1. Introduction

Schrödinger cat was born from the thought experiment of Schrödinger in 1935. However, after more than 80 years, we still do not know whether it is dead or alive in its sealed box. According to the modern quantum mechanics, based on Copenhagen interpretation, the fate of this cat is entangled with the Geiger counter monitor in its box, and the cat is in a "mixed state"—both dead and alive—if we do not open the box to look at it. It is a miserable and mystical cat, which seems its fate depends on our look. Yes, it just seems, because we deeply doubt that the power of our glimpse can really make the cat alive or dead. This doubt is not

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and eproduction in any medium, provided the original work is properly cited.

the business about human self-confidence, but the fear of our fate determination. If the glimpse of us can determine the cat's fate, who determines our fate? Trouble never singly comes; many researches find that "the moon is not there" in experiments [1] responding to what Albert Einstein said, "I like to think that the moon is there even if I don't look at it." According to the physicist's research, Albert Einstein seems worried because the world is quantum world and all things obey quantum mechanics. This means all the definite statuses we have observed are due to "a glimpse" of us or the god. Really? Is really the moon not here if we do not look at it, does really the cat not exist if we do not look at it, and do we not exist if the god does not look at us?

2. What is the quantum measurement?

basis state.

ment.

element of the basis-state set jφ<sup>k</sup>

object systems becomes <sup>r</sup><sup>O</sup> <sup>¼</sup> <sup>P</sup>

Qb under one quantum measurement is Qi ¼ φ<sup>i</sup> Qb

instantaneously collapse into the basis state jφ<sup>i</sup>

the output we get is the average value <sup>Q</sup><sup>~</sup> <sup>¼</sup> <sup>P</sup>

Quantum measurement is different from the classical measurement, in which the measurement accuracy is dependent on the measurement instruments. It means, we could infinitely approach the "absolute exact value" by upgrading instruments or improving methods in the classical measurement realm. However, the things change when we access to the quantum world. In quantum world, the "accuracy" does not exist. We cannot speak that the velocity of an electron is 1376:5 m=s or the distance of two electrons is 20 nm, etc., because these physical quantities exist in the form of quantum states in quantum world. Objects are always in the superposition states of these kinds of the basis state, such as momentum, position, energy, spin and so on. We can just get one of the basis states under every measurement, and the "absolute exact value" is never revealed under one measurement unless the state of the object is in the

> � � � � � �φi D E <sup>¼</sup> Tr <sup>P</sup>b<sup>φ</sup><sup>i</sup>

> > Qi <sup>¼</sup> Tr <sup>Q</sup><sup>b</sup> <sup>b</sup>r<sup>I</sup>

the initial density matrix of an object, after the quantum measurement. For multi-measurements,

This kind of measurement, to be exact, is the projective measurement. A more general formulation of measurement is the positive-operator valued measure (POVM), which can be seemed as the partial measurement in the subsystem of a projective measurement system. No matter what kind of quantum measurements there is, it is the kind of destructive manipulations and irreversible. It destroys the old state and rebuilds a new mixed state. The definition of the quantum measurement is simple and definite, but the problem is that we do not know why the quantum measurement acts as these strange behaviors. The irreversibility and unpredictability are incompatible with the smooth Schrodinger differential equation and are hated by physicists. What kind of objects has priority to do the quantum measurement? Taking the experiment of two-slit interference of electrons, for example, the detector behind the slits usually is regarded as a measurement tool, but the detector itself, which may be a microcavity or atom ensemble, is also a physical system and obeys the quantum mechanism. Therefore, it seems that the process of a quantum measurement is the interaction between the detector and electrons and should be a "quantum evolution process". However, the quantum evolution process is non-destructive and reversible. In fact, in the real world, it is hard for us to distinguish strictly which is the quantum evolution operation and which is the quantum measure-

The second problem is the space–time nonlocality in the quantum measurement process. This nonlocality exists not only in the correlation between particles but also in the wave

i pi ⟩⟨φ<sup>i</sup>

Stochastic Quantum Potential Noise and Quantum Measurement

http://dx.doi.org/10.5772/intechopen.74253

� �g. The measurement output for a mechanical quantity operator

⟨ with the probability pi ¼ Tr Pb<sup>φ</sup><sup>i</sup>

, which is very different from the initial state rI.

j, where jφ<sup>i</sup>

<sup>Q</sup><sup>b</sup> � �, and the initial state will

� �, and the final state of the many

brI � �, where <sup>b</sup>r<sup>I</sup> is

⟩ is an

137

In quantum mechanics, the projection operator is defined as Pb<sup>φ</sup><sup>i</sup> ¼ jφ<sup>i</sup>

i pi Pbφi

It must be something to worry because the moon exists more than 4.5 billion years as the astronomer finding, which is much more than human history. We are not going to discuss the superpower of human and if the god exists or not in this book. We return to the fundamental of quantum mechanics and find that the hidden actor, quantum measurement, is the crime culprit that causes these puzzling questions.

There is a confliction in modern quantum physics after its birth. The confliction is concerning the full description between the superposition state for the behavior of matter on the microscopic level and the definite-status appearance as what we can observe on the macroscopic level in the real world. Schrödinger proposed Schrödinger cat in his essay to illustrate the "putative incompleteness" of quantum mechanics, but many researches show that quantum mechanics is still the best one of these "not satisfied theories." To alleviate the theory-to-world confliction, a new conception, quantum measurement, is brought out. It is the basic assumption in quantum mechanics, thought that the superposition state will be collapsed into one of the eigenstates with the square of amplification probability if we do a quantum measurement. Although the quantum measurement bridges the gap of the different behaviors of subatomic level and the macro-world, some problems still remain. For example, its physical mechanism is dim. We do not know what will lead to the quantum measurement and how the process that the quantum measurement undergoes. The words "stochastic", "instantaneous" and "irreversible" torment us more than 70 years, and we still have no way to integrate them into the "determinate", "time-costed" and "reversible" quantum evolution. In fact, the manual division for the world into two parts, quantum world and quantum measurement apparatus, is not satisfied, and we are finding a uniform description.

In this chapter, we will overview the mechanism of quantum measurement and the main kinds of interpretation of quantum measurement. Among these interpretations, a promised theory which can well interpret the quantum measurement quandaries—why the quantum state collapses into some eigenstates with "stochastic" and "instantaneity", and what causes the "basis-preferred"—is detailed. The advantage of this theory is it is just an extension of Feynman path integral (FPI) and is obviously compatible with the classical quantum theory. According the conclusions of this theory, we show that the "noise" world (or apparatus here when we do an experiment) causes the "random" and "nonlocal" mechanism of the quantum collapse. Actually, the world exists due to itself, and the god can go to have a rest.
