Build your intuition for the quantum world that powers the next technological revolution — from wave-particle duality to photon entanglement.
Quantum optics applies quantum mechanics to light and its interaction with matter at the single-photon level. Unlike classical optics, it acknowledges that light is composed of discrete energy packets — photons — that exhibit fundamentally quantum behaviour.
At its core, quantum optics explores how photons exhibit superposition, entanglement, and interference — phenomena with no classical analogue — and how these properties enable quantum computation, communication and sensing.
Click any concept to expand a full explanation.
Light behaves as both a wave and a particle depending on how it is measured. In Young's double-slit experiment, a single photon passes through both slits simultaneously (wave) yet always arrives at a single location (particle).
This duality is a fundamental property of nature, not a measurement limitation. It underpins all quantum interference phenomena that power quantum computers and sensors.
The electromagnetic field is quantized into discrete photons. Fock states |n⟩ are eigenstates of the photon number operator n̂ = â†â — they describe light with exactly n photons.
Coherent states (laser light) are superpositions with Poisson photon number distributions, fundamentally different from classical waves.
Two photons are entangled when their joint state cannot be factored into a product of individual states. Measuring one instantly defines the state of the other, regardless of distance.
SPDC (Spontaneous Parametric Down-Conversion) in nonlinear crystals is the standard method for generating entangled photon pairs in the lab.
A quantum system exists in a superposition of states: |ψ⟩ = α|0⟩ + β|1⟩, where |α|² + |β|² = 1. Only measurement collapses it to a definite state.
The Mach-Zehnder interferometer splits a photon into superposition, lets paths acquire different phases, then recombines them — output probabilities are determined entirely by interference.
ΔxΔp ≥ ℏ/2 — position and momentum cannot simultaneously have zero uncertainty. For photons: ΔnΔφ ≥ ½ between photon number and phase.
Squeezed states exploit this: reduce phase noise below the standard quantum limit (at the cost of more number uncertainty) — enabling LIGO's sub-shot-noise sensitivity.
QKD distributes cryptographic keys with information-theoretic security. Any eavesdropping disturbs the quantum states, revealing the interceptor. The BB84 protocol sends single photons in random polarisation bases.
CV-QKD (continuous-variable) uses coherent states and homodyne detection, making it compatible with standard telecom fibre at speeds exceeding 1 Gbit/s.
Interaction with the environment causes decoherence — quantum superpositions collapse into classical mixtures. Maintaining coherence is the central engineering challenge of quantum technology.
Photons are naturally decoherence-resistant — they rarely interact with room-temperature environments — making optical channels ideal for quantum communication.
Lasers produce coherent light — photons sharing identical frequency, phase and direction. Coherent states |α⟩ are eigenstates of the annihilation operator: â|α⟩ = α|α⟩.
Modern quantum optics requires going beyond lasers — single-photon emitters and photon-number-resolving detectors probe the deepest nonclassical states of light.
Energy of a photon equals Planck's constant times its frequency. The founding equation of quantum theory.
Photon momentum equals reduced Planck constant times wave number. Links quantum and electromagnetic theories.
Position and momentum cannot simultaneously be exactly known. Fundamental limit in all quantum systems.
Maximally entangled two-photon state. Foundation of quantum cryptography and teleportation.
Adds one photon to a Fock state. The algebraic engine of quantum field theory.
Describes mixed quantum states — essential for modelling decoherence and open systems.
Quantum bit — basic unit of quantum information. Can exist in superposition of |0⟩ and |1⟩ simultaneously.
Elementary particle of light. Massless, travels at c, carries energy E = hν.
Non-classical correlation between particles. Measuring one instantly determines the outcome of the other.
Loss of quantum properties through environmental interaction. The central obstacle in quantum computing.
Quantum light state with noise in one quadrature reduced below vacuum level.
Optical device splitting a photon into superposition of transmitted and reflected paths — the fundamental quantum gate.
Spontaneous Parametric Down-Conversion: one pump photon → two entangled daughter photons in a nonlinear crystal.
Theorem: an unknown quantum state cannot be perfectly copied. Guarantees security of quantum cryptography.
Quantum state of exactly n photons. Written |n⟩. Eigenstate of the photon number operator.
Quantum Key Distribution — uses quantum mechanics to distribute cryptographic keys with provable security.
One of four maximally entangled two-qubit states. Foundation of quantum information protocols.
Fixed phase relationship enabling quantum interference. Coherence time T₂ limits qubit lifetime.