There is plenty of room at the quantum bottom
In 1959, Richard Feynman gave a now celebrated talk on miniaturization, a talk that would presage the field of nanotechnology. In his talk, he mentions that it should be possible to print, for example, the entire 24 volume Encyclopedia Britannica on the head of a pin since the smallest element, a single dot, would still be about 1000 atoms.
The talk was published in 1960 but gained little attention for two more decades until advances in the field, like a superhero comic series picking up steam, demanded an origin story. Digging through archives, Feynman’s paper was unearthed.
Feynman’s proposition has been bested repeatedly in the past 40 years with the most recent record set in 2009. In that year, Stanford university was able to encode information in subatomic particles, writing the letters “S” and “U” so small at 300 picometers (300 trillionths of a meter) that the entire, now 32 volume, Encyclopedia Britannica could be written on the head of a pin 2000 times over.
This shattered the long held record set in 1991 by Hitachi which was able to write letters 1.5 nanometers (1.5 billionths of a meter or 1500 trillionths) tall.
The long distance between records does not mean that nanotechnology has made no gains in between. After all, your computer operates precisely because it is filled with transistors, tiny circuit elements, that are themselves nanometers long. Every few years, computer chips can fit more transistors so that they can store more data and computer faster thanks to nanotechnology.
Nowadays, attention is on quantum nanotechnology. It isn’t how many bits you can fit onto the head of a pin but how many qubits.
This year IBM promises to unveil a 1000+ qubit quantum computer which would be an order of magnitude (over a fact of 10) more than we had two years ago and 4000 qubit machines by 2025.
I would argue that these days the bottom that Feynman spoke about in 1960 is now in the quantum realm.
That is not to say that there isn’t a lot of wonderful research going on in classical miniaturization including nanomachines, but that area is far more mature and understandable than quantum computing.
Now, I don’t see why we can’t have quantum computers with millions of qubits.
Typically, if you see a picture of a quantum computer, it looks like this:
Credit: IBM Research. IBM Quantum System One (CES 2020)
This is just the refrigeration unit that keeps the computer’s internal components ultracold. That is one of the main drawbacks of any quantum process involving either superconductors or ions. You have to keep them cold or they don’t behave in a “quantum way”. That means that you can’t get things like bits that are 0 and 1 at the same time.
Making a small quantum computer means making being able to cool something to within degrees of absolute zero temperature with very small cooling components. Right now, as computers in the 1960s, quantum computers, ones that actually can put atoms into quantum states like 0 and 1 at the same time, take up whole rooms.
Liquid helium is the typical choice for cooling quantum computers, but the future may be electronic cooling. A Finnish team developed a means of blocking phonons using thermonic junctions. Phonons are basically like particles of heat that move through a lattice of some solid.
A thermonic junction is a device that emits hot electrons by thermionic emission, also called the Edison effect although Thomas Edison was not the first to discover it. A classic example is the old cathode ray tube that powered old TVs. When the cathode emits electrons, those electrons carry heat away. This has a cooling effect on the cathode. A thermionic junction operates similarly but uses semi-conductors instead of vacuum for the electron emission.
Solid state cooling isn’t anything new. One method, called thermoelectric, uses electricity to generate a temperature gradient. You can also do the reverse and use temperature gradients to generate electricity. You may have seen small soda fridges that operate by thermoelectric cooling. These fridges aren’t very good compared to those that use a traditional compressor however.
So what the team has done is tried to make the process more efficient by blocking those heat particles. Not only could this lead to better solid state coolers in general, but miniature ones at that, including for quantum computers (and classical computers which tend to use breakable fans for cooling).
By doing this electrically, they can keep things very cool with no moving parts, and that is how we are going to see real, large scale quantum computers on our desks in the future.
In fact, there is no reason to think that one day computer chips will not only be quantum, but carry their own cooling mechanisms as semiconductors.
Mykkänen, Emma, et al. "Thermionic junction devices utilizing phonon blocking." Science advances 6.15 (2020): eaax9191.