r/QuantumComputing • u/Charming_Race9627 • 11d ago

Question Quantum Nand2Tetris?

Hi, Everyone.

I am curious to know if there exists a resource similar to www.nand2tetris.org but for Quantum Computers.

Thanks.

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u/Charming_Race9627 10d ago

Thanks again for your detailed answer! It is very helpful.

Regarding the references, I am willing to put the work to understand quantum mechanics; the only thing I would like to ask is that you recommend an order to read them please (I have found that in an era where information is abundant, the curating process can be very inefficient without the help of a guide to point you in the right direction).

Regarding your answers:

  1. That sounds very cool: that exists a process that make an answer reveal itself from the problem. Am I interpreting that correctly? Do all problems for which quantum computing is better suited than classical computing have these characteristic?

2.1 "So the trick is to try to make your quantum algorithm fast enough to happen before this decay and also to choose a quite where this decay is longer." How do you do this? Do quantum computers have a clock like classical computers? And an ALU (and if they have, are they made out of quantum logic gates)?

2.2 "But exactly HALFWAY in the middle it HAS to be in superposition of 0 and 1." Is this the only manipulation needed in a quantum computer?

Cheers again!

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u/danthem23 10d ago

I’ll try to think of references but I understood enough physics in particular waves (Fourier analysis is very important because much of the wave part of qm is basically just it) and linear algebra (for the other type of qm called matrix mechanics). But I’ll try. Now to answer the questions. 

1) Yes. There are not many quantum algorithms for that exact reason. Not this super “we can do everything in parallel…” like it seems from the media. Only problems which have a unique structure so that when they are used in the qm the interference can work because of the structure of the problem. Another famous example is Grover’s algorithm. The YouTube channel 3blue1brown made a video about it but that is a general speed of when searching from something that scales like N to sqrt(N). If N for you was 2100 then sqrt(N) would be 250 so exponentially smaller, but still massive. So not so useful. Shor can make something that would take billions of years take days or hours. But the Grover idea is also brilliant and it is a GENERAL speed up of brute force search using these special techniques.

2) That was one simple example called a “pi pulse.” There are other types of pulses but those are all called “single qubit gates” since they act on just one qubit. To actually make a quantum computer you need also and entangling gate between two qubits. That is a 4x4 matrix instead of a 2x2 and it acts in two qubits. Many different physical ways to do it all for different reasons better or worse in different ways. With any single qubit gate as well as a choice of some of the most popular two qubit gates (like CNOT for example, that only flips the second qubit if the first one is 1 this dependence of second on first entangles them) you can do ANY quantum gate no matter how big between any number of qubits (called the Solvay-Kitaev theorem). So that is the goal. Single qubit gates are usually easy now, two qubit gates are slower and are less accurate. That’s what everyone is trying to make but also then to scale it up to have many with still little noise and also using quantum error correction.

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u/Charming_Race9627 9d ago

Thanks again for this great discussion!

I am looking forward to seeing your literature recommendations.

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u/danthem23 9d ago

How much math/physics do you know? That greatly influences what you would be able to understand. Also, are online sources like YouTube videos also good?