system
Given the following post, respond as if you're commenting on Reddit:
human
What is Quantum Computer? How do they work? What are the differences between that at the computer I'm asking this question from?

And anything else interesting about the field.

Thanks in advance!

(Just noticed the small derp in my first question and the larger derp in my last. I'm a tired man.)
gpt
A quantum computer uses the key concepts of quantum mechanics—superposition and entanglement—to solve some problems more efficiently than a classical computer.

At the heart of it, you have quantum (as opposed to classical) bits, which can be realized in many different physical architectures, such as photons, trapped ions, neutral atoms, spins in solid states and superconducting circuits. The difference between these quantum bits and the classical bits in your computer is that they don't just assume two discrete logical states—'0' or '1'—but arbitrary superpositions of those (e.g. a qubit can simultaneously be in a state "'0' and '1'"). If you have many qubits in collective superpositions you get entanglement.

These quantum bits are the quantum information carriers and they have to interact with each other to realize quantum logic gates and eventually an algorithm. The best known quantum algorithm is Shor's; it factors large numbers into its constituents in polynomial time, while the best known classical algorithm scales exponentially.

We haven't got many interesting algorithms yet where quantum computers will really shine, but one area we're really interested in is to use them to simulate other quantum systems. Say you've got a quantum systems that's hard to understand and even harder to control, like a big molecule, or even a protein. So instead of trying to use that thing directly, we can simulate it's Hamiltonian (that best describes the system according to our understanding) on a quantum computer which is easier to set up. These simulations than help us figure out how the system behaves in reality.

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EDIT, you may also be interested in the technological status quo: single photons have achieved up to 8-qubit entanglement, but at terrible quality and really low rates. They do not currently constitute a very scalable approach to quantum computing. 

Trapped ions are doing much better: up to 14 entangled qubits have been realized, and they have done nice quantum simulations with 6 and more qubits. The quality is quite good and their immediate future in terms of scaling this to higher numbers is looking quite good. 

Superconducting circuits are catching up very quickly, they're currently playing with three-qubit gates, teleportation, error correction and so on—the usual first steps which are necessary to demonstrate the level of coherent quantum control you need for quantum computation. 

Solid-state qubits have long looked too hard to realize but all of a sudden they're also addressing and reading out single qubits, and the first entangling gates are in the pipeline. Once they've got that sorted, they could overtake the other technologies really quickly because their technology is based on a multi-billion dollar device fabrication industry.

Unfortunately though, *all* of these technologies will have a really hard time to move beyond a few tens of qubits. It's not even quantum coherence or other fundamental physics that's holding us back, it's things as profane as you can't get enough lasers into a single room to set up, manipulate, and read out 20000 ions. Or, alternatively, a few tens of thousands of superconducting qubits would need a whole power plant to stay cool.