- Physicists have cooled down a nanoelectric chip to 2.8 milliKelvin, setting a world record.
- They used magnetic cooling techniques to reduce the temperature of chip and electrical connections.
- With some optimizations, the same technique could reach to 1 milliKelvin limit.
Everyone loves to compete for records and nothing is better than the feeling of achieving something extraordinary. Even scientists like to break records, which is why several teams from all around the world are working on a high tech cooling system to reach temperatures as close to absolute zero as possible.
The absolute zero (0 K or -273.15°C) is the point at which particles of nature have minimum vibrational motion, retaining only quantum mechanical zero-point energy-induced particle motion. These extremely low temperatures provide an ideal condition for quantum experiment and allows us to study a whole new physical phenomena.
Scientists at the University of Basel have cooled down a nanoelectric chip to 2.8 milliKelvin. To achieve this record they used magnetic cooling techniques to reduce the temperature of the chip and its electrical connections. Let’s find out in details about what they have used to build the coldest nanoelectronic chip.
The physicists utilized the principle of magnetic refrigeration in nanoelectronics in order to cool devices close to absolute zero. In this technique, a system is cool down by applying magnetic field while preventing the external heat flow. However, the heat magnetization should be eliminated before magnetic field is ramped down.
Specifically, magnetic cooling technology is based on magnetocaloric effect – a magneto thermodynamic mechanism in which a temperature change of an appropriate material is caused by exposing the material to varying magnetic field.
In this process, a drop in external magnetic field strength allows the magnetocaloric material’s magnetic domain to get disoriented from the magnetic field via thermal energy (photons) present in the material. If the material is isolated so that no energy can remigrate, the temperature reduces as the domains absorb the thermal energy in order to perform their orientation.
For example, Praseodymium alloyed with nickel has a very powerful magnetocaloric effect – it allows to physicists to reach within 1 milliKelvin.
Achieving The Minimum Temperature Level
To get to the one thousandth of a degree of absolute zero, physicist used a combination of two cooling systems, both of which are based on magnetic refrigeration. They reduced the temperature of all of the electrical connections to 150 microKelvin.
The next step is to integrate the second cooling system into the chip and place a Coulomb blockade thermometer on it. The material compositional and overall construction of the system enabled them to reach temperature almost as low as absolute zero.
The metallic Coulomb blockade thermometer (CBT) is a reliable and accurate electronic thermometer capable of operating down to 10 milliKelvin and slightly below. Usually it contains linear arrays of Al/AlOx/Al tunnel junctions with copper metallic islands in-between.
This figure shows the schematic with CBT enclosed in a copper box (yellow), attached to Ag-epoxy microwave filters (gray), and stuck onto a Cu plate (orange) with Ag-epoxy. Figure B is an electron micrograph of the CBT island with tunnel junctions. Figure C is just a magnified view of tunnel junction.
Particularly, adiabatic demagnetization of both the electronic leads and the large metallic islands of a Coulomb blockade thermometer reduced the external heat leak via leads while providing on-chip refrigeration. The temperatures came down to 2.8 ± 0.1 milliKelvin.
For now, physicists can maintain these extremely low temperatures for nearly 7 hours, which is enough time to perform a wide range of experiments that will help us to better understand the physics properties close to absolute zero.
Chip with CBT, prepared for experiments | Source: University of Basel
Achieving such low temperatures in electronic devices could be key to novel quantum states of matters like helical nuclear spin phases, quantum Hall ferromagnets, fragile fractional quantum Hall sates, or full nuclear spin polarization.
Moreover, hybrid Majorana devices and the coherence of semiconductor and superconducting qubits can benefit from lower temperatures. We can also develop a parallel network of nuclear refrigerators to adapt well-known methodology of Adiabatic Nuclear Demagnetization for electronic transport experiments.
To get better results we can improve microwave filtering, decrease vibration induced eddy current heating due to active damping, fix the support structure of the nuclear stage to the magnet support assembly and mixing chamber shield.
This would help us to enhance the inefficient precooling process as well as decrease the large dynamic heat leak, reducing the final temperature after Adiabatic Nuclear Demagnetization. The research team claims that the same technique could reach to 1 milliKelvin limit.