Looking for introduction on QND: quantum non-demolition measurements

In summary, a recent news article on physicsweb discusses a study on quantum non-demolition (QND) measurements on single photons. These experiments are named as such because they allow for the observation of a single photon without destroying it. The paper on this study is available on arxiv and contains references for further reading on the topic.
  • #1
lalbatros
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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
 
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  • #3
, 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.
 

1. What is QND measurement in quantum mechanics?

QND, or quantum non-demolition, measurement is a type of measurement in quantum mechanics that allows for the precise determination of a quantum system's state without disturbing it. This is achieved by using indirect methods of measurement that do not collapse the quantum state of the system.

2. How is QND measurement different from traditional measurements?

Traditional measurements in quantum mechanics involve direct interactions with the system being measured, which can cause the system's state to collapse and change. QND measurements, on the other hand, use indirect methods that do not disturb the system's state, allowing for repeated measurements without altering the system.

3. What are some applications of QND measurements?

QND measurements have applications in a variety of fields, including quantum computing, quantum communication, and quantum metrology. They are also useful for studying and manipulating individual quantum systems, such as atoms and photons, in order to better understand their behavior.

4. How are QND measurements performed?

QND measurements are typically performed using specialized equipment, such as quantum non-demolition detectors and quantum logic gates. These tools allow for the precise measurement of quantum systems without directly interacting with them.

5. What are the benefits of using QND measurements?

The main benefit of QND measurements is the ability to accurately determine the state of a quantum system without altering it. This allows for repeated measurements and precise control of the system, making it useful for a variety of applications in quantum technology and research.

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