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In the rapidly advancing realm of quantum technologies, the quest for secure communication systems, powerful quantum computers, and ultra-sensitive quantum sensors is intensifying. At the heart of these cutting-edge applications lies a critical technology: High-speed, on-demand Single Photon Sources or Emitters (SPEs). This article delves into the core concepts of quantum cryptography, shedding light on its workings, applications, and the groundbreaking role played by Single Photon Sources (SPS) in advancing this field.
The Quantum Communication Revolution:
Secure communication has long been a paramount concern, and quantum communication stands as a revolutionary solution. Unlike classical communication systems, quantum communication leverages the principles of quantum mechanics to ensure the absolute security of transmitted information. At the core of this security is the use of single photons to carry quantum information.
Quantum cryptography, often referred to as Quantum Key Distribution (QKD), harnesses the fascinating properties of quantum mechanics to create an unbreakable shield for secure data transmission. QKD employs the quantum nature of photons to generate a shared secret key between remote parties, ensuring absolute security against quantum computer attacks. The process involves two parties, “Alice” and “Bob,” securely generating secret key material over a passive optical link. This key can then be used securely with conventional cryptographic algorithms. The more correct name for quantum cryptography is therefore Quantum Key Distribution.
An intriguing aspect of quantum cryptography is Heisenberg’s Uncertainty Principle, which introduces a high error rate in quantum transmissions when eavesdropping is attempted. This anomaly acts as a built-in alarm system, allowing parties to detect any unauthorized interception.
The Role of Single Photon Sources (SPS)
Single Photon Sources are devices that emit individual photons one at a time. These sources play a foundational role in quantum communication, quantum computing, and quantum sensing applications. The ability to produce single photons on-demand and at high speeds is a game-changer for several reasons.
Key Devices for Quantum Key Distribution:
Quantum key distribution (QKD) employs single or entangled photons to generate a shared secret key between the parties that is perfectly secure even against quantum computer attacks. Single-photon sources (SPSs) and single-photon detectors (SPDs) are key devices for enabling practical quantum key distributions (QKDs).
Single photon sources are instrumental in Quantum Key Distribution, a technique that uses quantum properties to secure communication channels. Single photons, emitted on demand by SPSs, serve as the carriers of quantum information. The use of single photons ensures that intercepted messages cannot be decrypted, guaranteeing the security of communication. High-speed, on-demand SPEs enhance the efficiency and reliability of QKD protocols. This holds immense potential for various applications beyond cryptography, and the production of truly random numbers.
Applications Beyond Cryptography:
Beyond secure communication, SPSs find application in quantum computing, where photons can act as quantum bits (qubits). The availability of coherent single photons is fundamental for optical quantum information manipulation and enables phenomena like entanglement.
In quantum computers, qubits—the fundamental units of quantum information—are often encoded using single photons. High-speed SPEs contribute to the rapid processing capabilities required for quantum computations.
Quantum Sensing: Quantum sensors, which exploit quantum principles for unprecedented sensitivity, benefit from high-speed SPEs. 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. These sensors find applications in fields such as precision measurements, imaging, and environmental monitoring.
Benefits of High-Speed SPEs:
“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.
- Enhanced Data Rates:
- High-speed SPEs enable faster quantum communication protocols, resulting in increased data transmission rates for secure communication systems.
- Scalability in Quantum Computing:
- Quantum computers with scalable architectures require efficient single photon sources for the simultaneous manipulation of multiple qubits. High-speed SPEs contribute to the scalability of quantum computing systems.
- Real-Time Sensing:
- Quantum sensors equipped with high-speed SPEs provide real-time data acquisition, improving the responsiveness and accuracy of quantum sensing applications.
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.
Categories of Single Photon Sources:
Single photon sources (SPS) are classified into two categories: deterministic and probabilistic. Deterministic SPS emit photons at specific, experimentally determined times. Examples of deterministic SPS include color centers, quantum dots, single atoms, and atomic ensembles. Color centers involve a nitrogen atom in a diamond lattice, quantum dots are formed through molecular beam epitaxy, and single atoms are designed for cavity quantum electrodynamics. In this category, the experimenter controls when a photon is emitted, enhancing the dynamics of the system.
