# Polarisation Rotators and Measurement

• StevieTNZ
In summary, when using polarisation rotators to measure polarisations for entangled photons, the photon is usually directed to a polarised beam splitter after. The polarisation of the photon can be in an eigenstate of the basis after passing through a polariser, even if it was not in that state before. In the case of a pair of entangled photons, the output of the second photon's polarisation can be undefined until the first photon's polarisation is measured. In another scenario, if one photon is measured in a certain polarisation and the other goes through a quarter wave plate, the outcome can be written as either the output polarisation or the initial entangled state, depending on the context.
StevieTNZ
When we use polarisation rotators to measure certain polarisations for entangled photons, the photon usually meets a polarised beam splitter after.

When we detect in the |H> or |V> basis, does that mean the photon is in |H> or |V>? What has happened to the polarisation we were measuring? Shouldn't it pass/fail into that polarisation?

Any photon that comes out of a polarisation filter, splitter or other polarising device will be in an eigenstate of that basis, i.e. it will have a collapsed wave function. That does not have to be true for the photon/wave function going into the polariser.

For example, a circularly polarized photon contains both |H> and |V> components. It will have a 50% chance of passing a |H><H|filter and coming out as |H> eigenstate.

(1) I wonder: If we have a pair of entangled photons (entangled as HH+VV), we send photon #2 to a polarising rotator orientated at 22.5 degrees. Prior to photon #2 reaching the subsequent PBS (in the H/V basis) - so all it has done is gone through the polarising rotator - we measure photon #1, also with a polarising rotator orientated at 22.5 degrees. Photon #1 also reaches the PBS and we detect the photon as H polarised.

Because the input of photon #2 into the polarising rotator was undefined at the time (photon #1 hasn't been detected as H), we get an undefined output. Now that photon #1 has acquired H polarisation, does the output at the polarising rotator that photon #2 went into now become: input H polarisation -> output 45 degree polarisation?

(2) In another case, if we had two photons entangled as HH+VV, and we send both through quarter wave plates, then only one goes onto PBS in H/V, do we write the outcome as H(output) + V(output) OR HH + VV, where (output) is the output polarisation of the 2nd photon not yet reached a subsequent PBS? Or because photon #1 takes on H polarisation, we write photon #2 as taking on polarisation H as well (even though it hasn't reached a PBS)?

## 1. What are polarisation rotators?

Polarisation rotators are optical devices that are used to rotate the polarisation direction of a light wave. They are typically made of birefringent materials, such as crystals, and can rotate the polarisation of light by a specific angle depending on the material and design of the rotator.

## 2. How do polarisation rotators work?

Polarisation rotators work by introducing a phase shift between two orthogonal polarisation components of an incident light wave. This phase shift causes the polarisation direction to rotate as it travels through the rotator. The amount of rotation depends on the material properties and the physical structure of the rotator.

## 3. What are the applications of polarisation rotators?

Polarisation rotators have various applications in optics and photonics, including telecommunications, imaging, and spectroscopy. They can be used to manipulate the polarisation of light for signal processing, polarization-based sensing, and controlling the polarization state of light in optical systems.

## 4. How are polarisation rotators measured?

The performance of a polarisation rotator can be measured using various techniques, such as polarimetry, ellipsometry, and Mueller matrix analysis. These methods involve measuring the changes in polarisation state of light before and after passing through the rotator and can provide information about the rotator's efficiency and accuracy.

## 5. Are there different types of polarisation rotators?

Yes, there are different types of polarisation rotators, including waveplate rotators, prism rotators, and fiber-based rotators. Each type has its own advantages and limitations, and the choice of which type to use depends on the specific application and requirements.

• Quantum Physics
Replies
99
Views
4K
• Quantum Physics
Replies
13
Views
838
• Quantum Physics
Replies
80
Views
4K
• Introductory Physics Homework Help
Replies
5
Views
1K
• Quantum Physics
Replies
25
Views
2K
• Quantum Physics
Replies
38
Views
3K
• Quantum Physics
Replies
61
Views
5K
• Quantum Physics
Replies
5
Views
1K
• Quantum Physics
Replies
19
Views
2K
• Quantum Physics
Replies
4
Views
532