Photon Entanglement Applications: From Quantum Cryptography to Teleportation

Photon entanglement has long fascinated scientists and laypeople alike. Pairs or groups of photons can share a connection that goes beyond anything classical physics allows, and that connection is now a practical tool rather than only a theoretical marvel. This article explains what entanglement is, corrects a few popular misconceptions, and walks through the applications that are reshaping computing, communication and measurement.

Understanding photon entanglement

Entanglement occurs when two or more photons are generated in a single joint quantum state that links a property such as polarisation, phase, time of arrival or direction. Neither photon has a definite value of that property on its own, yet the results of measurements on the two photons are correlated more strongly than any classical model allows. Einstein, uneasy about this, called it “spooky action at a distance”.

A common description says that measuring one photon “instantly influences” its partner, however far apart they are. The correlations are indeed independent of distance, and they challenge our classical intuitions about space and time. But they cannot be used to send a message: each side, on its own, sees completely random results. The correlation shows up only when the two sets of results are brought together over an ordinary, light-speed channel. This no-signalling property is what makes entanglement useful and consistent with relativity.

In the laboratory, entangled photon pairs are usually produced by spontaneous parametric down-conversion (SPDC) in a nonlinear crystal: one pump photon splits into two lower-energy photons that conserve energy and momentum. Our article on nonlinear optics and its quantum impact explains the process, and understanding photon entanglement covers how the correlations are measured and how Bell tests prove them.

Applications of photon entanglement

Four application areas have moved from proof-of-principle experiments to working systems: quantum computing, quantum cryptography, quantum imaging and sensing, and quantum teleportation.

Quantum computing

Quantum computers use superposition and entanglement to perform certain computations far faster than classical computers. In photonic quantum computing the qubits are photons, and entanglement between them is the resource that links operations together. A register of entangled qubits is described by amplitudes over many possible states at once, which lets a quantum algorithm process large amounts of information more efficiently.

The speed-up is not automatic: it comes from algorithms that make the wrong answers interfere away and the right ones add up. For specific problems the gain is dramatic, up to exponential, for example in simulating quantum phenomena and in some cryptographic problems; for unstructured database searching it is quadratic. Entangled photons are therefore a key ingredient for tasks such as database searching, cryptography and the simulation of quantum systems.

Quantum cryptography and quantum key distribution

Secure transmission of information is a major concern in the digital age. Quantum cryptography uses entangled photons, or single photons, to create shared encryption keys whose security rests on the laws of physics rather than on the difficulty of a mathematical problem.

This is quantum key distribution (QKD). Any attempt to eavesdrop on the photons disturbs their quantum state, which shows up as extra errors and alerts the sender and receiver to the breach. With a sound protocol and a well-engineered implementation, a QKD key is secure against any computing power, including future quantum computers, which makes QKD a strong answer to rising cyber threats. In practice, security also depends on the hardware: detectors, sources and timing electronics must be characterised so that they leak no side information.

Quantum imaging and sensing

Quantum imaging with entangled photons can reach a resolution and sensitivity beyond what classical light allows in the same conditions. It is especially valuable where light must be kept faint, such as medical and biological imaging, and where signals are naturally faint, such as astronomical observation.

Because the photons are correlated, an image or a measurement can beat the shot-noise (standard quantum) limit for the same number of detected photons. Examples include observing biological processes with so little light that the tissue is not damaged, and detecting faint objects against a bright background. Our articles on what makes quantum sensing so special and on NV centers and Zeeman splitting go deeper into quantum-enhanced measurement.

Quantum teleportation

Quantum teleportation is real, though not in the way popular culture depicts it. Researchers have teleported the quantum state of one photon onto another photon at a different location, using a shared entangled pair and a classical message. What moves is the quantum state, not a physical object, and because the classical message is needed, nothing travels faster than light.

Teleportation is a building block for secure communication channels, for long-distance quantum communication and for a future quantum internet, where quantum repeaters will extend entanglement between network nodes. Read more in quantum teleportation: beyond sci-fi into reality.

Challenges and the future

Two challenges dominate. The first is maintaining entanglement over long distances: every decibel of fiber or free-space loss removes pairs, while dark counts and accidental coincidences stay, so the signal-to-noise ratio falls with distance (try a 20 dB link in the calculator above). Quantum repeaters, satellite links and better detectors are the answers under development. The second is building scalable quantum systems for practical use: bright, pure pair sources, low-loss integrated photonics, fast feed-forward electronics and stable, well-aligned optics.

Both are active research areas, and the rapid progress of recent years is a strong indicator of a transformative future. Much of that progress is engineering: stable mounts and alignment for nonlinear crystals, characterised sources and detectors, and deterministic timing electronics (see the best use of FPGAs in electro-optics).

Conclusion

Photon entanglement has moved from a perplexing quantum oddity to a cutting-edge technology. It is bolstering cybersecurity through QKD, redefining computing, enabling advanced imaging and sensing, and deepening our understanding of the universe. As the engineering matures, many of these developments may soon become everyday realities.

IZAK products for this application

Instruments from our catalog that match the measurements and set-ups in this article.

Quantum opticsQuantumAlign NLO Kinematic StagePosition nonlinear crystals with X–Y translation, tip and tilt.View product →Laser metrologyLaser Beam ProfilersUV to LWIR families, packaged, OEM or software, with Test mode and reports.View product →

Need something that is not in the catalog? We design custom electro-optical systems →

References

1. Einstein, A., Podolsky, B., & Rosen, N. (1935). Can quantum-mechanical description of physical reality be considered complete? Physical Review, 47, 777–780.
2. Bennett, C. H., & Brassard, G. (2014). Quantum cryptography: Public key distribution and coin tossing. Theoretical Computer Science, 560, 7–11 (first presented in 1984).
3. Bouwmeester, D., Pan, J. W., Mattle, K., Eibl, M., Weinfurter, H., & Zeilinger, A. (1997). Experimental quantum teleportation. Nature, 390, 575–579.
4. Giovannetti, V., Lloyd, S., & Maccone, L. (2004). Quantum-enhanced measurements: Beating the standard quantum limit. Science, 306, 1330–1336.
5. Aspect, A., Grangier, P., & Roger, G. (1982). Experimental realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A new violation of Bell’s inequalities. Physical Review Letters, 49, 91–94.

Frequently asked questions