How to build a quantum computer06/11/2019
Building the most powerful quantum computer requires specific technology, but also a super specialized team that makes its operation possible. Credit: IBM.
ISABEL RUBIO ARROYO | Tungsteno
Google may have just achieved one of the greatest advances in the history of quantum computing. The company from Mountain View, California claims to have successfully completed in three minutes and 20 seconds an operation to calculate random numbers that would have taken the world's most powerful supercomputer about 10,000 years. The research, published in the scientific journal Nature, would be the first empirical demonstration of the concept of quantum supremacy, which is the ability of a quantum computer to perform a task impossible for all the classical computers to accomplish working together.
But like Google, there are other companies also participating in a never-ending race to build the most powerful quantum computer. IBM, which has disputed Google’s claim to have achieved quantum supremacy, has almost a dozen devices of different sizes open to the public through contracts or via its quantum computing open software. The companies Rigetti Computing and IonQ also offer such computers under specific collaboration agreements. But Juan José García Ripoll, a researcher at the Institute of Fundamental Physics at the Spanish National Research Council, explains that it is difficult to know the exact number of quantum computers that exist in the world: "There are research laboratories to which we don’t have access, either in defence companies or linked to the U.S. government."
These computers need very special conditions to work, from extremely low temperatures to an isolated environment. Credit: IBM.
Beyond the speed of operation
Nowadays these devices only serve to test algorithms in small problems, but it is hoped that in the future they can be applied to solving larger and larger questions. They could be useful in everything from designing molecules or studying chemical reactions to solving specific and complex problems in sectors such as medicine, financial risks or materials science.
Although the quantum computer tends to be thought of as an evolution of the traditional computer, Ripoll stresses that it does not even have the same architecture: "A quantum computer does not have a RAM memory, a hard disk and a processor. Instead, it consists of microscopic and macroscopic elements. The largest element, as the researcher explains, is the dilution refrigerator, a huge fridge that uses helium-3 and helium-4 to reach extremely low temperatures —dozens of millikelvin— at which quantum effects occur.
This refrigerator is connected to a rack where generators and microwave detectors are stored: "These electronic devices are not very different from those used to design and calibrate mobile phones and are responsible for telling the quantum computer the operations it has to do and measuring what state it is in."
Finally, inside the refrigerator and connected to the outside world through coaxial cables and filters, is the superconducting chip, "which is the quantum computer itself." "This chip also has many elements. There are these small microwave oscillators or qubits that, with a non-linear response, can store 0 or 1 photon, or a quantum superposition of both states," he explains.
While traditional computers use bits, quantum computers use qubits. Traditional bits store information as 0 and 1. Qubits can be 0 and 1 at the same time thanks to a phenomenon known as superposition. In this way, the amount of information that can be accumulated grows exponentially. Ripoll adds that qubits are connected to each other through capacitors and connect to the outside world through antennas —resonators— that can alter their state or measure it.
These computers require elements of the most standard technology to the most innovative, such as the superconducting chips composed with qubits. Credit: Google
Technologies and manufacturing costs
To manufacture each part mentioned and the quantum computer as a whole, there is no single technology. For example, the technology used by Google "is quite standard and very similar to other types of chips we use in our daily lives." While there are quantum computers that trap atoms in a vacuum chamber and operate with them, others employ defects in semiconductors.
Superconducting quantum computers use electrical circuits built from aluminium: "If we work at room temperature, these circuits are electrical processors, capable of generating, transmitting, and filtering energy in the form of microwaves, typically at gigahertz frequencies. However, when we cool the entire system, the circuit becomes superconducting and demonstrates quantum effects. In particular, the circuit absorbs and emits energy in the form of microwave photons, the particles that make up this radiation. With these photons we carry out quantum computation."
Contrary to what one might think, Ripoll says that building a quantum computer is not "too expensive". A refrigerator and the electronics to control about five qubits can cost around two million euros —the amount of a starting grant from the European Research Council (ERC). If you want to make larger devices, with more qubits, the cost may go up because of the need to use more microwave generators and a larger refrigerator, but not that much," he explains. For him, the main cost is the skilled labour and know-how. In other words, it is the codes and routines that have to be developed for such a complex experiment to work that are expensive.
Low temperatures and super-specialized equipment
These computers need very special conditions to work. Apart from the extremely low temperatures —less than – 273 ºC — already provided by the refrigerator, it is important to have an environment isolated from intense electromagnetic disturbances. The researcher explains that superconducting chips fluctuate and the qubits can change their energy over time due to fluctuations in temperature, tensions in the material, environmental effects or simply the aging of the material. All this makes it necessary to carry out a periodic calibration every few minutes or hours to make sure that the parameters are correct.
To ensure that the conditions under which a quantum computer operates are ideal and to find possible applications, there is a whole team of professionals behind it. Quantum physicists, materials engineers, microwave engineers, electronic engineers and programmers are in charge of the hardware development. Meanwhile, theoretical physicists, computer scientists and mathematicians are in charge of the development of applications. They all work in laboratories with a common goal: to achieve a new era of computing.
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