# How to create a quantum algorithm that produces 2 n-bit sequences with equal number of 1-bits?

I am interested in a quantum algorithm that has the following characteristics:

1. output = 2n bits OR 2 sets of n bits (e.g. 2 x 3 bits)
2. the number of 1-bits in the first set of n-bits must be equal to the number of 1-bits in the second set. E.g. correct output = 0,0,0, 0,0,0 (both 3-bit sets have zero 1-bits); 1,0,0, 0,1,0 (both 3-bit sets have one 1-bit); 1,1,0, 0,1,1 (both 3-bit sets have two 1-bit)
3. Each time the quantum algorithm runs it must randomly return one of the possible solutions.

Any idea how I can best implement such an algorithm on a quantum computer ?

FYI I have tried the following algorithm (where n = 2 ) but it missed the 2 answers 0110 and 1001:

There are probably better ways than this, but here’s one you could try:

Start as you have done, with Hadamards on every qubit of the first register, then controlled nots between matching pairs of qubits across the two registers. This creates a uniform superposition of terms $|x\rangle|x\rangle$.

Now you need to somehow perform a random permutation on the second register. Introduce $\binom{n}2$ ancillary qubits. Apply Hadamard on each, and use each qubit to control the application of a swap between a different pair of qubits on the second register. Then forget about the ancillary qubits, and just measure the first two registers. (I’m guessing this gives you a sufficiently random permutation.)

• In the MathOverflow StackExchange: I have received a different answer on the same question (see mathoverflow.net/questions/301733/…). Jun 3, 2018 at 8:35
• @JanVdA I see how that solution works for n=2. It’s not obvious to me how you generalise it to larger n, but that might just be me. Also, I inferred from your question that you wanted all outputs with equal probability, which isn’t the case there. Jun 3, 2018 at 10:32
• @JanVdA Perhaps the trick is to come up with something that works just like the quantum Fourier Transform, but just within fixed excitation subspaces. I'll have a think about this... Jun 3, 2018 at 12:30
• thanks for the answer, I have created a separate question for how to best handle the permutation issue : quantumcomputing.stackexchange.com/questions/2222/… Jun 3, 2018 at 12:43

I know we already have an answer here, but going back to the problem specification, there's a much simpler way to achieve this if the only thing that's important is the output, comprising a binary string. All the protocol actually has to do is:

• Select an $n$-bit string, $x\in\{0,1\}^n$ at random. If you don't want to trust classical randomness, use $n$ qubits, starting in $|0\rangle$, with Hadamard acting on them, and measure in the computational basis. In fact, just use 1 qubit, and repeat the same thing $n$ times with it.

• Let $w_x$ be the Hamming weight of $x$ (i.e. the number of 1s). Select a random $y\in\{0,1\}^n$ such that $w_y=w_x$. There are several ways you might do this, but, for example, use $m=\lceil\log_2\binom{n}{w_x}\rceil$ bits. Generate an answer $z\in\{0,1\}^m$ uniformly at random (i.e. $p_z=1/2^m$). $\binom{n}{w_x}$ of these strings $z$ can be mapped onto a suitable $y$. If a given answer cannot, just keep choosing a random $z$ until it can. On average, you won't need more than 2 gos.

• Give the answer $x,y$.

• thanks for the update. It is not yet fully clear to me. What do you mean by ... can be mapped onto a suitable y and how should this mapping be implemented on a quantum computer ? Jun 4, 2018 at 17:26