最近被要求学习量子，所用教材是Berkeley的Vazirani在2004年所开的Intro, Qubits, Measurements, Entanglement的notes。下面是这套讲义的第一章的开头部分：

There are several reasons why we might wish to study quantum computation. Here are a few:

• Moore’s Law
  Moore’s Law states that the density of transistors on a chip roughly doubles every eighteen months. Current estimates say that in about a decade this should be down to single electron transistors. This is the end of the road for further miniaturization of classical computers based on electronics. Long before that chip designers will have to contend with quantum phenomena. Quantum computation provides a method of bypassing the end of Moore’s Law, and also provides a way of utilizing the inevitable appearance of quantum phenomena.
• Factoring, Discrete log, Pell’s equations, etc..
  There are certain problems that quantum computation allows us to solve more efficiently that any classical computational method. A few examples are listed above. We may wish to exploit this feature of quantum computation. 
• Cryptography
  Quantum computation allows us to do cryptography in a way that doesn’t require assumptions about factoring primes, etc.. It also allows us to break classical cryptography schemas. Obviously, if we are interested in cryptography, we’ll also have to be interested in quantum computation.
  Above are the three standard reasons for studying quantum computation. There are other reasons as well that are perhaps just as compelling. 
• Quantum Mechanics is a model of computation
  We can study quantum mechanics as a model of computation. 
• Quantum Entanglement

In particular, the detailed study of entanglement is the most important point of departure from more traditional approaches to the subject. For example, quantum computation derives its power from the fact that the description of the state of an n-particle quantum system grows exponentially in n. This enormous information capacity is not easy to access, since any measurement of the system only yields n pieces of classical information. Thus the main challenge in the field of quantum algorithms is to manipulate the exponential amount of information in the quantum state of the system, and then extract some crucial pieces via a final measurement.

Quantum cryptography relies on a fundamental property of quantum measurements: that they inevitably disturb the state of the measured system. Thus if Alice and Bob wish to communicate secretly, they can detect the presence of an eavesdropper Eve by using cleverly chosen quantum states and testing them to check whether they were disturbed during transmission.