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Comment by azatris | Hacker News Reader

I do not know much about quantum computing. But could you explain what makes these computers quantum? Is it the configuration of these transistors to invoke some quantum phenomena?

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In a classical computer, every bit of information in the system is in a definite state- 1 or 0. In a quantum system with such definite possible states, what you actually have most of the time of the system in some interpolation of the possible states- so in the quantum computer case each bit is usually in a state a1 + b0, where a and b are complex numbers such that |a^2|+|b^2| = 1.

Most of the time, the 'weight' flows back and forth between a and b according to certain equations over time. When you measure the system- that is, when the bit interacts with the outside world, hopefully your measuring apparatus- you see a 1 or a 0, with probabilities |a^2| and |b^2| respectively.

So what you can do is get a whole bunch of these quantum bits- qubits together, and set things up so that the time-evolution of their quantum state is correlated and probabilistically moves towards something you're interested in. Say you can set things up so the bit array- which, at first, will give you a mere perfectly random bit string on measurement- becomes more and more likely to give you, say, a prime factor, or the answer to some other question.

So yes, the quantum phenomenon is that the bits of the computer are quantum objects as opposed to classical.

this is the most comprehensible entry-level description of quantum computers I've ever read. thank you.

(qubits I've seen explained many times, but setting things up so that qubits are probabilistically correlated is the part I've never understood anyone else to be saying)

They're built off fundamentally different basic units. Contemporary computers use transistors, which occupy traditional physics and do logic with voltage thresholds. Quantum computers use a quantum phenomenon -- one easy-to-understand (and fairly easy to construct) quantum computer substrate uses the spin of electrons in superconducting loops. Electron spin is a quantum phenomenon, in that the spin isn't deterministically positive or negative, it's a probability distribution -- initially, equally likely to be positive or negative, but you can't tell what it is until you actually read it. It's not 0.0, it's either -0.5 or 0.5, both with a 50% chance. Equally importantly, you can perform (physical) operations on electrons singly or in pairs in order to manipulate these probabilities. Quantum computing is turning these probability fields and operations into useful computational results.

This is exactly what I wanted to hear! Awesome, thank you!

Here is a long-ish video that explains quantum computers without using pop-science hand-wavy terminology

https://www.youtube.com/watch?v=F_Riqjdh2oM

A quantum computer is defined as a machine that can implement an arbitrary "unitary evolution" (up to arbitrary precision). If you don't know enough math to understand what a unitary evolution is, think of it as a quantum computer doing basically any possible thing that you can do with a given number of qubits. As another comment said, the idea is to (ab)use this to do some operations on qubits such that they end up in a state where measuring it will give you an answer to something you're interested in. It has very little to do with classical programming.

The math on quantum computers checks out, it's "just" an engineering challenge at this point, and many are doubtful whether these challenges will ever be overcome to build a quantum computer of sufficient complexity.

Essentially, some "unitary evolutions" are complex to implement, as in requiring a lot of quantum "gates". This causes an accumulation of error and a whole lot of other problems, which limits the complexity of the calculations that can currently be performed.

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