Refrigeration in quantum computers: how does it work?

Adequate cooling of quantum computers is crucial to ensure the correct functioning of these devices, which are among the most promising and innovative of our time.  They have the potential to revolutionise key sectors such as medicine, energy, cryptography and artificial intelligence.

Their peculiarity lies in their use of the qubit, a different base unit from the traditional bits used in conventional computers.  In contrast with the latter, which can only represent a single binary state at a time (either 0 or 1), qubits exploit the principle of quantum superpositioning, which enables them to exist in multiple states simultaneously.

However, in order to function, these computers require specific environmental conditions, including sufficient cooling to be able to reach temperatures close to abolute zero (-273.15°C), which is essential to ensure that errors are minimised and calculations are stable.

The role of refrigeration in quantum systems

The need for extremely low temperatures inside quantum computers derives mainly from the physical properties of the superconductor circuits used in the construction of the qubits.  Under these conditions, the circuits do not present any electrical resistance, a feature which enables information to be stored for long periods.  At higher temperatures however, any disturbances to the environmental conditions can interfere with the qubits and risk causing errors which could render the system unusable.

Currently, in order to reach such low temperatures, quantum computers rely on dilution refrigerators: highly specialised machines which are able to bring the qubits to a temperature of around 50 milliKelvin (mK) above absolute zero.  Getting any closer to zero degrees Kelvin poses a considerable challenge in terms of engineering, as lowering the temperature by each single degree means higher energy consumption and the need to use increasingly sophisticated technologies.  Thermodynamic laws have established that it is currently impossible to reach absolute zero within a finite system.  This limitation means that innovative systems are required in order to improve the efficiency of the cooling systems.

A new type of quantum refrigerator

Recently, researchers at the Chalmers University of Technology and the University of Maryland have designed an autonomous quantum refrigerator, aimed at bringing qubits to record low temperatures, thereby overcoming the limitations of traditional refrigeration techniques previously used in quantum computers.

This approach uses pre-existing dilution refrigerators and exploits an innovative architecture based on superconductor circuits.  The device is distinguished by its capacity to operate independently, without the need for external intervention), thanks to its use of ambient heat as the energy source.

The principle forming the basis of this refrigerator is heat transfer between separate qubits.  The system is composed of three qubits: the first is the qubit to be cooled while the other two act as components of the refrigerator.

Via interaction between these three elements, heat is transfered from the target qubit towards the auxiliary qubits, which dissipate it in a cold environment.  This process is guided by an autonomous thermal gradient, determined by the difference in temperature between two thermal pools (one hot and one cold).

The device described above is definitely a major development in the refrigeration of quantum computers, thanks to its many unique characteristics, which can be analysed as follows:

1. autonomy: once set up, the system operates without the need for external inputs, making it highly efficient. This autonomy is guaranteed by the energy generated by the difference in temperature between the thermal pools;
2. record low temperatures: the device is able to bring the target qubit to temperatures of 22 milliKelvin, significantly lower than those achievable with traditional dilution refrigerators;
3. energy efficiency: by using its capacity to exploit ambient heat, the refrigerator considerably reduces total energy consumption, contributing to reducing the hardware overload involved in maintaining the systems at such extremely low temperatures;
4. practical applications: this technology is not limited to demonstrating a principle but is a very useful method of improving the precision and reliability of quantum computers. This opens the way to new applications in the field of information processing.

How the system works and possible future implications

The quantum refrigerator’s cooling system is based on the interaction of three separate bodies.  Each auxiliary qubit is connected to a physical heat pool via a microwave guide.  The warm pool provides energy to one of the refrigerator’s qubits, which uses it to transfer heat from the target qubit to the second auxiliary qubit.

The latter, in turn, disperses the heat into the cold environment, completing the cycle.  This method exploits the qubits’ quantum properties to obtain more efficient cooling compared to conventional methods.

One interesting aspect is that the system combines the principles of classic thermodynamics with those of quantum mechanics, representing one of the first practical applications of quantum thermal machines.  According to researchers, the technology in question could be developed even further in order to explore new frontiers in subatomic physics and data processing.

In light of the developments mentioned above, it is feasible that the introduction of this new type of device for the refrigeration of quantum computers could have real potential to open up new opportunities for the future of these devices.

The capacity to cool the qubits to such low temperatures increases the stability and precision of the calculations and, at the same time, significantly reduces the probability of errors.  These advances could accelerate the implementation of quantum computers in practical contexts, making them more accessible for use in scientific research or by companies and other organisations.

In addition, the system developed by the researchers demonstrates how quantum thermal machines can be employed in many useful ways and can overcome the limitations of previous technologies significantly.

While traditional thermal machines contributed to the industrial revolution, this innovative development signals an important step towards a new era in which quantum technologies could become the engine of future mathematical, scientific and industrial revolutions.

Translated by Joanne Beckwith