Photonics, the study and development of devices that use photons, has significantly shaped today’s technological landscape. Just as J.J. Thomson’s discovery of the electron laid the foundation for electronics and, eventually, modern computing, the exploration of photons has opened up a comparable path of innovation and practical applications.
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The theoretical foundation for photonics was laid when Max Planck formulated the quantization of energy, defining them as finite packets. Subsequently in 1905, Albert Einstein’s explanation of the photoelectric effect introduced the concept of photons. Several decades of theoretical developments and experimental verifications ultimately led to the invention of the maser and laser between 1958 and 19601. The development of these coherent light sources marked the formal birth of photonics in the 1960s.
Fundamentals of Photonics
At its core, photonics is the study and manipulation of photons, the fundamental building block of all electromagnetic radiation2. These elementary light particles are the quantized carriers of energy and momentum, spanning the full electromagnetic spectrum from high-energy gamma rays to low-frequency radio waves.
Within an atom, photons are emitted when an electron transitions from a higher energy level to a lower one, releasing energy in the form of a discrete packet. Because photons are massless and electrically neutral, they travel at the speed of light in a vacuum.
With the invention and commercialization of the laser and subsequently laser diodes, the term “photonics” gained popularity in the 1980s as fiber optic communication became widespread1. Coherent laser beams play a critical role in modern telecommunications. Laser pulses are used to encode binary data, representing "1"s and "0"s, which are transmitted through optical fibers. At the source, lasers or LEDs convert electrical signals into optical pulses. On the receiving end, photodetectors translate these optical signals back into electrical form, enabling the high-speed data transmission that constitutes the basis of today’s digital communication.
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Quantum Nature of Photons
A fundamental principle in quantum mechanics, wave-particle duality, shows that photons exhibit both particle-like and wave-like behavior. This dual nature is key to understanding how light interacts with matter at the quantum level.
The photon is classified as a bosonic particle, carrying an integer spin of 1. This quantized intrinsic angular momentum is closely tied to polarization, a fundamental property in optics. As photons travel through space, they exhibit two transverse circular polarizations, meaning the electric field of circularly polarized light rotates in a circle around the direction of propagation. The polarization of photons was experimentally confirmed in 1931 by C.V. Raman, marking a key moment in deepening our understanding of light and its quantum characteristics.
What Is Quantum Photonics?
Quantum photonics is a field that applies the principles of quantum mechanics to light to build advanced technologies2,3. It is an intersection of quantum optics and photonics, focusing on the generation, manipulation, and detection of individual photons and their unique quantum properties. Unlike classical photonics, which deals with large numbers of photons, quantum photonics works at the single-photon level, exploiting phenomena like entanglement, superposition, and non-locality to create groundbreaking technologies such as quantum computers, secure communication networks, and highly sensitive quantum sensors.
Central to quantum photonics is the ability to create entangled photons, whose quantum states are linked regardless of the distance between them. The procedure known as spontaneous parametric down-conversion (SPDC) is usually used to accomplish this4,5. SPDC method involves directing a single high-energy photon into a non-linear optical crystal, which causes it to split into two lower-energy photons. The resultant pair of photons are entangled to exhibit essential quantum correlations, including polarization and momentum. This guarantees that the state of a remote partner is immediately affected by a measurement on one photon.
In addition to producing and entangling photons, integrated photonics is essential for handling quantum information. Qubits encoded in photons are transported and manipulated by devices like waveguides, modulators, detectors, filters, amplifiers, and beamsplitters. This integrated approach allows for the miniaturization and scalability of quantum systems, moving them from complex laboratory setups to more practical, real-world applications.
Applications of Quantum Photonics
The unique capabilities of quantum photonics enable a new class of technologies across multiple sectors. One of the most significant applications is in quantum computing, where entangled photons are used to carry out computations based on quantum algorithms. By leveraging the principles of superposition and entanglement, quantum computers have the potential to solve complex problems that are intractable for even the most powerful supercomputers.
Using one of the fundamental ideas of quantum mechanics, quantum key distribution (QKD) is a novel method for securely transferring data over networks. QKD makes good use of the quantum nature of photons, which states that decoherence results from the observation of a state. This ensures that encryption keys are kept secret via an optical network. Any attempt to eavesdrop on the network can be detected, as QKD makes data interception virtually impossible without altering the system in a measurable way. This technology will serve as a pillar for next-generation communication networks designed to be inherently secure and resistant to hacking.
Additionally, quantum photonics provides powerful new instruments for medical and biological study. It makes it possible to use extremely sensitive quantum biosensing to find certain chemicals and biomarkers with previously unattainable accuracy. What’s more, quantum imaging allows cellular structures and processes to be observed at a resolution beyond the limits of classical microscopy. This enhanced detail could lead to more accurate diagnoses and improved therapeutic outcomes. To deepen our understanding of biological processes, quantum technologies are also being used to enhance imaging and to study fundamental quantum effects in living systems, such as photosynthesis.
Many companies are actively developing technologies based on quantum photonics, aiming to harness its potential for applications ranging from secure communication to advanced computing and imaging. While established industry leaders like IBM have adopted photonics-based quantum technologies, new spinout companies have emerged offering different services and devices. Some examples include Quandela, Nu Quantum, Xanadu and Qubit Pharmaceuticals.
Challenges and Future of Quantum Photonics
Despite its immense promise, the field of quantum photonics faces several significant challenges. One of the primary hurdles is the efficient and reliable generation of high-quality single photons and entangled pairs. The sources often produce photons at random times, which is a major limitation for building scalable quantum circuits. Additionally, maintaining the fragile quantum states of photons is difficult, as they are susceptible to noise and loss, particularly over long distances. Miniaturizing and integrating these complex optical components onto a single chip is another ongoing challenge that needs to be solved to move from a laboratory setting to a commercial product. As these challenges are addressed, quantum photonics is set to power the next wave of advancements in computing, communications, and sensing, reshaping the technological landscape in profound and lasting ways.
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References and Further Reading
- Photonics Spectra“Photonics Definition and Historical Timeline. [Online] Photonics.com. Available at:https://www.photonics.com/LinearChart.aspx?ChartID=3
- Sivarajah, I. (21 November 2023) What to Know About Photonic Quantum Computers. [Online] AZOQuantum. Available at:https://www.azoquantum.com/Article.aspx?ArticleID=471
- Sigov, Alexander, Leonid Ratkin, and Leonid A. Ivanov. "Quantum information technology." Journal of Industrial Information Integration 28 (2022): 100365.
- Sivarajah, I. (02 November 2023) What to Know About Optical Quantum Information Processing [Online] AZOOptics. Available at: :https://www.azooptics.com/Article.aspx?ArticleID=2498
- Sergei Slussarenko, Geoff J. Pryde; Photonic quantum information processing: A concise review. Appl. Phys. Rev. 1 December 2019; 6 (4): 041303. https://doi.org/10.1063/1.5115814
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