Joanthan Schwinger

Let $f: \mathbb{F}_2^n \rightarrow \mathbb{F}_2^n$ be a Boolean function with period $\vec s$. It is well-known that Simon's algorithm finds $\vec s$ in time polynomial in $n$ on quantum devices that are capable of performing error-correction. However, today's quantum devices are inherently noisy, too limited for error correction, and Simon's algorithm is not error-tolerant. We show that even noisy quantum period finding computations lead to speedups in comparison to purely classical computations. More precisely, we implemented Simon's quantum period finding circuit on the $15$-qubit quantum device IBM Q 16 Melbourne. Our experiments show that with a certain probability $\tau(n)$ we measure erroneous vectors that are not orthogonal to $\vec s$. We propose new, simple, but very effective smoothing techniques to classically mitigate physical noise effects such as e.g. IBM Q's bias towards the $0$-qubit. After smoothing, our noisy quantum device provides us a statistical distribution that we can easily transform into an LPN instance with parameters $n$ and $\tau(n)$. Hence, in the noisy case we may not hope to find periods in time polynomial in $n$. However, we still obtain quantum advantage even for large errors $\tau(n)$ close to $\frac 1 2$. Thus, period finding does not necessarily require full quantum error correction capability. keywords: Noise-tolerant Simon period finding, IBM Q 16, LPN algorithms, quantum advantage