Friday is here, and the 'Weekend Science Bite' corner is back — number 96.
This time: how do you cool a quantum computer, and what does it have to do with the Sun and nuclear missiles?
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The quantum computer shown in the video is housed at Google's labs, and the vast majority of the apparatus is a cooling system that brings the chamber at its base to near absolute zero — making it one of the coldest places in the universe.
A quantum computer is built on qubits that exist in superposition, with their precise state being probabilistic rather than definite. To allow them to exist in such a state, they must be brought to near absolute zero. The problem is that even helium, which serves as an excellent coolant, can only cool down to just below 1 Kelvin — a temperature still too high for creating qubits.
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The key lies in helium-3, whose nucleus contains 2 protons and 1 neutron, as opposed to the common helium-4, which contains 2 protons and 2 neutrons.
Helium-3 is produced through nuclear fusion in the cores of stars like our Sun. The fusion merges two protons from hydrogen atoms to form deuterium. One of the protons emits a positron and a neutrino and becomes a neutron, and an additional proton that joins converts the atom into helium-3.
The Sun emits enormous quantities of helium-3 into space, but Earth's magnetic field and atmosphere prevent it from reaching the surface, making it an extremely rare and nearly unobtainable element. The lunar surface, by contrast, absorbs tonnes of helium-3, which is one of the reasons behind the great-power race to the Moon.
To cool a quantum computer, helium-3 and helium-4 are mixed together. Once mixed, the helium-3 floats above the helium-4 because it is lighter — but quantum properties keep 6.6 percent of it trapped within the helium-4.
During the cooling process, helium-3 is filtered out of the helium-4, and as the helium-3 is drawn back into the helium-4 to restore the 6.6 percent concentration, it absorbs a large amount of energy from the quantum chip, cooling it down.
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Helium-3 also plays a role in detectors used to identify radioactive materials at ports and airports, and has potential as a fuel for nuclear fusion reactors (a topic for another time).
Currently, the only way to produce it is from tritium reservoirs found in nuclear missiles.
Tritium serves as a critical yield booster, but since it decays and produces helium-3 — which reduces the missile's effectiveness — it must be replaced with fresh tritium every few years.
During the replacement process the helium is separated out, yielding a total of only a few tens of kilograms worldwide each year.
In the meantime, efforts are underway to develop cooling methods that do not rely on helium-3, such as cooling through cyclic changes in the magnetic field of a metal — a process that draws heat away from the chip.
Despite the vast helium-3 deposits in the lunar soil, extracting it would require highly complex and expensive equipment, and it is far from certain that recovering the helium and launching it back to Earth would be either economically viable or practical.
Shabbat Shalom 😊
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Video credit: Good Morning America | ABC
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