Looking for introduction on QND: quantum non-demolition measurements

[lalbatros]

I just read a news on physicsweb:

http://physicsweb.org/articles/news/11/3/9"

This is called a "quantum non-demolition (QND) measurement on single photons".

I would like to know a little bit more about QNDs.
Why they are called so.
What is special about these QND experiments, or maybe paradoxal (I guess nothing).
Why this single photon experiment is called a QND.
Why it is possible to -apparently- observe a single photon without destroying it.
If and how the photon observed in this experiment has been perturbed?
I guess that non-demolition does not mean non-interaction ...

Web sites, papers, comments, ideas, ... welcome.

Michel

 

[Edgardo]

The paper is available on arxiv:
http://arxiv.org/abs/quant-ph/0612031
Have a look at the references given at the end of the paper.

 

[denian]

, thank you for your interest in QND measurements. QND, or quantum non-demolition, measurements are a type of measurement in quantum mechanics that allows for the observation of a quantum system without destroying its state. This is in contrast to traditional measurements, which typically collapse the quantum state of a system and destroy any information about its previous state.

In QND measurements, the system is measured in such a way that the outcome of the measurement does not affect the system's state. This is achieved by using a probe that interacts with the system in a way that does not change its state, but still provides information about it. This allows for repeated measurements on the same system, providing more accurate and precise information without destroying the system's state.

The term "non-demolition" refers to the fact that the system's state is not demolished or destroyed by the measurement. This is an important concept in quantum mechanics, as it allows for the study and manipulation of delicate quantum systems without altering their behavior.

In the experiment mentioned in the news article, scientists were able to perform a QND measurement on single photons by using a special type of detector called a superconducting transition-edge sensor. This allowed them to observe the photons without destroying them and gather information about their properties, such as their polarization.

One of the key advantages of QND measurements is their potential for use in quantum computing and communication. By allowing for repeated measurements on a quantum system without destroying it, QND measurements could help improve the accuracy and reliability of quantum information processing.

It is important to note that QND measurements do involve some level of interaction with the system, but this interaction is designed to be minimal and non-destructive. This is why the photon in the experiment was not perturbed in a significant way.

If you would like to learn more about QND measurements, I recommend checking out scientific papers on the topic, such as "Quantum Non-Demolition Measurements in Quantum Optics" by M. Brune et al. or "Quantum Non-Demolition Measurements: The Route from Enhanced Sensitivity to the Quantum Nondemolition Principle" by M. Brune and J.M. Raimond. Additionally, there are many online resources available that explain QND measurements in more detail. I hope this helps to answer your questions and further your understanding of this fascinating topic in quantum mechanics.