Deterministic Single Photon Sources: Deterministic SPS, such as color centers, quantum dots, single atoms, and atomic ensembles, enable precise control over photon emission times. Color centers involve a nitrogen atom in diamond, quantum dots are created using molecular beam epitaxy, and single atoms are designed for cavity quantum electrodynamics. In this category, the experimenter has control over when a photon is emitted, influencing the system dynamics and aiding photon emission.
Probabilistic Photon Sources: Probabilistic photon sources generate correlated photon pairs through spontaneous nonlinear optical processes. The first photon “heralds” the creation of the second single photon. Examples include parametric down conversion and four-wave mixing. Parametric down conversion involves a pump laser aimed at a nonlinear optical material, creating correlated photon pairs. Detection of one photon gates its partner, producing a time-tagged single-photon source. Four-wave mixing, although less reliable, is an alternative based on enhanced four-wave mixing induced by silicon susceptibility in photonic circuits.
Parametric Down Conversion: Parametric down conversion involves a pump laser directed at a nonlinear optical material, creating correlated photon pairs with determined wave vector relations. The process occurs with small probability in materials like KDP, LiIO3, and BBO. The detection of one photon is used to gate its partner, creating a source rich in time-tagged single photons. However, randomness in emission times and contributions from multi-pair events are challenges, and these sources are operated in a regime with low mean pairs per pump pulse to minimize multi-photon noise.
Four-Wave Mixing as an Alternative: Integrated single-photon sources in silicon-on-insulator photonic circuits have been explored as an alternative, based on enhanced four-wave mixing. This process converts two uncorrelated photons into two correlated photons in centrosymmetric materials like glass. However, the reliability of this source is limited due to false heralding from dark current and stray light, making it less dependable than deterministic photon sources. Research emphasis has shifted toward more reliable sources like deterministic SPS.
spatial multiplexing : An innovative approach to address the challenges associated with non-deterministic photon sources involves the active multiplexing of photons in distinct spatial or temporal modes, ultimately amplifying the probability of achieving a single-photon output. While spatial multiplexing has been successfully implemented in certain architectures, its scalability diminishes rapidly as the number of photon sources and heralding detectors grows proportionally with the multiplexed modes. In contrast, temporal multiplexing, as proposed in literature references, offers a more resource-efficient and scalable solution by reusing the same detectors and photon-generation components. Although the efficiency of integrated sources can potentially benefit from spatial multiplexing enhancements, compactness and scalability persist as open challenges demanding further exploration.
The photon blockade mechanism offers an alternative avenue for generating single photons, relying on a strong third-order nonlinearity within an optical resonator. This mechanism involves a shift in the resonant frequency by more than its linewidth when a single photon is present, allowing the device to absorb a photon only after the previous one has been emitted. However, the implementation of this mechanism requires a higher optical nonlinearity than what is currently achieved in state-of-the-art Silicon-On-Insulator (SOI) devices.
Another approach to single-photon generation involves utilizing a single atom or molecule. Researchers have harnessed the inherently quantum mechanical nature of the emission process, demonstrating that an atom excited by an optical pulse shorter than its lifetime can emit only a single photon. On-demand single-photon sources can be realized with artificial two-level emitters, such as semiconductor quantum dots or color centers in diamond, which have shown improved radiative efficiency over recent years, emitting photons nearly on-demand with high indistinguishability.
Overcoming Challenges: Towards Perfect Single Photon Sources
Developing high-speed, on-demand SPEs poses technical challenges, including maintaining quantum coherence and achieving high photon generation rates. Researchers and engineers are actively addressing these challenges, leading to remarkable advancements in SPE technology.
Ideal Characteristics: An ideal SPS emits a single photon at a time chosen by the user, with indistinguishable properties in central frequency, bandwidth, spatial mode, and polarization. Achieving this perfection remains a challenge, but recent progress has been promising.
Advancements in Technology:
Recent advancements include integrated sources in silicon-on-insulator (SOI) circuits, leveraging the Purcell effect, and breakthroughs in squeezing quantum dots to emit identical photons. These innovations address challenges related to predictability, efficiency, and scalability.
Realizing the Quantum Advantage: The recent theoretical breakthrough in achieving deterministic polarization single photons represents a significant leap in SPS technology. Overcoming challenges related to random emissions and background laser elimination, this achievement propels optical quantum information technology towards new horizons.
