问题
While we are waiting for our quantum computers, is it possible to write a software simulation of one? I suspect the answer is no, but hope the reasons why not will throw some light on the mystery.
回答1:
Implementing it isn't that hard. The problem is that the computational and memory complexity is exponential in the number of quantum bits you want to simulate.
Basically a quantum computer operates on all possible n-bit states at once. And those grow like 2^n.
The size of an operator grows even faster since it's a matrix. So it grows like (2^n)^2 = 2^(2*n) = 4^n
So I expect a good computer to be able to simulate a quantum computer up to about 20 bits, but it will be rather slow.
回答2:
They do exist. Here's a browser based one. Here's one written in C++. Here's one written in Java. But, as stated by CodesInChaos, a quantum computer operates on all probability amplitudes at once. So imagine a 3 qubit quantum register, a typical state for it to be in looks like this:
a1|000> + a2|001> + a3|010> + a4|011> + a5|100> + a6|101> + a7|110>+ a8|111>
It's a superposition of all the possible combinations. What's worse is that those probability amplitudes are complex numbers. So an n-qubit register would require 2^(2*n) real numbers. So for a 32 qubit register, that's 2^(2*32) = 18446744073709551616 real numbers.
And as CodesInChaos said, the unitary matrices used to transform those states are that number squared. Their application being a dot product... They're computationally costly, to say the least.
回答3:
My answer is yes:
You can simulate the behaviours of a quantum machine by simulating the quantum machine algorithm
D-Wave quantum machine using a technique called quantum annealing. This algorithm could be compared to simulated annealing algorithm.
References:
1.Quantum annealing
2.Simulated annealing
3.Optimization by simulated annealing: Quantitative studies
回答4:
As Wikipedia state:
A classical computer could in principle (with exponential resources) simulate a quantum algorithm, as quantum computation does not violate the Church–Turing thesis.
回答5:
Years ago I attended a talk at a Perl conference where Damian Conway (I believe) was speculating on some of this. A bit later there was a Perl module made available that did some of this stuff. Search CPAN for Quantum::Superpositions.
回答6:
There is a very big list of languages, frameworks and simulators. Some simulate at low level the quantum equations, other just the gates.
- Microsoft Quantum Development Kit (Q#)
- Microsoft LIQUi>IBM Quantum Experience
- Rigetti Forest
- ProjectQ
- QuTiP
- OpenFermion
- Qbsolv
- ScaffCC
- Quantum Computing Playground (Google)
- Raytheon BBN
- Quirk
- Forest
It would be great to know your opinions on their capabilities and easiness of use.
https://quantumcomputingreport.com/resources/tools/ https://github.com/topics/quantum-computing?o=desc&s=stars
回答7:
Quipper is full blown simulation EDSL for Quantum Computing, implemented in Haskell I have experince to simulate behaviour of several QC algorithms such as Deutsch, Deutsch–Jozsa, Simon's, Shor's algorithms and it's very straightforward.
回答8:
Another reason why classical simulation of quantum computation is hard: to keep track you may want to know after each action of a n-qubit gate (n>1) whether the outgoing qubits are entangled or not. This must be calculated classically but is known to be NP-hard.
See here: https://stackoverflow.com/a/23327816/363429
回答9:
Yet another reason why classical simulation of quantum computing is hard: you need almost perfect - i.e. as perfect as possible - random number generators to simulate measurement.
来源：https://stackoverflow.com/questions/4595156/software-simulation-of-a-quantum-computer