Finland brings cryostats and other cool things to quantum computing

Low-temperature physics yields a market-leading spin-off

Bluefors started business with the idea of adding closed-loop dilution refrigeration after pulse tube cooling. “In 2005 and 2006, pulse tube coolers became more powerful,” says David Gunnarsson, CTO at Bluefors. “We used pulse tube coolers to pre-cool at the first two stages, which takes you down to around 3 kelvin. We get the pulse tube coolers from an American company called Cryomech.

“Bluefors’ key differentiator is a closed-loop circulation system, the dilution refrigerator stages, where we circulate a mixture of helium 4 and helium 3 gas. At very cold temperatures, this becomes liquid, which we circulate through a series of well-designed heat exchangers. This approach can get the temperature down to below 10 milli-kelvin. This is where our specialty lies – going below the 3 kelvin you get from off-the-shelf coolers.”

Cryogenic systems are tailored for use cases

Bluefors has more than 700 units on the market that are used for both research in publicly funded organisations, and for commercial research and development. One big market that has driven the dilution refrigeration is quantum computing. Anyone currently doing quantum computing based on superconducting qubits is most likely to have a Bluefors cryogenic system.

When a customer recognises the need for a cryogenic system, they talk to Bluefors to decide on the size of the refrigerator. This depends on the tasks they want to do and how many qubits they will use. Then they start looking at the control and measurement infrastructure, which must be tightly integrated with the cryogenic system. Some combination of different components and signalling elements might be added, depending on the frequencies being used. If the control and measurement lines are optical, then optical fibres are included.

As soon as Bluefors and the customer reach an agreement, Bluefors begins to produce the cryogenic enclosure, along with a unique set of options tailored to the use case. Bluefors then runs tests to make sure everything works together – and that the enclosure reaches and maintains the temperatures required by the application.

The system has evolved since the company first started marketing its products in 2008. To cool down components with a dilution refrigerator, Bluefors uses a cascade approach, with nested structures that drop an order of magnitude in temperature at each level. The typical configuration includes five stages, with the first stage now bringing the temperature down to 50 kelvin. The temperature goes down to about 4 kelvin at the second stage, and reaches 1 kelvin at the third. It then drops to 100 milli-kelvin at the fourth stage, and at the fifth stage gets down to 10 milli-kelvin, or even below.

The enclosure can cool several qubits, depending on the power dissipation and the temperature the customer needs. A challenge here is that the more power dissipates, the higher the temperature is raised, and every interaction can increase the temperature.

“Our most powerful model today can probably run a few hundred qubits in one enclosure,” says Gunnarsson. “IBM has just announced it has a system with 127 qubits. We can handle that many in one enclosure using the most powerful system we have today.”

In most architectures, quantum programs work by sending microwave signals to the qubits. The sequence of signals constitutes a program. Then you have to read the outcome at the end.

“The user typically has a microwave source at room temperature,” says Gunnarsson. “Usually, when it reaches the chips, it’s at power levels of the order of pico-watts, which is all that is needed to drive a qubit. Pico-watts are one trillionth of a watt – a very small power requirement.

“That is also a power that is very hard to read out at room temperature. So to read the output from a chip, the signal has to be amplified and taken back up to room temperature. A cascade of amplification is required to get the signal to the level you need.”

The microwave control signals and the read-out process at the end constitute a cycle that lasts about 100 nanoseconds. Several such cycles occur per second, collectively making up a quantum program.

Running electronics is challenging at low temperatures

Another challenge for quantum computing is to get electronics inside the refrigerators. All operations are performed at very low temperatures, but then the result has to be taken up to room temperature to be read out. Wires are needed to start a program and to read results. The problem is that electrical wires generate heat.

This means that quantum computing lends itself only to programs where the results are not read out until the end – one of many reasons interactive application such as Microsoft Excel will never be appropriate for the quantum paradigm.

It also means that every qubit needs at least one control line and then one readout line. Multiplexing can be used to reduce the number of readout lines, but there is still a lot of wiring per qubit. The chips themselves are not that large – what takes up most space are all the wires and accompanying components. This makes it challenging to scale up refrigeration systems.

“Since Bluefors supplies the cryogenic measurement infrastructure, we developed something we call a high-density solution, where we made it possible to have a six-fold increase in the amount of signal lines you can have in our system,” says Gunnarsson. “Now you can have up to 1,000 signal lines in a Bluefors state-of-the-art system using our current form factor.”

One very recent innovation from Bluefors is a modular concept for cryostats, which is used by IBM. The idea is to combine modules and have information exchanged between them. “This modular concept is going to be an interesting development,” says Aalto University’s Hakonen, who since the 1970s has enjoyed a front-row view of the development of quantum technology in Finland.

Finland prepares to export quantum algorithms

Finland has a very strong tradition in quantum theory in general – and specifically, the quantum physics used in superconducting qubits, which is the platform used by IBM and Google. Now a large area of active research is in quantum algorithms.

“How one goes about making a program is a key question,” says Sabrina Maniscalco, professor of quantum information and logic at the University of Helsinki. “Nowadays, the situation is such that programming quantum computing is much more quantum theory-related than any software ever managed or developed. We are not yet at a stage where a programming language exists that is independent of the device on which it runs. At the moment, quantum computers are really physics experiments.”

Finland has long been renowned worldwide for its work in theoretical quantum physics, an area of expertise that plays nicely into the industry growing up around quantum computing. Two other factors that contribute to the growing ecosystem in Finland are the willingness of the government to invest in blue-sky research and the famed Finnish education system, which provides an excellent workforce for startups.  

The country’s rich ecosystem of research, stable political support and the education system have resulted in the birth and growth of many startups that develop quantum algorithms. This seems like quite an achievement for a country of only five million inhabitants. But in many ways, Finland’s small population is an advantage, creating a tight-knit group of experts, some of whom wear several different hats.

Maniscalco is a case in point. In addition to her research into quantum algorithms at the University of Helsinki, she is also CEO of quantum software startup Algorithmiq, which is focused on developing quantum software for life sciences.

“We are trying to make quantum computers more like standard computers, but it’s still at a very preliminary stage”

“As a researcher, I am first of all a theorist,” she says. “I don’t get involved in building hardware, but I have a group of several people developing software. Quantum software is as important as hardware nowadays because quantum computers work very differently from classical computers. Classical software doesn’t work at all on quantum systems. You have to completely change the way you program computers if you want to use a quantum computer.

“We are trying to make quantum computers more like standard computers, but it’s still at a very preliminary stage. To program a quantum computer, you need quantum physicists who work with computer scientists, and experts in the application domain – for example, quantum chemists. You have to start by creating specific instructions that make sense in terms of the physics experiments that quantum computers are today.”

Algorithm developers need to take into account the type of quantum computer they are using – the two leading types are superconducting qubits and trapped ions. Then they have to look at the quality of the qubits. They also need to know something about quantum information theory, and about the noise and imperfections that affect the qubits – the building blocks of quantum computers.

“Conventional computers use error correction,” says Maniscalco. “Thanks to error correction, the results of the computations that are performed inside your laptop – or any computer – are reliable. Nothing similar currently exists with quantum computers. A lot of people are currently trying to develop a quantum version of these error correction schemes, but they don’t exist yet. So you have to find other strategies to counter this noise and the resulting errors.”

Overcoming the noisiness of the current generation of qubits is one of many challenges standing in the way of practical quantum computers. Once those barriers are lifted, the work Maniscalco and other researchers in Finland are doing on quantum algorithms will certainly have an impact around the world.