This article is the fourth part. General knowledge of popular science quantum computing -3
Classic bit, abbreviated as c bit.
Classical bits have only two states: 0 or 1. No matter what we mean, 0 or 1, true or false, on or off, yin or yang ... even the vector (1, 0)(0, 1) used in our previous article is a classic bit, because it has only two states, and there is no semi-yin and semi-yang state.
Quantum bit, qbit for short.
Quantum bits can only be represented in the form of binary vectors, which are defined as follows:
It can be seen that both A and B are numbers from 0 to 1 or from 0 to-1. Here are some common qubits:
Classical bits are special cases of quantum bits.
In the macro reality we are familiar with, we can only kick football into a goal. Even if there are two goals on the opposite side, from the moment we set foot on it, the ball has decided that it can only fly to one of them.
In the double-slit experiment, we send a photon into two slits, but we can't know which slit it will fly to. In fact, it will pass through two slits at the same time like a water wave and interfere with itself.
Unless we install a detection device at the gap for observation, the result is that either photons pass through or they don't, and it is impossible to observe that half a photon passes through the gap.
Our observation behavior makes the uncertain photons deterministic, and changes the possibility from left to right into a certain gap.
If we regard two gaps as 0 or 1, it is uncertain before measurement, 50% may pass through the gap on the left and 50% may pass through the gap on the right. In this state, we say it is in a superposition state.
Our measurement leads to the uncertainty of superposition state becoming a definite reality. This process is called quantum collapse, and it is a certain reality that becomes 0 or 1.
Measurement will lead to quantum collapse and turn uncertainty into certainty.
For qubits, it is to find the square value of each term:
Here refers to how likely it is to be 0, or how likely it is to pass through the gap on the left; It also means how likely it is to be 1 or how likely it is to pass through the gap on the right.
It must be 1, or follow the definition of qubit qbit. We can also explain from probability that the sum of all possibilities must be 100%. No matter how likely it is to cross from the left or right, the sum of the probabilities must be 100%, and there can be no other circumstances.
Simple memory means that the square of the previous item represents the possibility of 0, and the square of the next item represents the possibility of 1. Because (0, 1) has a probability of 100%, the probability of 0% is 0, (0, 1) is a definite 1, and (1, 0) is a definite 0:
From here, we can also see that the classical bits represented by vectors are also special quantum bits.
In quantum computing, in more cases, the qubit cannot be determined whether it is 0 or 1 after measurement, and it is still in a probabilistic entangled state, such as:
This means that there is still a 50% probability of 0 and a 50% probability of 1, which is still uncertain. For a unified coin, there is no valid information in it. But the following situations are different, indicating that this is a cheating coin:
The definition of multi-bit still follows tensor product algorithm:
Please note that the rule that the sum of squares is 1 is still satisfied, that is:
It's like we emit two photons into a double slit, so there are four possible situations when they pass through the double slit, namely Zuo Zuo, left and right, left and right, and left and right. The sum of the last four possibilities must be 100%. For example:
So there is a 25% chance that it will collapse to | 00 >; Another 25% may collapse to | 01>; 25% may collapse to |10 >; And 25% may collapse to |11>; .
In reality, can quantum be operated without measurement? The answer is yes.
Scientists can use some lenses or instruments to manipulate the entangled quantum in flight, and the quantum is still entangled after the operation. This is actually the scientific experimental basis of quantum computer.
In quantum computing, we can also use matrix mathematical algorithm to calculate the quantum bits of entangled States, such as flip or CNOT gate operation introduced in the previous two articles. This is consistent with the experiments made by scientists in reality.
In fact, many important operations of quantum computing are carried out in superposition state, and we will only carry out the square measurement operation in the last step, trying to get the fixed value after collapse.
General knowledge of popular science quantum computing -5
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