International Leadership: Led by researchers like Pan Jianwei, the Chinese Academy of Sciences has pioneered high-performance SPSs, achieving deterministic polarization, high purity, high isotacticity, and high efficiency. This breakthrough cements China’s international leadership in scalable optical quantum information technology.
Researchers at the University of Sheffield have developed a waveguide-coupled quantum dot-photonic crystal cavity system for a single-photon source. This system involves placing a quantum dot inside a cavity within a larger semiconductor crystal. When a laser is shone on the quantum dot, it absorbs energy and emits photons, with the laser light bouncing around the cavity, speeding up photon production. To separate data-carrying photons from the laser light, the photons are funneled away from the cavity into the semiconductor chip. The technique, based on the Purcell effect, demonstrated a photon emission rate about 50 times faster than conventional methods, with the advantage of producing identical photons, crucial for many quantum computing applications. The technology could find applications in secure fiber optic telecom systems and environments where security is paramount.
EPFL’s Room Temperature Single Photon Emitters: Researchers at École Polytechnique Fédérale de Lausanne (EPFL) have developed Single Photon Emitters (SPEs) that operate at room temperature. Utilizing gallium nitride and aluminum nitride (GaN/AlN) quantum dots on cost-effective silicon substrates, these SPEs exhibit a single-photon purity of 95% at cryogenic temperatures and 83% at room temperature. The GaN/AlN quantum dots demonstrate excellent photon emission rates up to 1 MHz, making them highly promising for quantum applications.
NPL’s Tunable Single-Photon Microwave Source: A team of researchers, including those from the National Physical Laboratory (NPL) in collaboration with RIKEN, Moscow Institute of Physics and Technology, and Royal Holloway, has developed a tunable single-photon microwave source. The traditional circuits that produce single photons are coupled to a resonance circuit, limiting photon output to specific frequencies, requiring rebuilding for different frequencies. The new technology uses a super-cooled qubit bridging two open ends of a broken transmission line. By tuning the qubit energy, the frequency of the output photons can be altered.
This device, using a super-cooled qubit, can produce single photons over a wide range of frequencies on demand. The technology addresses limitations in existing circuits, allowing researchers to efficiently produce single photons without the need for rebuilding circuits for different frequencies. The device demonstrated an efficiency of over 80%, making it competitive with other sources.
University of Sydney’s Breakthrough in Single Photon Generation: The University of Sydney’s research team has achieved a significant breakthrough in generating on-demand single photons, fundamental to quantum information in security systems. In this innovative approach, photons are simultaneously generated in pairs within distinct streams, and the detection of photons in one stream provides timing information for the other. This timing data is then dynamically used to apply precise timing control, resulting in the appearance of photons at regular intervals. This technique enhances the rate of photons at these intervals, offering valuable applications in quantum secure communication and quantum photonic computation. The collaborative effort, involving physicists at the Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS) and electrical engineers from the School of Electrical and Information Engineering, was recently published in Nature Communications. Lead author Dr. Chunle Xiong emphasized the transformative potential of quantum communication and computing, citing the critical challenge of achieving on-demand, indistinguishable single photons. The technology leverages the CUDOS Photonic chip, providing a compact and manufacturable solution with existing infrastructure.
Qubitekk Licenses Oak Ridge Photon Production Method: Qubitekk Inc. has non-exclusively licensed Oak Ridge National Laboratory’s method for producing photons in a controlled, deterministic manner. This method promises improved speed and security for sharing encrypted data in quantum computing and cryptography. By leveraging multiplexing and controlling the quantum state of photons, the technology aims to produce identical photon pairs crucial for developing quantum key encryption technologies, enhancing protection against cyber threats.
In China, researchers led by Pan Jianwei at the University of Science and Technology of China have proposed a new theoretical scheme for a high-performance single-photon source. The team achieved deterministic polarization, high purity, high isotacticity, and high efficiency on a cavity, marking a significant advancement in optical quantum information technology. The breakthrough overcomes previous challenges in creating a perfect single-photon source and lays a foundation for optical quantum computers to surpass classical computers. The development is recognized for its importance in achieving “quantum hegemony” beyond traditional classical computers.
