{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Stablization via Symmetrization\n" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We've seen how we can implement the \"symmetrized tensor product\" of several qubits in terms of a quantum circuit: depending on the number of qubits we want to symmetrize, we adjoint a number of auxilliary qubits, prepare them in a special initial state, apply a sequence of controlled swaps on the auxilliaries and the original qubits, undo the initialization, and postselect on the auxilliary qubits all being $0/\\uparrow$. The key was that the algorithm requires post-selection, and so is only successful part of the time. By preparing our original qubits in the directions provided by a constellation of \"Majorana stars\", we were thus able to prepare spin-$j$ states in the guise of $2j$ symmetrized qubits. \n", "\n", "But nothing limits us to symmetrizing qubits. In fact, we can apply the same algorithm to symmetrize quantum systems consisting of several qubits: we simply repeat the controlled swaps on however many qubits comprise each subsystem. In fact, this played a role in the motivation behind the paper which provided our symmetrization algorithm: \"Stabilization Of Quantum Computations by Symmetrization\".\n", "\n", "The idea is the following: Suppose we have a quantum computer, and we're worried about noise. Instead of just running our quantum circuit, we could run $n$ copies of the circuit in parallel, while periodically using the symmetrization algorithm to project the $n$ copies as a whole into the permutation symmetric subspace. Suppose there were no errors: then naturally, each copy of the circuit would be precisely the same, and symmetrizing across them would have no effect. But if there are some errors that asymmetrically affect the different copies, then it's possible that projecting the copies into the permutation symmetric subspace will project off these errors! Indeed, the permutation symmetric subspace \"is the smallest subspace containing all possible error-free states. It thus corresponds to the 'most probing' test we can legitimately apply, which will be passed by all error-free states,\" since we can't just do any probe we like, since such poking around will generally disturb the quantum computation. Moreover, if we rapidly and periodically do this projection, which is probabilistic, then due to the \"quantum Zeno effect,\" we should be able to stay within the subspace with high probability.\n", "\n", "The problem, however, is that to do the symmetrization at all, we need to add in extra qubits and perform highly entangling operations, again and again. Given that NISQ-era quantum computers are notoriously unreliable about things like controlled swap operations, one might wonder if the errors introduced by the symmetrization algorithm itself would outweight the errors corrected by symmetrization! And in fact, that sadly seems to be the case--for now!\n", "\n", "But with an eye to the future, let's check out how it might work. \n", "\n", "(Incidentally, depending on one's particular quantum circuit, there may be other relevant subspaces that one could project into, without disturbing the computation, and which would provide some much needed stabliziation. The \"stabilization via symmetrization\" technique, however is nice insofar as it can help independently of the structure of one's circuits. Indeed, if such stabilization could be provided by the quantum computer itself that would be ideal.)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "