Quantum Computing and Cleantech
In the future, quantum computers could execute tasks that would take a lifetime even for a current supercomputer, from decryption of highly encrypted data to development of complex medicines. For cleantech, the advent of quantum computing is expected to offer a major benefit, as it will permit even larger more detailed simulation of everything from chemical reactions to traffic control patterns. Such modeling can, for example, reduce the need for energy expenditures used for real-life physical research, or it could offer major fuel savings through optimization of traffic flow.
Cooling is Key
Part of the challenge, however, is that intensive and effective cooling is needed to achieve reliable quantum computing results, which can be a drain on power, especially as the computers are scaled up for larger tasks. Currently available versions of early “quantum-style” computers such as those from Canadian firm D-Wave, consume as much as 25 kiloWatts at any given time to cool the processors — the equivalent of 250 100-Watt light bulbs. The D-Wave system uses tiny loops of niobium cooled to close to absolute zero (-459.6F) by liquid helium.
Now, scientists at Aalto University, Finland have invented a quantum-circuit refrigerator that is expected to speed up the process of developing a reliable true quantum computer, and which may succeed in raising the energy efficiency level as computers are scaled up to handle larger operations.
Quantum bits, qubits, are the key element of the quantum computer that can be utilized to store and process quantum information. Also, just like the bits in conventional processors in the computers that we use today, qubits need to be chilled in order them to function properly in quantum devices. However, cooling the qubits in the quantum computer isn’t as easy as cooling your laptop bits. The bits being squeezed in your laptop are either zeros or ones, whereas a qubit can exist simultaneously in both states. This versatility of qubits is essential for complex computing, but it also makes them sensitive to external disruptions like heat.
To calculate and analyze the enormous amount of data, quantum computers need thousands or even millions of logical qubits to be used simultaneously in computation and to obtain the correct result. However, every qubit has to be reset at the beginning of the computation. The problem is that if the qubits are too hot, they cannot be initialized (reset) because they are switching between different states and behaving – calculating and analyzing – unpredictably.
Quantum scientist Mikko Möttönen and his Quantum Computing and Devices (QCD) team in Aalto University, believe they have a solution to this problem – a nanoscale quantum-circuit refrigerator, which forces these too excited qubits to behave.
Watch the quantum physicists explain the operating principle of the refrigerator in two minutes.
The QCD team used a qubit-like superconducting resonator in their research, and now the plan is to test the refrigerator with actual quantum bits. They made the resonator out of a centimeter-long superconductor where electric current and voltage oscillate ten billion times a second. In this quantum-circuit refrigerator, the resonator works much like the actual bits.
Kuan Yen Tan, a postdoctoral researcher in Möttönen’s group who has been working on the project for over five years, cooled down a resonator utilizing the tunneling of single electrons through a two-nanometer-thick insulator. Tunneling refers to the quantum mechanical phenomenon where a particle moves through a barrier, which should not be possible based on classical physics. In conventional physics, if a particle doesn’t have enough energy to overcome a barrier, it simply won’t go through. However, particles often behave like waves in the quantum world.
Stealing Energy to Keep Devices Cool
Tan used an external source to give the electrons slightly less energy than what was required for direct tunneling. As a result, the electron captures the missing energy required for tunneling from the nearby quantum device, causing the quantum device to lose energy and cool down. The cooling can be switched off by adjusting the external voltage to zero. Then, even the energy available from the quantum device is not enough to push the electron through the insulator.
In other words, the refrigerator shoots the electrons towards the insulator in close proximity to the qubit, and the qubit pushes the electrons through. This sucks the energy out of the qubit and cools it down. The electron shooting can be then stopped, and the qubit is ready for reset. With the help of the refrigerator, most electrical quantum devices can be initialized quickly, and the devices become more powerful and reliable.
A Path to Energy Efficiency?
While the technique doesn’t necessarily reduce the amount of power needed for cooling, the Aalto quantum-circuit refrigerators could enable the ability to have more logical qubits on a chip to help the physical qubits work more effectively. This would reduce the amount of power consumed per logical qubit. Additionally, according to a theoretical paper published last month, the team believes that they have a method to avoid excess heat dissipation on the qubit chip. The energy savings here come from the same principle: that using reduced amounts of energy per gate operation means that with the same total system power consumption, you may have a more computationally powerful quantum computer.
“Our refrigerator keeps quanta in order,” sums up Professor Möttönen.
Kuan Yen Tan, Matti Partanen, Russell E. Lake, Joonas Govenius, Shumpei Masuda and Mikko Möttönen. Quantum-Circuit Refrigerator. Nature Communications 8, DOI:10.1038/ncomms15189
Joni Ikonen, Juha Salmilehto & Mikko Möttönen. Energy-efficient quantum computing. npj Quantum Information 3, Article number: 17 (2017), doi:10.1038/s41534-017-0015-5