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08 Jul 2024

In search of the best qubits, the 'bricks' of quantum computing

Imagine having a computer capable of performing calculations at stratospheric speed. Or of immediately solving problems that a 'normal' computer would take the same age of the universe to solve. Sounds interesting, doesn't it?

Candelabros dorados hacen un prisma para refrigerar ordenador cuántico

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Article written by María Benito, CSIC's  predoctoral researcher at the Institute of Microelectronics of Barcelona (IMB-CNM), for the "Ciencia para llevar" blog (20 minutos).

Imagine having a computer capable of performing calculations at stratospheric speed. Or of immediately solving problems that a 'normal' computer would take the same age of the universe to solve. Sounds interesting, doesn't it?

The quantum computer promises to make all this possible by replacing the 'ordinary' bits, the basic units of information or 'bricks' with which conventional computers operate, with 'magical' qubits. Unlike bits, which can only take values 1 or 0, qubits can be simultaneously in a superposition of states or intertwined... And that would allow very complex calculations to be performed immediately and efficiently.

And why don't we already have quantum computers in our homes? Well, because, although work has been going on for more than 20 years to make them a reality, their bricks are more delicate than traditional ones. I'm sure you've dropped your cell phone on the floor, but when you picked it up, you were relieved to see that it was still working normally. I'm sorry to say that today's quantum computers are much more fragile...

The quantum computer promises to make all this possible by replacing the 'ordinary' bits, the basic units of information or 'bricks' with which conventional computers operate, with 'magical' qubits. Unlike bits, which can only take values 1 or 0, qubits can be simultaneously in a superposition of states or intertwined... And that would allow very complex calculations to be performed immediately and efficiently.

And why don't we already have quantum computers in our homes? Well, because, although work has been going on for more than 20 years to make them a reality, their bricks are more delicate than traditional ones. I'm sure you've dropped your cell phone on the floor, but when you picked it up, you were relieved to see that it was still working normally. I'm sorry to say that today's quantum computers are much more fragile...

Obsessed with noise

There are different types of qubits. At the Institute of Microelectronics of Barcelona (IMB-CNM, CSIC), we manufacture superconducting qubits. To have this property, superconductivity, they have to be cooled below their critical temperature: 1.2 degrees Kelvin in the case of aluminum.

The shiny golden chandeliers you see in the picture fulfill this function: they hide several floors or levels at falling temperatures: 4 degrees Kelvin, 1 degree Kelvin... up to 0.02 degrees Kelvin! To give you an idea, the vacuum of outer space has the lowest possible temperature: 0 degrees Kelvin, or -273.15 degrees Celsius, a state characterized by the absence of energy.

Those of us who work on the quantum computer are obsessed with reaching these very low temperatures, which require careful thermal insulation from the outside. And also with 'shielding' the inside of these computers to avoid any other type of interference, or what we consider noise: cosmic radiation, nuclear radiation, photons, magnetic fields...

And you will think: "what paranoid people...". Maybe so, but there is an explanation.

Inside these candlesticks there can be thousands of qubits performing continuous operations. And, unfortunately, it often happens that these qubits experience a phenomenon called 'decoherence', whereby they lose their quantum state. In other words: what makes them 'magical', systems capable of being in a superposition of states, vanishes completely.

Without going into too much detail, all these sources of noise accelerate, through different mechanisms, the decoherence of superconducting qubits. We therefore seek to suppress them as much as possible in order to have coherent qubits for as long as possible.

Quality qubits

They have tried everything from shielding with materials that do not allow the magnetic field to penetrate to building these computers underground to curb cosmic radiation. But they keep failing... Could the answer lie within the cage itself? That is, could we improve the quality of the qubits from their constituent materials?

"You can paint your house with the best paint that, if the foundations are not good, you will never get the best result". This sentence can help us to exemplify what happens in a quantum computer: "You can apply thousands of algorithms to avoid decoherence, but, if the qubit is not good, you will never achieve the computing power you need".

That's why at IMB-CNM we are working to find the best recipe for developing qubits that resist or minimize decoherence.

Our superconducting qubits consist of a chip that includes a pair of 'Josephson junctions'. The chip is made of a substrate and a thin layer of aluminum or other superconducting material. Josephson junctions also consist of two layers of aluminum or other superconducting material separated by an insulator, which can be aluminum oxide itself.

To find the qubit we are looking for, we use different substrates (the 'foundation' of an integrated circuit) or change the superconducting material we deposit. It is not simple, because something as 'silly' as the type of cleaning you do beforehand, or the seconds you take at each stage of the process, are decisive for the final result. That's why we have to work in a clean room, a very controlled environment that serves to minimize the number of contaminating particles. Each test is a challenge, but also an exciting experience.

Will we succeed? It is difficult to predict, but if humans were able to build skyscrapers using reinforced concrete instead of wood, one day we will be able to make the best bricks for quantum computing.