKEN in Japan, the Moscow Institute of Physics and Technology and Royal Holloway, University of London, has solved this problem by creating a new device which is tuneable and is able to produce single photons over a wide range of frequencies on demand.


The technology developed by the consortium uses a super-cooled qubit that bridges two open ends of a broken transmission line. One end, through which microwave photons are outputted, is strongly coupled to the qubit. The other end of the transmission line is weakly coupled and is used as the input port to trigger the emission of a single photon from the qubit. An input pulse is used to excite the qubit into a higher energy-state similar to an electron orbiting an atom. After being excited, the qubit immediately relaxes, producing a single photon. The qubit energy can be tuned, thus altering the frequency of the output photons. The demonstrated device has an efficiency of above 80%, which is highly competitive when compared with other sources.


In addition to being a necessary part of prospective quantum computers, single photon sources can be used to shed light on the fundamental interactions between light and matter, which is vital for our understanding of quantum physics and the development of quantum and solid-state technologies. The team is hoping to build on its new single-photon source to develop the field of quantum information even further.

On Demand Single photon generation

But a research team at the University of Sydney has made a major breakthrough in generating single photons – the single quanta of light particles – as carriers of quantum information in security systems.

Photons are generated simultaneously in pairs, each in one of the photon streams. The detection of photons in one stream indicates the timing information of those in the other. Using this information, a proper timing control is dynamically applied to those photons so they appear at regular intervals. This new technique increases the rate of photons at the regular interval, which is extremely useful for quantum secure communication and quantum photonic computation.


The collaboration involving physicists at the Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), an ARC Centre of Excellence headquartered in the School of Physics, and electrical engineers from the School of Electrical and Information Engineering, has been published last night in Nature Communications. Lead author Dr Chunle Xiong, from the School of Physics, said: “Quantum communication and computing are the next generation technologies poised to change the world.”


Among a number of quantum systems, optical systems offer particularly easy access to quantum effects. Over the past few decades, many building blocks for optical quantum information processing have developed quickly,” Dr Xiong said.  “Implementing optical quantum technologies has now come down to one fundamental challenge:  having indistinguishable single photons on-demand,” he said.


“Our demonstration leverages the CUDOS Photonic chip that we have been developing over the last decade, which means this new technology is also compact and can be manufactured with existing infrastructure.”

Qubitekk Licenses Oak Ridge Photon Production Method

Quantum computing and cryptography technology developer Qubitekk Inc. has non-exclusively licensed a method developed by Oak Ridge National Laboratory (ORNL) to produce photons in a controlled, deterministic manner that promises improved speed and security when sharing encrypted data.


The team built upon existing ideas of multiplexing, an approach that uses a series of light source systems comprising components common in fiber-optics. The ORNL system switches the speed and frequency of the heralded photon, carrying out the switching in the frequency domain that potentially reduces single-photon loss.


“The goal is to specify and control every aspect of the photon’s quantum state, constraining everything to a single mode so that the photons emitted from the single-photon source are identical – each one indistinguishable from the next,” said co-inventor Nicholas Peters at ORNL.

The identical photon pairs can be used in developing quantum key encryption technologies that protect information from cyber threats when shared over existing machine-to-machine networks.






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