Why Quantum Physics Should Fear
“If quantum physics doesn’t scare you, then you don’t understand it,”
At the end of the 20th century, many researchers realized that quantum physics could be used to create a new type of computer.We can say that researchers who deal with the issues of quantum computing are preparing a theoretical basis for teleportation, time travel, or to parallel worlds.
In the context of classical computing, there is such a thing as 1 bit - this is a unit of representation or storage of information.Similar to the classical bit, one can definequantum bit, which is a unit of quantum information. One classical bit can store one of two states at any moment in time: either zero or one. From a physical point of view, this is the presence or absence of an electrical signal. As in the classical case, in the quantum case there are states - 0 and 1. But, unlike classical calculations, 1 qubit can store a superposition of these states. That is, the state of a quantum bit is generally determined by two characteristics, or two parameters. The first parameter is responsible for the probability of the zero state, and the second is responsible for the probability of the first state. A quantum bit is in some ways a probabilistic state, but classical information can be extracted from it. To do this, a special operation called measurement is used.
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Basis states in the quantum case are not the only possible states.There is also a state, for example, plus or minus, and it should be noted that the basic state depends on the physical implementation of the quantum bit.
Quantum computing and how it differs from classical computing
Any classical calculations are based on some classical transformations.That is, these are some actions that we canundertake with a classic look. For example, the NOT operator inverts the value of a classic bit. That is, if we get 0 at the input, then we get 1 at the output, and vice versa. To work with a quantum bit, quantum transformations are used. There is one difference that separates quantum transformations from classical ones. Quantum transformations are reversible. The action of any of them can be reversed using some other quantum transformation. And, unlike classical calculations, for quantum calculations one can define another operation called “measurement”. With this transformation we can extract classical information from a quantum bit.
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The operation of a quantum computer can be determined using, respectively, a quantum circuit.If a classical circuit consists of classical transformations, then a quantum circuit consists of quantum ones.
Quantum computing, unlike classical, is a young science, but there are already interesting examples of their application.For example, an area such as cryptography -information protection, optimization problems are well solved using quantum computers. By creating a real quantum computer comparable to classical computers, we will be able to solve some problems faster than classical computers.
The idea of ultra-dense coding is to transmit two classical bits using one quantum bit.Why is this coding calledsuper dense? Let's remember a black hole - this is a kind of physical body, the entire mass of which collapses into one singularity point. However, in the quantum case, everything is much more prosaic, we are talking about data compression, and not even so impressive - simply transmitting two classical bits using one qubit.
Two qubits are said to be entangled if, by measuring or extracting classical information from the first qubit, we can accurately determine the state of the second qubit.Simple example:Let's say there are brother and sister Bob and Alice. Every day for breakfast or lunch, their mother prepares a container of food for them. She either puts on a salad or a cheese sandwich. Moreover, neither Alice nor Bob know the contents of the container when they go to school. And only when they come to school, they open their containers: Alice sees the salad, and already knows exactly what is in Bob’s container. Another more interesting example is a pair of socks. Let's say you wake up in the morning and want to put on socks, by putting one of the socks on your right foot, you will know for sure that the second sock belongs to your left foot or will be the left sock. Ultra-dense coding is based on the phenomenon of entanglement.
Teleportation is the physical movement of objects from one place to another in a short period of time.This phenomenon is invented in quantum computing,and in quantum physics it is experimentally demonstrated. However, in this case we are not moving the entire physical body, but only the state of one qubit. It can be noted that the matter is already small; now you need to learn how to split physical bodies into elementary particles, and then, after transmission using a quantum communication channel, put physical bodies back together from them. This phenomenon is also based on the phenomenon of entanglement.
“Let’s say there is a Soviet spy…”
The next example is the BB84 protocol, which belongs to the field of cryptography.Suppose we have a certain Soviet spy,the purpose of which is to exchange information with the general staff. There are several options for solving this problem. One option is to use a key that the spy can use to encrypt the message and the receiving party to decrypt it. There are two problems: how to obtain a given key so that no one can forge it, and secondly, how to exchange the key in such a way that no one can intercept it. The BB84 protocol solves this problem.
In the beginning, the spy has some kind of random bit generator and uses it to generate random bits.It uses as a quantum bitsingle photons. With their help, he encrypts or stores classical information into a single photon, let's just call it a qubit. In this case, when writing a classical bit into a qubit, two types of bases can be used. Different polarizations of a single photon are used as bases. To simplify the action, let's call these bases the white and yellow bases. What this means: With white and yellow, we can encrypt both the value 0 and the value 1. If we use a yellow basis, then the polarization of the photon is diagonal, and it will store the value 0; if we receive 1 at the input, then antidiagonal polarization is used, and, therefore, we transmit 1 with its help. If a white basis is used, then the state 0 is transmitted with the help of horizontal polarization, and 1 with the help of vertical polarization. The spy chooses these bases arbitrarily: neither he, nor anyone else, knows which one he will choose. The resulting photons with a certain polarization are transmitted to the general headquarters, which also has these bases: with their help, the resulting quantum bit is measured there. The General Staff does not know which bases the Soviet spy used, therefore, they randomly choose these bases. But, from the point of view of probability theory, in half the cases they will guess these bases. And, therefore, in about half the cases, the bases used - and the received and transmitted classical bits - will coincide. Next, the General Staff transmits the bases that it used, and the spy, in turn, reports in which positions the match occurred. The string that was obtained from the squeezed states becomes the key. That is, if a spy sends 1,000 bits of classical information, then in the end the key will be about 500 characters, or 500 bits.
