Rapid readout of a register of qubits using open-loop quantum control

Abstract

Measurements are a primitive for characterizing quantum systems. Reducing the time taken to perform a measurement may be beneficial in many areas of quantum information processing. We show that permuting the eigenvalues of the state matrix in the logical basis, using open-loop control, provides an O(n) reduction in the measurement time, where n is the number of qubits in the register. This reduction is of the same order as the (previously introduced) locally optimal feedback protocol. The advantage of the open-loop protocol is that it is far less difficult experimentally. Because the control commutes with the measured observable at all times, our rapid measurement protocol could be used for characterizing a quantum system, by state or process tomography, or to implement measurement-based quantum error correction.

Zooming in on quantum trajectories

“…Introduction -Quantum monitoring equations play a key role in modern theoretical quantum physics and are used widely in control [1][2][3], quantum information [4][5][6], and even foundations [7,8]. They describe a system subjected to iterated or continuous measurements and can be used to treat a large variety of experimental setups e.g.…”

Efficient progressive readout of a register of qubits

“…In any case, these numerical simulations show that the asymptotic speed-up should not be the only metric used to assess the efficiency of rapid measurement schemes in the future as the asymptotic regime may be irrelevant in realistic setups. Note that the previous schemes of Combes et al [1,4] which could only be probed numerically for small registers, also showed lower performances for finite infidelity targets. All these reserves being made, the GC algorithm does provide a large speed-up in absolute value for all register sizes and reasonably small infidelity targets.…”

“…One expects that, on average, the states will be at a Hamming distance of order n/2 from each other yielding the same kind of speed-up as before but for a different pre-factor. And indeed, in [4] the authors prove that the speed-up S RP for the open-loop random permutation scheme verifies for large n:…”

“…It would be more convenient to have a predefined strategy implementable in open-loop. In [4] the authors provide such a scheme. The idea is simply to do rapid random permutations of the pointer basis.…”

“…In a recent series of articles [1][2][3][4], Combes et al have proposed control schemes which increase the measurement speed of qubit registers. The methods they proposed provide an impressive speedup rate proportional to the register size.…”