Quantum computers. Why are they not yet, although they already exist?

Fifty years ago, smartphones would have seemedtotally magical computers. Just as classic computers were almost unimaginable for previous generations, today we are faced with the birth of a completely new type of computing: something so mystical that it can be called magical. These are quantum computers. If the word "quantum" is unfamiliar to you, you are not alone. This very cold, small, sensitive and very strange world may seem a dubious system on which it is proposed to build a commercial computing machine, but this is exactly what IBM, Google, Rigetti Computing and other companies are working on.

In January at CES as part of the IBM Q initiativeshowed System One (see above): a dazzling, elegant and chandelier-like machine that became the first integrated universal quantum computing system for commercial use that everyone could play with.

About the potential of quantum computers heard,probably everyone: the properties of quantum physics open up massively parallel computational schemes, which are likely to provide tremendous leaps in computing power and outrun any transistor supercomputers that we may encounter today and tomorrow. They will revolutionize chemistry, pharmaceuticals, materials science, and machine learning.

But what exactly makes quantum computers so powerful? Let's figure it out.

What are qubits?

First, let's remember how quantum computers work.

The secret of their skill is that they manipulatequbits. Everything that processes a classic computer — text, images, video, and so on — consists of long lines of zeros and ones, or bits. In its essence, a bit represents one of two states: on / off, or an electrical circuit is connected, or not. In modern computers, bits are usually represented by electrical voltage or a current pulse.

Quantum computers, on the other hand, rely onqubits. Like binary bits, qubits underlie calculations, with one big difference: qubits are usually superconductors of electrons or other subatomic particles. It is not surprising that manipulations with qubits represent a complex scientific and engineering problem. IBM, for example, uses several layers of superconducting circuits that are in a controlled environment and are gradually cooled to temperatures that are lower than deep space — near absolute zero.

Since qubits live in quantum reality, they have amazing quantum properties.

Superposition, entanglement and interference

If a bit is presented as a coin with an eagle (0) orwith tails (1), qubits will be represented by a spinning coin: in a sense, they are both eagles and tails at the same time, each state having a certain probability. Scientists use calibrated microwave pulses to put qubits in superposition; in the same way, other frequencies and the duration of these pulses can turn the qubit so that it is in a slightly different state (but still in superposition).

Because of the superposition, a separate qubit canrepresent much more information than the binary bit. This is partly due to the fact that with the initial input qubits can iterate through the brute force method a huge number of possible results simultaneously. The final answer appears only when scientists measure qubits — also using microwave signals — which causes them to “collapse” into a binary state. Often, scientists have to make calculations several times to check the answer.

Confusion is an even more awesome thing. Applying microwave pulses to a pair of qubits can confuse them so that they will always exist in the same quantum state. This allows scientists to manipulate pairs of entangled qubits, simply changing the state of one of them, even if they are physically separated by a large distance, hence the "eerie action at a distance." Due to the predictable nature of entanglement, adding qubits exponentially increases the computational power of a quantum computer.

Interference is the last of the properties thatimplement quantum algorithms. Imagine rolling waves: sometimes they push each other (act constructively), sometimes quench (destructively). The use of interference allows scientists to control states, amplifying the type of signals leading to the correct answer, and canceling those that give the wrong answers.

How are quantum computers programmed?

The main goal is to encodeportions of the problem into a complex quantum state using qubits, and then manipulate this state to bring it to a solution that can be measured after the collapse of superpositions into deterministic sequences of zeros (0) and ones (1).

Unclear? Re-read again.

It sounds difficult, but since we have already understood all the terms, it is possible to understand.

As with classic programming,Scientists are developing low-level assembly languages ​​that the machine understands better in order to move from them to high-level languages ​​and graphical interfaces that are more appropriate for the human mind. IBM Qiskit, for example, allows experimenters to create tasks and drag and drop logic elements.

Daemon decoherence

Why are quantum computers not yet being sold?on every corner? In a sense, scientists are trying to build perfect machines from imperfect parts. Quantum computers are extremely sensitive to disturbances, noise and other environmental influences that cause their quantum state to oscillate and disappear. This effect is called decoherence.

For some experts, decoherence isproblem holding back quantum computing. Even with all the measures taken, noise can leak into the calculations. Scientists can store quantum information until it loses its integrity under the influence of decoherence, which limits the number of calculations that can be performed in a row.

The delicate nature of quantum computing is alsothe reason why blind adding qubits to a system will not necessarily make it more powerful. Fault tolerance is thoroughly investigated in the area of ​​quantum computing: logically, adding qubits can compensate for some problems, but to create a single, reliable qubit for data transfer will require millions of corrective qubit errors. And today we have no more than 128 of them. Perhaps smart algorithms that are also being developed will help.

Quantum imitation with quantum computers

Since big data is now a hot topic, one would expect quantum computers to better handle large data sets than classic ones. But it is not.

Instead, quantum computers will be especiallygood at modeling nature. For example, quantum computing could be used to more efficiently build drug molecules, because they mostly work on the same basis as the molecules that they are trying to model. Calculating the quantum state of a molecule is an incredibly difficult task, which is almost impossible for our computers, but quantum computers can do it with a bang.

Similarly, quantum computing canflip the field of materials science or information transfer. Due to entanglement, qubits, physical separated by a large distance, can create a channel for transmitting information that is scientifically safer than our existing channels. Quantum Internet is quite feasible.

But the most interesting thing is: we do not even know the whole variety of amazing questions that quantum computers can try to solve. Just by having a commercial quantum computer and allowing people to work with it, we could map out new interesting areas suitable for this amazing new technology.

And what problems would you try to solve on a quantum computer? Tell us in our chat in Telegram.