What is a quantum computer and what does it consist of? Not all computers are entitled to such a name. Why this is so and why such installations are needed, explains Christopher Monroe, professor at the University of Maryland and one of the leading players in the global "quantum race".
The Russian Quantum Center regularly holds major international conferences in Moscow dedicated to the development of quantum technologies and their practical application. Not only leading researchers take part in its work, but also representatives of large Russian and foreign business and government officials.
This year, the conference was attended by the leaders of three scientific teams leading in the creation of complex quantum computing systems. In addition to Mikhail Lukin, a professor at Harvard University (USA), who first announced the creation of a record-breaking 51-qubit computer at the previous conference, Professors Christopher Monroe and Harmut Neven took part in it.
Monroe, who works today at the University of Maryland (USA), created a machine of similar power almost simultaneously with his Russian-American counterpart, using similar, but slightly different principles.
He spoke about the direction in which this system is developing, how it differs from "competitors" and where the border lies between real quantum computers, which fully correspond to this term, and computing systems that are built on the basis of classical principles.
Quantum superiority
Quantum computers are special computing devices whose power grows exponentially due to the use of the laws of quantum mechanics in their work. All such devices consist of qubits - memory cells and at the same time primitive computing modules capable of storing a range of values between zero and one.
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Today, there are two main approaches to the development of such devices - classical and adiabatic. Proponents of the first of them are trying to create a universal quantum computer, in which qubits would obey the rules by which ordinary digital devices work. Working with such a computing device, ideally, would not be very different from how engineers and programmers operate conventional computers.
An adiabatic computer is easier to create, but it is closer in its principles of operation to adding machines, slide rule and analog computers of the early 20th century, and not to digital devices of our time. There are also hybrid approaches that combine the features of both machines. Among them, according to Monroe, can be attributed to the computer of Mikhail Lukin.
According to Monroe, this is due to the fact that the memory cells in his machine are built on the basis of ions of the rare earth metal ytterbium, whose state does not change when manipulated with laser beams. Lukin's quantum computer, in turn, is built on the basis of the so-called Rydberg atoms, which are not protected from such influences.
They are atoms of rubidium-87 or other alkali metals, whose free electron has been "pushed" a great distance from the nucleus using special laser or radio wave pulses. Because of this, the size of the atom increases by about a million times, which turns it into a qubit, but, as Monroe explained, does not allow it to be moved without deforming this structure and destroying quantum states.
The absence of such problems in ions, according to the American physicist, allowed his team to create not a hybrid, but a fully controlled quantum computer, whose qubits scientists can manipulate directly in the course of computing.
For example, three years ago, long before the creation of larger machines, Monroe and his team announced that they had managed to create the first reprogrammable quantum computer, which consisted of five memory cells. This modest machine, thanks to its high flexibility, allowed physicists to execute several quantum programs on it at once.
In particular, they managed to run the Deutsch-Jozy, Bernstein-Vazirani algorithms on this mini-computer, as well as create a quantum version of the Fourier transforms, the cornerstone of cryptography and its breaking.
These successes, as well as the difficulties in keeping large numbers of ions in traps, Monroe notes, prompted him to think that quantum computing systems should be built modular rather than monolithic. In other words, "serious" quantum computers will not represent a single whole, but a kind of network, consisting of many similar and fairly simple modules.
Imperfect vacuum
Such systems, as noted by the American professor, already exist, but are not yet used in prototypes of quantum computers for one simple reason - they work about a hundred times slower than the qubits themselves. Nevertheless, he believes that this problem is completely solvable, since it has an engineering rather than a scientific nature.
Another potential problem that will interfere with the operation of monolithic or just large quantum computers is that the vacuum, as Monroe put it, is not perfect. It always contains a small number of molecules, each of which can collide with atomic qubits and interfere with their work.
The only way to overcome this is to further cool the quantum computer, as close as possible to absolute zero. Monroe's team is not yet engaged in this, since the number of qubits in their machine is small, but in the future this problem will definitely have to be solved.
The modular approach, as suggested by the American professor, will be another way to solve this problem, since it will allow breaking the computer into many independent parts containing relatively small numbers of qubits. In theory, it will not run as fast as a monolithic machine, but it will circumvent the problem of "imperfect vacuum", since the modules will be easier to cool and control.
When will this time come? As Monroe suggests, in the next three to five years, machines will be created that include several hundred qubits. They will be capable of performing several tens of thousands of operations and will not require extreme cooling or error correction systems to operate.
Such machines will be able to solve many complex practical problems, but they will not be full-fledged computers in the classical sense of the word. To do this, you will need to increase the number of qubits and “teach” them to independently correct mistakes in their work. This, according to the physicist, will take another five years.
Final stretch of the race
The first complex quantum computers, according to Monroe, will be built on the basis of ionic or atomic technologies, since all other variants of qubits, including promising semiconductor memory cells, have not yet reached a similar level of development.
“So far, these are all university laboratory experiments. These qubits cannot be used to create complete logic gates. Therefore, I agree with Mikhail that our colleagues from Australia, Intel and other teams will have to solve many practical problems before they can create a full-fledged computing system,”the physicist notes.
How to determine the winner in this "quantum race"? Two years ago, Monroe and his colleagues tried to answer this question by organizing the first comparative testing of quantum computers. They chose an IBM quantum computer based on superconducting qubits as a competitor for the first version of their machine.
To compare them, physicists and programmers from the University of Maryland prepared the first set of "quantum benchmarks" - simple algorithms that measure both the accuracy and speed of these computers. The test did not reveal a direct winner - the computer of Monroe and his team won exactly, but lost in speed to the IBM machine.
At the same time, Monroe believes that the so-called quantum superiority - the creation of a quantum computer, the behavior of which cannot be calculated by other methods - will not be some serious scientific or practical achievement.
“The problem lies in the concept itself. On the one hand, our experiments with five dozen qubits, like Mikhail's experiments, helped to calculate those things that cannot otherwise be calculated. On the other hand, this cannot be called superiority, since we cannot prove that it really cannot be calculated in other ways. Quantum superiority will appear sooner or later, but personally I am not going to chase after it,”the scientist emphasized.
Another difficulty lies in the fact that we cannot yet say for sure what problems quantum computers can solve and where their application will be most justified and useful. For this, it is necessary that both the scientific environment and the whole society begin to perceive such machines as an affordable and universal tool.
Quantum mysteries of the universe
For this reason, the American professor does not believe that adiabatic computing systems like the D-Wave devices can be called quantum computers. Their work, according to the physicist, is based on completely classical physical principles that have nothing to do with real quantum mechanics.
“Despite this, analog computers like these are extremely interesting from a practical point of view. You can simply take a few magnets, attach them to a triangular mesh, and trace their behavior. These experiments will have nothing to do with quantum physics, but they will allow for some complex optimization calculations. Investors are interested in them, which means that this is not done in vain,”continues the professor.
What tasks can a "real" quantum computer solve? As Monroe noted, in recent years, many other teams of physicists have contacted his team. They plan to use their machine to solve many important scientific problems that cannot be calculated on a conventional computer.
So far, the same experiments, as the physicist admitted, can be carried out on ordinary supercomputers. On the other hand, in the coming years, the number of qubits in quantum machines will increase significantly, which will make their work uncountable.
This will expand their applicability and make such experiments one of the most interesting and unique ways to study the largest and most mysterious objects in the Universe, as well as solve many everyday tasks, such as finding routes or managing the economy, the researcher concludes.
