### What is a quantum computer?

The term Quantum Computer is very good but; What is it? Are they powerful? Are there quantum computers already in operation? Will they replace classical computers? What advantages do they offer?

To answer these questions it is necessary to previously introduce the concept of a quantum system . Without trying to go into technical details, a quantum system is one in which the states of its particles (or elements) are not predictable with certainty. If you drop a coin vertically and you know its mass, the time it takes to reach the ground and the distance to the ground, you can know the acceleration to which it is subjected. If you repeat the same experiment under the same conditions, you will get very even results (g = 9.81 m / s ^ 2 on the earth's surface).

Not in a quantum system. An experiment may give you a value and then, under the same circumstances, give you another value for nothing like the first. This state that you have just measured can also be a superposition of the rest of the possible states . That is why the possible results of a measurement of a quantum system are based on probabilities, that is, what options are there for a certain value to come out and what range of values we are considering. Therefore, a priori, in quantum all the values are valid and they are not, at the same time, until you carry out the measurement. The topic of quantum superposition is better to assume so as not to go into more detail and continue with the article.

### The qubit replaces the bit

This, extrapolated to the topic at hand, which is quantum computers, can be applied to qubits, which are the equivalent of bits in classical computers . Classical computation is based on bits, that is, the only two possible states of a measurement. These states are 0 or 1, there are no more possibilities, and 0 and 1 cannot occur at the same time, or it is one or the other. By having two possible states, we will only have one true result, since both are mutually exclusive. With one bit there can be two possible states, 0 or 1 (one result) , with 2 bits there can be 4 possible states, 0 or 1 for the first bit, and again 0 or 1 for the second bit (two results in total) .

Qubits are the analogs in quantum computing and, extrapolating what we have commented in the previous paragraph, a quantum state cannot be predicted with certainty, so with a qubit we now have as possible states 0 and 1 (two results) , both at the same time (quantum superposition). With two qubit we will have 0 and 1 for the first qubit, plus 0 and 1 again for the second, a total of 4 results. Generalizing, with bits we have a relationship of n: n (bits: results) while with qubits we have a relationship of n: 2 ^ n (qubits: results).

This is where the great computing potential of quantum computers lies, the qubit: results ratio increases exponentially, while the bit: results ratio increases linearly . That is, from a certain number of qubits, the computational capacity of the quantum computer increases compared to that of a traditional or classical computer.

Furthermore, the fact that a qubit contains several values at the same time makes a computational parallelism possible , that is, the possibility of simultaneously computing several operations, whereas a classical computer can only do them one at a time. To give an example, if we want to find out a password with a classic computer , brute force is used, that is, trying all the possible combinations character by character one by one until the right one is found. In a quantum computer, the same method is followed, but not one at a time, but several at a time, and also at the same time . This implies that a quantum algorithm using qubits as the fundamental unit of computation will break any current ciphers in the blink of an eye.

RSA encryption is currently used to keep us safe when surfing the net. This cipher is based on Shor's algorithm, which is a method of factoring whole numbers into products of prime numbers. Today these numbers are generated large enough that by brute force it is unfeasible to get anything out, but theoretically quantum computing could solve it without getting messy. From the previous sentence, the word "theoretically" should be highlighted, since right now there are no quantum computers with that capacity, that is known, so it has not been proven.

### What quantum computers exist today?

We have already seen some of the advantages of quantum computers over the classics and now it is worth wondering about the prototypes and developments that are being carried out in this field. Big tech companies are developing their own quantum computers . Companies like IBM, Intel and Google have a kind of race to make a computer of these characteristics viable. The calculation power is based on the number of qubits that they are capable of handling and we have already seen some models such as Sycamore, Google's, which with a capacity of 54 qubits has been able to perform a calculation that a normal computer would take 10,000 years to perform, in just 200 seconds. Intel, on the other hand, has shown this year 2020 its first quantum control chip, Horse Ridge , emerged through a collaboration between two other companies in the sector. This chip allows the integration of quantum processors of up to 128 qubits . On the other hand, D-Wave has proposed to researchers to use their quantum computers to fight covid-19.

### Quantum computer problems

There are many companies involved in this world, as it aspires to far exceed the computing power of today's computers, but some things should be clarified about these innovative computers. Although the computing capacity (what we call computing power) is measured according to the qubits that each QPU (Quantum Processing Unit, analogous to CPU) can handle, it must be made clear that these computers maintain their operation as long as certain terms. The most notable is the fact that the temperature to be maintained in the quantum environment must be very close to absolute zero. This implies that, at present, these computers are large machines highly thermally isolated (adiabatic) with cooling according to the requirements. To keep components close to absolute zerotoday it requires considerably large and expensive refrigeration, in other words: it is not feasible for domestic use (which we will talk about later). To get an idea, IBM quantum computers work with temperatures around 0.015 K, that is, -273 degrees Celsius. The integrity of the quantum system has recently been maintained by increasing the temperature to 1.5 K, which means a brutal jump from the 0.015 K mentioned above.

The fact that such low temperatures are needed is associated with quantum, of course. These systems are highly sensitive to disturbances , so each qubit must be almost perfectly isolated from the environment around it. This isolation is achieved by superconductors , which are achieved, in turn, by applying very low temperatures to certain materials. A disturbance, or interference, can cause the state of the qubit in question to change (due to its high sensitivity), which leads to erroneous results and, obviously, to a computer that is useless in practice. That is why such cooling is needed and, with it, a large physical space where to place the computer. Curiously, this is reminiscent of the beginnings of the processors that we carry in our hands today (the SoCs of smartphones), which previously occupied large rooms and entire floors of buildings, and also were much less powerful than current SoCs.

### The quantum computer replacing the classic

Having seen all the above, we end up wondering if a quantum computer can replace a classical computer. Quantum computing begins from a different base than the classical one , its operation is radically different and this entails developing a structure that supports all this new computing system. It is necessary to develop completely new algorithms for solving problems in quantum computers, and therefore in the short or medium term they are not expected to replace the current ones. Moreover, the quantum computers that exist today are capable of solving very specific problemsThey have nothing to do with the many years of development that we have had with classical computing, so no, at the moment it is not possible to imagine them in our homes. In the short or medium term they are not expected to be useful for the common population, but they are expected to be useful for research that requires endless calculations for normal computers.

Similarly, on the subject of computer security they aim quite high and, if developed as planned, they will represent a radical change in this field, providing perfectly secure systems if the ideal operating conditions are met (something not physically possible), although if they are not fulfilled, they will continue to represent an unimaginable advance in security.

After reading all these lines we hope that it has become somewhat clearer what quantum computers are, how they work (broadly speaking), their possible uses and if we will be able to see them in our homes for the rest of our lives. Of course, this field is in full development and, in addition, well promoted by the largest companies on the planet, so it is to be expected that in a few years we will have a much clearer vision of the possible uses of quantum computers in tasks of everyday life.

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