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Programming Quantum Computers

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This is really interesting technology.
How far away are we from Quantum computers being mainstream?

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Indeed, when does this become mainstream? I found the largest QC is about 72 qubits.
Authors - how many qubits would be required to do create say a calculator?
How many qubits would we be looking at before it can join the main stream?
What are some contemporary thoughts about the timelines when this is going to be bread and butter?
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Hi Paul! Thanks for this question.
Looking forward to QPUs (quantum processing units) becoming "mainstream", the critical thing to watch is not the number of qubits, but the number of fault-tolerant qubits. This is what separates a mainstream QPU from an advanced science experiment. To illustrate, I'll use Mainak's example...

Mainak Biswas wrote:Authors - how many qubits would be required to do create say a calculator?

I really like this example.

Imagine it's 1950, and you're trying to implement a calculator using a "digital electronic computer" which is new and exciting. It's a big advanced model, so it has 64 bits of RAM. So you've got 8 bytes to work with. Is that enough to implement a calculator? For sure, if just a simple one. You can increment, add, divide-by-2 by bit-shifting, it'll be a fun project. But then you find out that the eight bytes aren't error-corrected or fault-tolerant. You can clear a register to zero, but as the next instructions execute some of the bits start losing their values, and after about 40 instructions it's gone completely random. Suddenly, making a useful calculator becomes a lot more difficult, so this computer might not be ready to be a "mainstream" product.

In 2019, the current state of the art in QC is 50-72 raw (non-FT) qubits, which is challenging (not impossible) to build a quantum calculator with. it's really the number of FT qubits we need to watch when deciding when something will be mainstream. These are coming, but it's hard to peg a date until we can see them working.

Still, we can simulate FT Qubits with no problem:

In the book, chapter 5 (right near the beginning) contains everything you need to implement and simulate a calculator on a QPU using very few qubits. Basic math functions which work in quantum superposition are specified and demonstrated, with code samples that run in one click.

For example, Example 5-3 shows how to implement "a = a + b * b" in quantum superposition, and you can run it right now, on the sample code page.

This simple example can also be run for free on a physical QC, since the page contains Qiskit and OpenQASM versions. If you do that, you'll notice that non-FT qubits get really fuzzy.

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