Qubit

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What is it?
A figure that compares two elementary units of information measurement: a bit, used in the work of ordinary computers, and a qubit, on the properties of which quantum computers are based.

A classic bit takes two values, that is, its physical medium can only be in two specific states. For example, if a transistor in the processor passes an electric current, then it takes on the value 1, if it does not pass it – 0. The bit is in a strictly defined state, there are no intermediate values ​​between 0 and 1 for it.

A qubit (from the English q-bit, quantum bit) can also take on the values ​​0 and 1, but, unlike a simple bit, it is not limited by them. If a qubit can be in any two basic states, then it can also be in a superposition of these states, that is, take on a huge set of intermediate values. It is convenient to represent the space of states of a qubit in the form of a Bloch sphere. At the north pole of the sphere, the value is 0, at the south pole – 1. But there is also the rest of the surface, which represents all kinds of states.

You can create a qubit from any quantum objects that have two basic states. For example, a spin ½ electron can be in two states: spin up and spin down. Any particle with this property, be it a photon, a neutral atom, or an ion, can act as a qubit. However, at this point in time, the most technologically advanced quantum computers operate on superconducting qubits – microcircuits made of superconductors with nanoscale discontinuities (Josephson junctions). A key advantage of superconducting qubits is the ability to fabricate them using streamlined processes used to create microelectronics.

Why is this interesting for science?
The main problem facing the development of quantum computers is the loss of coherence by qubits. Any quantum system will inevitably interact with the environment, as a result of which uncontrolled changes in the states of qubits occur. As a result, the likelihood of errors in calculations increases significantly. In addition, the low coherence of the qubit as a whole severely limits the number of operations a quantum computer can perform.

Scientists are trying to solve this key limitation of qubits by creating “complex” logical qubits, which will consist of several physical ones. If a few of them lose coherence, then the rest will continue to complete the task anyway. If such complex systems can be obtained, then as technology develops, there will be a chance to obtain error-free quantum computers capable of an unlimited number of operations.

The efficiency of quantum computers in solving problems of this type is so great that it is called quantum superiority. A quantum computer can take several minutes to solve certain brute-force problems, while the most powerful classical supercomputer can take over a year. This superiority can be most useful for modeling the chemical and physical properties of particles, optimizing the construction of complex graphs, creating advanced encryption and decryption methods.

Why is it important to know?
Because the qubits are in superposition, quantum computers can perform certain tasks many times faster by performing multiple operations in parallel. A good example of the benefits of parallelization is pathfinding in a maze. A conventional computer sequentially goes through all possible options, running into dead ends and returning, while a quantum computer can check all possible moves in one go.