# Coherent state between a fermion and a photon

• sugeet
In summary, the conversation discusses the possibility of a coherent state between a fermion and a photon, specifically a fermionic polariton. The person asking the question clarifies that they are referring to a situation where the photon interacts with an atomic system and flips the electron's spin, but the electron remains a fermion. The respondent provides a link to a Wiki page that states the polariton is a bosonic quasiparticle and should not be confused with a polaron, which is a fermionic quasiparticle. They also mention the univalence superselection rule, which states that a superposition of states with an even and odd number of fermions is not possible.
sugeet
Can we have a coherent state between a fermion and a photon! I mean can there ever be a fermionic polariton?

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You may have to clarify your question a bit. Imagine a simple atomic system with one electron in a magnetic field. The photon can interact with the system by flipping the electrons spin from down to up, total change of spin by 1, but the electron remains a fermion. What situation were you thinking of? What part of the system should be fermionic?

Thank you Zarqon for replying, I did some search and got my answer.
The Wiki page says :"The polariton is a bosonic quasiparticle, and should not be confused with the polaron, a fermionic one, e.g. an electron plus attached phonon cloud. ".

Precisely something I was looking for, if you are still interested we can discuss

Note also that you cannot have a superposition of states with an even and odd number of fermions.
This is calles univalence superselection rule. It is easiest to see from the fact that such a superposition is not invariant (not even up to a phase) under a rotation by 2π.

## What is a coherent state between a fermion and a photon?

A coherent state between a fermion and a photon is a quantum state that describes the simultaneous presence of a fermion and a photon in a coherent manner. This means that the fermion and photon have a fixed relationship between their properties, such as energy and momentum, and have a well-defined phase relationship.

## How is a coherent state between a fermion and a photon created?

A coherent state between a fermion and a photon can be created through a process known as parametric down-conversion. In this process, a photon is split into two photons with lower energy, one of which interacts with a fermion to create the coherent state.

## What are the properties of a coherent state between a fermion and a photon?

A coherent state between a fermion and a photon has several important properties. It has a well-defined energy and momentum, and its wavefunction is highly localized in both space and time. Additionally, it exhibits interference effects, similar to those seen in quantum systems.

## What are the applications of coherent states between a fermion and a photon?

Coherent states between a fermion and a photon have several potential applications in quantum information and technology. They can be used to create entangled states, which are important for quantum computing and communication. They can also be used in quantum teleportation and for precise measurements in quantum metrology.

## What are some current research topics related to coherent states between a fermion and a photon?

Current research topics related to coherent states between a fermion and a photon include exploring their applications in quantum information, studying their behavior in different physical systems, and investigating their potential for quantum simulation. Additionally, scientists are working on developing new techniques to create and manipulate these states more efficiently and accurately.

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