Scientists in Finland have succeeded in transporting maximal heat effectively 10,000 times further than ever before, possibly a major step in the development of super-efficient quantum computers. A research group at the Aalto University in Finland was able to use a photon carrier method and to measure quantum-limited heat transport over distances up to a meter. Previously scientists had been able to measure such heat transport only up to distances of about 100 microns, which is comparable to the thickness of a human hair.
The Quantum Computing and Devices (QCD) group led by Prof. Mikko Möttönen has been able to show that quantum-limited heat conduction is possible over long distances. For the developers of quantum computers this is groundbreaking news.
A fully functional quantum computer is still many years away. However, it would be useful to speed up this progress, since there are several tasks for which a quantum computer would be more useful and more efficient than the conventional computer. Quantum computing uses the quantum nature of matter, the atoms themselves, as computing devices, to execute tasks that would take a lifetime even for a current supercomputer. The quantum machines would be useful for scientists in conducting virtual experiments, handling huge amounts of data or rapidly finding a single data point in a vast quantity of data.
A solution for cooling the quantum computer?
However, a quantum computer could be prone to errors due to external noise and interference including heat, and will require efficient cooling in order to operate properly. The QCD research group’s remarkable innovation may be utilized to take heat away from quantum processors so cleverly that the operation of the quantum computer is not disturbed.
Cooling the quantum computer is more difficult than cooling standard computers. Fans won’t work since many of the conventional cooling systems can create more heat and disturb the delicate processes. This means that quantum computers must work at very low temperatures in order to keep interference to a minimum.
Aalto University and QCD team’s innovation revolutionizes heat conduction through efficient heat transport from point A to point B. The scientists utilized photons as heat carriers rather than the electrons used in the past to transfer heat. They built a superconducting transmission line, which works in ultra-cold temperatures, on top of a silicon chip just a square centimetre in size. Resistors made of aluminum wire were placed at the ends of the line in order to measure the temperature variation. Results were obtained by measuring induced changes in the temperatures of these resistors. Möttönen and his team reported measurement of quantum-limited heat transport over distances up to a meter, which is an extremely long distance for the computer processors. Möttönen says “nobody wants to build a larger processor than that.”
“Our research started…in 2011 and advanced little by little. It feels really great to achieve a fundamental scientific discovery that has real practical applications,” Möttönen said.
Based on the results of the experiment, the researchers suggested that quantum-limited heat conduction has no fundamental distance cutoff. Their work established the integration of normal-metal components into the framework of circuit quantum electrodynamics. According to Möttönen, this technology would be well suited to cooling or initializing quantum devices, which need to operate at or close to the single-quantum level, but how exactly?
One of the most promising architectures for quantum computing is based on superconducting qubits, which are the key ingredients in circuit quantum electrodynamics. In such systems, the control of heat at the quantum level is extremely important, and remote cooling may be a viable option. The long-distance heat transport through transmission lines may be a useful tool for certain future applications in the quickly developing field of quantum technology.
In the future, Möttönen´s team plan to use the technology that they have now developed to cool down, or initialize, quantum memories. It works so that the electromagnetic excitation of the quantum bit in its state 1 will release its energy to a tiny resistor.
”One can think that when the qubit is excited, it induces some electric current to flow through the resistor and according to Ohm’s law, some power will be dissipated in the resistor. This is dissipated power is exactly the energy that the qubit releases to the resistor when it is initialized from state 1 to state 0,” he said.
Currently, his team has shown that they can reliably fabricate these tiny resistors on the chip and integrate then with circuitry similar to the one that is used in the superconducting quantum computer. Now they only need to find a way to switch the coupling between the resistor and the qubit on when they want to initialize the qubit and off when they want to run the quantum algorithms.
”Bear in mind that if one brings the qubit from the state 1 to 0 during the computation, it induces errors in the end result. Thus one really needs to be able to switch the coupling off. We have some ideas how to implement this switch but are also working on new ones. One of the ideas is based on a so-called superconducting quantum interference device (SQUID), and has been already published in Ref. [P. J. Jones et al. 3, 1987 (2013)].
In the beginning of 2017, Möttönen will start his European Research Council (ERC)Consolidator Grant to develop and implement the necessary switches. The project lasts five years and has a budget of 2 M€. Möttönen is confident that after five years, they will have achieved great results on these quantum coolers.
The discovery was published on February 1, 2016, Nature Physics.