World Quantum Day丨What is the difference between quantum measurement and ordinary measurement?

April 14 is World Quantum Day. Why was this date chosen? This is because one of the most fundamental constants in nature, Planck's constant, is approximately equal to 4.14 times 10-15 electron volt seconds. It is Planck's constant that brings us into the quantum world.

Today we will look at one of its applications: quantum measurement. What is the difference between it and ordinary measurement? If you stare at someone on the street, you may annoy that person and he will ask you, "What are you looking at?" Can this be avoided? Yes. You can take a sneak peek. In this way, others will not easily notice that you are looking at him.

That is to say, classical measurement can minimize the impact on a system. Therefore, in classical physics, measurement is a technical means, not part of physical theory. However, in the quantum world, things are not so simple. Your measurement will change the system. This is because when you use a specific wavelength of light to probe the system,

Light has the smallest unit of energy, which is the photon, and you cannot break a photon into two halves.

The picture comes from Tuchong.com

What does this have to do with Planck's constant 414? The light quantum is the reason for the introduction of Planck's constant. Planck's world-famous formula: energy equals Planck's constant multiplied by frequency. Planck's constant is the ratio of light quantum energy to frequency. The existence of Planck's constant 414 tells us that, in general, measurement always has some interference with the system, rather than the classical measurement, which can minimize the impact on the system. Therefore, we must regard measurement as part of the theory of quantum mechanics.

Here, we say that measurement generally interferes with the system because, in fact, you can also design sophisticated experiments so that measurement sometimes does not affect the system. Although we cannot break a photon into two halves, we can use the superposition state of one photon and zero photons in the measurement. This is called quantum non-destructive measurement. At this time, a conclusion that is even philosophically surprising will be produced: sometimes we can detect some properties of an object without touching it at all. This is the famous quantum bomb defusing photon bomb test thought experiment.

Although quantum measurement is very important to quantum mechanics and opens the door to the wonderful quantum world for us, it is not as magical as it is said on the Internet. Some people on the Internet say that whether we want to observe or not, this desire affects the objective world. Although quantum mechanics was born not long ago, some physicists have also thought about similar "consciousness determines the measurement result". But this is unreliable.

Here I want to emphasize that quantum measurement is the interaction between the measuring instrument and the quantum system, the interaction between matter and matter, not our desire to "measure", nor our thought activity of "seeing the test results", not our thinking directly interacting with the quantum system, nor our spirit directly acting on matter. Some people on the Internet have linked quantum measurement with discussions on basic philosophical issues of materialism and idealism, which is an over-interpretation of quantum measurement.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Wang Yi

Reviewer: Zhang Wenzhuo (Founder and CEO of Quami Quantum, former member of the Mozi satellite team)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.