Feature Story | 9-Jul-2025

How quantum light is shaping tomorrow’s technologies

De Gruyter

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**Volume 14, Issue 11 - Special Issue: Quantum Light: Creation, Integration, and Applications**
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Credit: De Gruyter Brill/Sciencewise Publishing

What Is Quantum Light?

In the classical world, a beam of light is a smooth, continuous wave. In the quantum realm, however, photons manifest as the discrete constituents (quanta) of light. These photons can be carefully controlled – emitted one at a time, entangled across long distances, or shaped into states with unusual polarization, amplitude or phase characteristics. This ability to manipulate individual quanta of light underpins a new generation of technologies with capabilities far beyond those of classical optics and photonics.

Key Terms

Photon: The fundamental particle of light, carrying discrete packets of energy that exhibit both wave-like and particle-like behavior. They can be fired one at a time in discrete streams.
Wave–Particle Duality: A core quantum principle stating that particles such as photons (and electrons) can display the properties of both waves (e.g., interference and diffraction) and particles (e.g., localized impacts), depending on how they’re measured.
Entanglement: A quantum linkage in which two or more particles share a common state so that measuring one instantly determines properties of the other, regardless of the distance separating them.
Cat States: Named after Schrödinger’s famous paradox, these are superpositions of two macroscopically distinct states (e.g., “alive” and “dead” in the thought experiment). In quantum optics, cat states refer to light fields that exist simultaneously in two phases or amplitudes, enabling enhanced sensing and error-resilient encoding.
Non-Gaussian States: A quantum state of light whose statistical distribution cannot be described by a simple Gaussian function – examples include squeezed light , which features reduced noise and nonclassical correlations. In everyday terms, a Gaussian distribution is the familiar “bell curve,” where most outcomes cluster around an average with fewer occurring as you move farther out.
Purcell Enhancement: The increase in spontaneous emission rate of an emitter, which becomes brighter when placed inside a resonant cavity and can channel more photons into a desired mode.

From Quantum Dots to Rare-Earth Ions: Crafting the Perfect Photon

Low-Dimensional Materials

Researchers are investigating the unique optics of materials that are just a few atoms thick. Zero-dimensional quantum dots, one-dimensional carbon nanotubes, and two-dimensional crystals like hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDCs) confine electrons so tightly that they emit one photon at a time with dazzling brightness. By embedding these emitters in high-quality cavities, scientists achieve large Purcell enhancement – boosting emission rates and funneling photons into desired directions.

Telecom-Band and Tunable Emitters

Quantum networks allow information to travel in the form of quantum bits, or qubits. Photons can act as travelling qubits, with the information encoded in one of their physical characteristics, such as their polarization. To ensure information protection over long distances, it’s critical to generate photons in the near-infrared bands used by existing fiber-optic cables. Innovations such as serrodyne frequency tuning of quantum dots and fiber-pigtailed, cavity-enhanced devices now enable on-demand single photons at telecom wavelengths – which can remain indistinguishable even after kilometers of fiber.

Rare-Earth Ion Platforms

Rare-earth ions like europium (Eu³⁺) embedded in solid hosts (such as Y₂O₃) bring exceptionally narrow linewidths in photon emission/absorption spectra – down to sub-megahertz – and long spin and optical coherence times. When coupled to on-chip cavities, these ions serve as long-lived quantum memories and interfaces between flying and stationary qubits.

Synthesizing Exotic States with Free-Electron Modulation

Beyond material emitters, theoretical breakthroughs propose using free electrons—common in electron microscopes—as a canvas for quantum light. By shaping the electron’s energy distribution, one can generate non-Gaussian photon states such as squeezed or Schrödinger ‘cat’ states with near-perfect fidelity. These methods open a new route to examining highly nonclassical light without bulky optical setups.

Building the Quantum Photonic Highway

To turn individual photons into practical qubits or into interconnects among other qubits, researchers are weaving emitters, waveguides, cavities, and metasurfaces into compact, chip-scale platforms:

Metasurfaces & Photonic Crystals
Engineered ‘meta-atoms’ sculpt the flow of light, enabling directional emission, frequency conversion, and enhanced coupling.
Plasmonic Nanostructures
Metallic resonators concentrate light into nanometric volumes, squeezing more interaction out of each photon.
Inverse-Designed Lithium Niobate
Advanced algorithms craft crystal geometries that guide, modulate, and frequency-convert quantum light with minimal loss. This integrated approach paves the way for scalable quantum photonic circuits that can be mass-produced using semiconductor fabrication techniques.

Seeing, Sending and Sensing with Quantum Light

Quantum light is already finding its way into practical applications across three powerful domains:

Super-Resolution Imaging
By exploiting correlations between entangled photon pairs – known as biphotons – researchers can beat the classical diffraction limit (the theoretical limit of image resolution imposed by the wave nature of light). This quantum-enhanced microscopy promises sharper, less invasive imaging of living cells and nanomaterials.
Long-Distance Quantum Networks
Quantum repeaters – nodes that store, entangle, and re-emit single photons – extend entanglement across hundreds of kilometers. Combined with microwave-to-optical transducers, these repeaters could interlink superconducting quantum processors in a future ‘quantum internet.’
Quantum Sensing & Thermometry
Quantum sensors take advantage of single-photon sensitivity to detect tiny changes in magnetic fields, temperature (from cryogenic to room temperature), and biological markers with unprecedented precision. These devices could lead to breakthroughs in material science, medical diagnostics, and environmental monitoring.

Looking Ahead: Toward a Quantum-Enabled World

The journey from individual photon emitters to fully integrated quantum photonic chips exemplifies the rapid evolution of nanophotonics. As this technology matures, we can anticipate:

Ultra-secure global communications, immune to eavesdropping thanks to quantum cryptography.
Revolutionary medical imaging tools, revealing subcellular structures in living organisms.
Precision sensors for geophysics, navigation, and fundamental tests of physics.

In the International Year of Quantum 2025, quantum light stands as both a symbol and a driver of transformative progress. Whether you’re a scientist, engineer, or simply a curious mind, the photons of tomorrow promise to illuminate a new era of discovery.