There is a third person, the fictitious Muller, whose goal is to eavesdrop on the key exchange process.How does he do it?Suppose he also knows all those bases that are used by the spy and the general staff. It gets in the middle and starts accepting single qubits with its bases. He, too, does not know which bases the Soviet spy used, and arbitrarily chooses between the yellow and white bases. In 50% of cases, he will guess. Consequently, 50% of the qubits will leave in the same state in which they were received. However, about 50% will leave in a changed state. As a result, when receiving these qubits, the general staff will receive exactly the states that were sent only in a quarter of the cases, in principle, this will be a signal that someone is eavesdropping on them. If no one overheard them, then 50% of their keys would match. However, if someone eavesdrops on them, only a quarter of the time the keys will match. Therefore, the first problem that we voiced with you is that how exactly to generate a key so that no one eavesdrops will be solved in this way. As soon as they find out that someone is eavesdropping on them, they can change the communication channel. That is, to choose a different quantum channel. The second problem: how exactly to exchange a key so that no one can intercept, in this case is solved by itself, since there is no key exchange problem in this case.
When will real quantum computers appear?
At the moment, quantum computers already exist and are even practically used industrially.In fact, these are computers that in some wayleast use of quantum effects. These computers solve a limited range of problems and are mainly used to solve some optimization problems. For example, the d-wave company is one of the developers of almost quantum computers. Among the clients of this company are such giants as Google; several automakers also use almost quantum computers.
To date, several developments are already known that are being carried out in the creation of real quantum computers.Literally a year ago it was developedexperimental model of a quantum computer that works with two qubits. These quantum computers are also not suitable for solving real problems, but it is important to note that their work well demonstrates the operation of the principles on which quantum computers are theoretically based.
In 2019, a quantum computer was presented, consisting of and working with 20 qubits.This computer is used purely fordemonstrating that the principles of quantum computing work. This can be compared to two megabytes, for example, of RAM in the modern world, that is, in principle, it’s nothing.
Now there are hypotheses that quantum entanglement and the phenomenon of wormholes are one and the same phenomenon.Moreover, wormholes themselves are basedon such a phenomenon as quantum entanglement. This suggests that in the future, as an option, it will be possible to create wormholes already artificially. That is, entangling some quantum bits with each other.
How to measure the quantum bit
There are three views on measuring a quantum bit.First look is Copenhagen theory,a classic view of the measurement process. It says that with the help of measurement, we, receiving a certain classical result, influence the measured qubit. If we consider in the context of an electron, then the measurement of an electron is represented in the form of a certain wave - that is, it is a certain wave function. But the measurement leads to the fact that the given wave function collapses, and we are already dealing with a particle. It is important to mention the Heisenberg uncertainty, which states: that we cannot know about the wave function and the location of the electron at the same time. That is, if we measure an electron, we will lose the characteristics of the wave function. Conversely, knowing the characteristics of the wave function, we cannot determine the location of the electron.
The second view is the theory of David Bohm, which says that we simply do not have all the information about the system, but in reality both before measurement, and after measurement, the wave function does not disappear anywhere.There are simply some hidden parameters that wewe don't know. And knowing these additional characteristics, we can establish both the exact location of the electron and the characteristics of the wave functions. This can be compared to tossing an ordinary coin. If we consider it from a classical point of view, a coin toss is considered a random process, that is, the result cannot be predicted. However, from the point of view of physics, we can accurately determine, knowing some additional characteristics, which side the coin will fall on. For example, the initial force of impact or the force of air resistance, and so on.
And a third way of looking at the measurement process is the multiple worlds theory.This theory was expressed by Hugh Everett.It says that when measuring, a kind of splitting of the physical world occurs. And the hypostasis that we observe, the location of the electron, is real only in our world. In parallel, other worlds are created, in which another hypostasis of the electron is real. Developing Everett's theory, one of the creators of quantum computing once said that, thus, the Universe itself is a kind of quantum computer and performs calculations.
The reason for the emergence of post-quantum cryptography was a theoretical quantum algorithm that allows you to crack existing encryption systems.One of them is the basis for the security of manyInternet banking, as well as the basis for website encryption. Suppose there is a Soviet spy whose goal is to transmit information to the General Staff, and there is a third party who can eavesdrop on all this. Previously we looked at encryption using a single key, but in this particular case a different method is proposed. There is the RSA protocol, the purpose of which is as follows: two keys are generated - a public key and a private one; The private key is used to decrypt the received message, and the public key is used to encrypt it. This protocol allows you to implement this algorithm, that is, create public and private keys.
At the end of the 20th century, a new algorithm was proposed by Peter Shor to break the core of the RSA algorithm.This algorithm is completely quantum, and,therefore, the emergence of a really working quantum computer will make it possible to hack modern security systems. As a result, a new science has emerged that is looking at new algorithms to make encryption methods resistant to cracking by a quantum computer.
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