# Quantum Photonics

Author: the photonics expert Dr. Rüdiger Paschotta

Definition: the science and technology using quantum optics for applications like quantum communications, quantum computing and quantum metrology

More general term: photonics

Quantum photonics is the science and technology which uses quantum optics for certain applications where quantum effects play an essential role. It is an area within photonics, and the term is more frequently applied in the context of technology, i.e., applications, than in fundamental science.

## Applications of Quantum Photonics

*Quantum communications*is mostly about much improved information security. For example, one exploits the non-cloning theorem for preventing any undetected eavesdropping. See also the articles on quantum cryptography and quantum key distribution.- In
*quantum computing*, one encodes information with*quantum bits*, in short*qbits*, as linear superpositions between two states of a photon or an atom, for example. (In contrast, a classical bit can always be only in one of the two states, not in a superposition.) If a combination of <$N$> qbits is used, that can be in a superposition of <$2^N$> states. The coherent evolution of such qbits under well controlled conditions depends on all involved quantum states and can thus be used to perform certain operations with far higher speed than is possible with classical computers. For example, it should be possible to perform certain operations which allow one to crack current cryptography systems which would appear to be completely safe against the use of conventional (classical) technologies. Both analog and digital realizations are considered and developed; for example, there are*quantum logic gates*for digital quantum computations (involving quantum measurements projecting states to basic quantum states) and methods of quantum simulation, quantum annealing and adiabatic quantum computation for analog operations. - Photonics-based quantum computing is still under development, and it competes with other technologies, such as superconducting devices.
- A more general field is
*quantum information processing*, which does not only include quantum computing, but also related technologies such as quantum error correction, quantum cryptography and quantum teleportation.

While quantum optics serves as the underlying basic science, quantum photonics denotes its technological use. It could also be called applied quantum optics. It belongs to the broader field of photonics.

## Technical Aspects

Generally, quantum photonics applications involve the generation, manipulation and detection of light and matter with control on the quantum level, often involving single photons. Often, the interactions of light with matter – for example, single atoms or ions in an optical trap or a quantum – also plays an essential role. One may also exploit *quantum entanglement*, e.g. between photons within photon pairs or between atoms, or between photons and atoms. Compared with atoms and ions, photons have the advantage of exhibiting much weaker tendencies for loss of quantum coherence since they react much less to external influences such as electric and magnetic fields. Also, they can be relatively easily manipulated. However, interactions with matter are often required for important processing steps.

Some critical devices for quantum photonics are:

- Single photon sources are devices that can reliably produce individual photons on demand. They are essential for quantum communication and computing.
- Photon pair sources generate pairs of entangled photons, i.e., with linked quantum states.
- Various kinds of lasers are required, often with special performance specifications, for example with ultra-low laser noise and special emission wavelengths.
- Photon counting detectors can register single photons. They are important for measuring quantum states and for quantum key distribution in quantum cryptography. A crucial quality in that context is a very high quantum efficiency.
- Quantum dots may be used to manipulate single photons, e.g. in quantum computing and quantum communication systems.
- Some types of optical resonators are suitable for storing photons for a certain period, also for enhancing light-matter interactions. They may thus be used for controlling and manipulating quantum states.
- Integrated photonic circuits can contain many miniaturized optical components that can manipulate photons for various quantum operations. For more complex operations, such integration is crucial due to demands concerning compactness, power consumption and fabrication cost. This field is called
*integrated quantum photonics*. - Various nonlinear materials can be utilize for manipulation quantum states.

Many other types of components are also utilized, although not being specific for quantum photonics; examples are waveguides including optical fibers, beam splitters and dichroic mirrors.

## Levels of Maturity

While some areas of quantum photonics have already been developed to a state where first practical applications have become possible – in particular, this is true for certain aspects of quantum communications such as quantum cryptography – other fields such as quantum computing are still under intense investigation in the search for practical implementations. This includes the development of relatively robust miniature devices for functions like single photon sources, quantum gates and quantum memories. The development could lead to a technology which is similar to that of classical photonic integrated circuits, which can perform complex functions in a rather small volume, and exhibit good manufacturability. However, serious technological obstacles need to be overcome. For example, essential parts of so far developed quantum computation technology are not scalable to larger numbers of qbits; it becomes extremely hard to avoid various kinds of processes causing quantum decoherence. Therefore, further research and development must not only further optimize various kinds of components for quantum photonics but also work out improved concepts and principles.

## More to Learn

Encyclopedia articles:

### Bibliography

[1] | J. Wang et al., “Integrated photonic quantum technologies”, Nat. Photonics 14, 273 (2020); https://doi.org/10.1038/s41566-019-0532-1 |

[2] | G. Moody et al., “2022 roadmap on integrated quantum photonics”, J. Phys. Photonics 4, 012501 (2022); https://doi.org/10.1088/2515-7647/ac1ef4 |

[3] | W. Luo et al., “Recent progress in quantum photonic chips for quantum communication and internet”, Light Sci Appl. 12, 175 (2023); https://doi.org/10.1038/s41377-023-01173-8 |

[4] | T. P. Pearsall, Quantum Photonics, Springer (2020); https://doi.org/10.1007/978-3-030-47325-9 |

[5] | Z. Zhang et al., “Entanglement-based quantum information technology: a tutorial”, Adv. Opt. and Photonics 16 (1), 60 (2024); https://doi.org/10.1364/AOP.497143 |

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