Quantum States of Light
Quantizing the electromagnetic field yields photons and states such as coherent, number, squeezed, and entangled light with no classical counterpart.
Definition
The states available to the quantized electromagnetic field, characterized by their photon-number distributions, coherence properties, and quantum correlations, including coherent, number, squeezed, and entangled states.
Scope
This topic covers the quantum description of the light field and its states. It includes the quantization of field modes as harmonic oscillators, photon number (Fock) states, coherent states that most resemble classical light, and non-classical states such as squeezed light with reduced fluctuations in one quadrature and entangled photon pairs. It treats photon statistics and the distinction between sub-Poissonian, Poissonian, and super-Poissonian light, the second-order coherence function, antibunching as a signature of single photons, and the role of these states in quantum information and metrology.
Core questions
- How is the electromagnetic field quantized into photons?
- What distinguishes coherent, number, squeezed, and entangled states?
- How do photon statistics reveal the quantum nature of light?
- What makes a state of light non-classical?
Key concepts
- field quantization
- photon
- coherent state
- number state
- squeezed light
- entangled photons
- photon antibunching
- second-order coherence
Key theories
- Field quantization and photon-number states
- Each mode of the electromagnetic field is quantized as a harmonic oscillator whose excitation quanta are photons; number states have definite photon count, while coherent states are minimum-uncertainty superpositions that behave most like classical waves.
- Non-classical light: squeezing and entanglement
- Squeezed states redistribute quantum noise below the standard limit in one quadrature at the expense of the other, and entangled photon pairs share correlations stronger than any classical fields, enabling quantum metrology and information.
Clinical relevance
Non-classical light promises improvements in biomedical imaging and sensing, with squeezed light able to push optical measurements below the classical noise limit and entangled photons explored for low-dose and enhanced-resolution imaging of delicate biological samples.
History
Glauber's 1963 quantum theory of optical coherence, recognized by the 2005 Nobel Prize in Physics, established the framework of coherent states and the correlation functions used to classify light. Mandel, Wolf, and others developed the experimental study of photon statistics, and the generation of squeezed and entangled light followed in the 1980s.
Key figures
- Roy J. Glauber
- Leonard Mandel
- Emil Wolf
Related topics
Seminal works
- loudon2000
- glauber1963
Frequently asked questions
- What is squeezed light?
- Squeezed light is a quantum state in which the uncertainty of one field quadrature is reduced below the usual vacuum level, at the cost of increased uncertainty in the conjugate quadrature, allowing measurements with less noise in the squeezed variable.
- How can light be entangled?
- Certain nonlinear processes generate pairs of photons whose properties, such as polarization or arrival time, are correlated in a way that cannot be explained by independent classical states, so measuring one photon instantaneously constrains the other.