Advancements in Integrated Quantum Photonics: Integrated quantum photonics holds promise for quantum technology, offering the ability to generate and control single photons in miniaturized optical circuits. The challenge has been the lack of on-chip sources capable of producing high-quality single photons, crucial for reliable quantum computation. A team at Bristol’s Quantum Engineering Technology Labs addressed this challenge, developing the first integrated photon source compatible with large-scale quantum photonics. They employed a novel technique, “inter-modal spontaneous four-wave mixing,” creating optimal conditions for generating single photons.
Overcoming Limitations: The researchers, in collaboration with the University of Trento in Italy, conducted a heralded Hong-Ou-Mandel experiment, a key component of optical quantum information processing. The integrated photon source demonstrated exceptional performance with 96% visibility—the highest on-chip photonic quantum interference observed. The Silicon photonic device, fabricated via CMOS-compatible processes in a commercial foundry, can be easily integrated on a single chip, paving the way for scalable quantum circuits.
Scalability and Applications: The breakthrough addresses critical noise limitations in photonic quantum information processing, enabling the scaling of quantum circuits. The Silicon platform’s scalability allows the integration of tens to hundreds of sources on a single chip. This development opens avenues for near-term noisy intermediate-scale quantum (NISQ) photonic machines, capable of solving specialized tasks like molecular dynamics simulation or optimization problems related to graph theory. The research, funded by EPSRC and ERC, marks a significant step toward building scalable quantum circuits with diverse applications.
Quantum Dots
Quantum dots, tiny semiconductor structures mimicking the behavior of individual atoms, are considered ideal single-photon sources due to their stability, compatibility with chip technology, wide spectral range, electrical pumping capability, high repetition rate, and strong interactions. However, challenges persist, including the difficulty of fabricating identical emitters at the nanoscale, resulting in issues such as low temperature operation, non-uniformity, low device production yield, decoherence, and reduced efficiency.
Addressing the challenge of creating identical quantum dots, researchers at the U.S. Naval Research Laboratory developed a technique in July 2019. By shrink-wrapping quantum dots with laser-crystallized hafnium oxide after their creation, the researchers could precisely tune the wavelength of the quantum dots. This innovative method allowed for the emission of single photons with the same color and positioned less than a millionth of a meter apart, marking a significant advancement in the quest for uniform and controllable quantum dot emissions.
Qubitekk Inc., a developer in quantum computing and cryptography, has secured a non-exclusive license for a photon production method developed by Oak Ridge National Laboratory (ORNL). This method enhances the control and determinism of photon production, promising improved speed and security for sharing encrypted data.
The ORNL system, based on multiplexing principles, focuses on specifying and controlling every aspect of a photon’s quantum state, ensuring that emitted photons from a single-photon source are indistinguishable. 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.
This capability holds significant potential for applications in quantum key encryption technologies, enhancing information protection against cyber threats in machine-to-machine networks.
These developments collectively signify significant progress in the field of quantum technologies, ranging from room temperature operation of single photon emitters to tunable microwave sources and breakthroughs in generating single photons on demand. Each innovation contributes to the advancement of quantum information processing, quantum communication, and the development of secure quantum systems.
Towards a Quantum-Enabled Future:
As the demand for secure communication, powerful computing, and precise sensing grows, the role of high-speed, on-demand Single Photon Sources becomes increasingly pivotal.
With the achievement of near-perfect photon sources, researchers anticipate integrating tens to hundreds of sources on a single chip in the coming months. This scalability enables NISQ photonic quantum machines to address industrially relevant problems beyond current supercomputing capabilities. Furthermore, by optimizing and miniaturizing the photon source, the technology could lead to fault-tolerant quantum operations in the integrated photonics platform, unlocking the full potential of quantum computers.
Researchers and industry leaders are collaboratively driving innovations in SPE technology, propelling us towards a future where quantum-enabled systems redefine the boundaries of what is possible.
Conclusion:
In the quantum landscape, where information security and computational power are paramount, High-speed Single Photon Sources emerge as the unsung heroes. Their contribution to secure communications, quantum computing, and quantum sensing positions them at the forefront of quantum technology advancements. As we harness the potential of these tiny emissaries of light, the quantum-enabled future awaits, promising a transformative era in communication, computation, and sensing.
References and Resources also include:
https://phys.org/news/2020-05-photon-discovery-major-large-scale-quantum.html
https://phys.org/news/2022-05-photon-emitter-closer-quantum-tech.html
