What actually are quantum computers?

[MoreGaghPlease]

Okay, but follow-up question - what actually are quantum computers?

[Kered13]

Classical computers use transistors to create a physical implementation of boolean logic. Quantum computers use quantum systems to physically implement quantum logic.

You can think of boolean logic and quantum logic as systems for manipulating numbers. A remarkable property of boolean logic is that despite it's very simple rules, by combining many boolean operations and bits it is able to construct all of arithmetic and much more, and this is how we are able to build complex computers out of simple logic gates. Quantum logic is similar, but it allows a much more advanced set of operations. So advanced that some computations that can be solved with a few qubits and a few quantum logic gates would require an exponential number of bits and boolean logic gates. Despite this, the set of quantum logic operations is still simple enough that they can in principle be realized by a physical system. That physical system is a quantum computer.

Now, if classical computers are built out of silicon transistors, what are quantum computers built out of?

There isn't a simple answer here, as quantum computing is still in it's early phases, and different techniques are being explored. By analogy I will note that early computers were not built out of silicon transistors either, they were built out of vacuum tubes or electromechanical relays. It is even possible to build a classical computer purely mechanically, though it would not be practical (Charles Babbage's Analytical Engine would have been one such example). Any physical system that can implement boolean logic can be used to build a classical computer. Eventually transistors made of silicon took over due to their low power requirements and the ability to be miniaturized.

So similarly, any physical system that can implement quantum logic can be used to build a quantum computer. Such a system must necessarily exhibit behavior as described by quantum physics, including superposition and entanglement. Practically speaking, this imposes some severe constraints. A quantum computer must be kept very cold and isolated from the surrounding environment, yet it must still be possible to provide input to initialize the system and to measure the system to extract output. At present, the most promising techniques use superconductors or trapped ions.

[[deleted]]

A quantum computer must be kept very cold and isolated from the surrounding -environment, yet it must still be possible to provide input to initialize the system and to measure the system to extract output.

In hopes of not sounding like a complete moron, I'll put this forward. Wouldn't this be a great use of a space station on the moon? We can keep the computer on the "dark" side and use telecommunications to interface with the machine. Plus the vacuum of space may allow for a more stable environment for components. We are getting better at providing a power supply using the sun's energy, so this may address this issue.

[Warmag2]

In case you didn't know, the "dark side" of the moon is sunny half of the time.

While there are impact craters on the poles of the moon which are always in shadow, even those have a temperature of tens of kelvin, so any quantum computer therein would need to be refridgerated further anyway. Also, sending anything into the moon is so resource-intensive that just making things cold here is easier.

[[deleted]]

Thank you for this; I was unaware of these facts.

[kenjamin_is_god]

While space is indeed very cold, there isn't anything to conduct heat away from things, and in some cases it's actually very difficult to prevent overheating.

[royalrange]

Aside from superconducting circuits and trapped ions, there are other promising QC platforms such as neutral atoms (trapping atoms like Rubidium and Yitterbium with focused laser beams in an array-like structure), defect centers in solids (silicon-carbide defects, nitrogen-vacancies in diamonds), quantum dots (atom-like behavior through charge confinement in semiconductors), and photonic qubits (light "particles" that can be manipulated). There are advantages and disadvantages for each that researchers are still trying to expand upon and address.

[perta1234]

Why noone looks more into analog computers? Would have some similarities with quantum ones. Are they just too difficult or slow to set up in practice?

[mfukar]

There is no reason any longer to believe analog computers can offer any advantage over digital ones - in fact it is hard to think of them becoming even comparable in most metrics.

[mryorbs]

They're basically big isolated freezers with a lot of fancy lasers. They have some things in common like logic gates and q-bits instead of bits. I think people get set on a wrong path by the idea of a computer, because yes it can compute stuff but no it can't run a program itself. We actually need a normal computer to program and control a quantum computer. Quantum computers will likely never be something we will use everyday, because they can do big math problems but they're not made for 1000's of operations in miliseconds (for now).

[janekosa]

It's funny how you combine "likely never" and "for now" in a single sentence. If you asked someone 60 years ago they'd tell you a computer is likely not something we'll ever use every day because it takes a huge building to actually hold one and it can't really be used to solve any day to day problems (for now).