arXiv:1011.3646 · Physical Review A
Quantum abacus for counting and factorizing numbers
M. V. Suslov, Gordey B. Lesovik, G. Blatter
Abstract
We generalize the binary quantum counting algorithm of Lesovik, Suslov, and Blatter [Phys. Rev. A 82, 012316 (2010)] to higher counting bases. The algorithm makes use of qubits, qutrits, and qudits to count numbers in a base-2, base-3, or base-$d$ representation. In operating the algorithm, the number $n<N={d}^{K}$ is read into a $K$-qudit register through its interaction with a stream of $n$ particles passing in a nearby wire; this step corresponds to a quantum Fourier transformation from the Hilbert space of particles to the Hilbert space of qudit states. An inverse quantum Fourier transformation provides the number $n$ in the base-$d$ representation; the inverse transformation is fully quantum at the level of individual qudits, while a simpler semiclassical version can be used on the level of qudit registers. Combining registers of qubits, qutrits, and qudits, where $d$ is a prime number, with a simpler single-shot measurement allows us to find the powers of 2, 3, and other primes $d$ in the number $n$. We show that the counting task naturally leads to the shift operation and an algorithm based on the quantum Fourier transformation. We discuss possible implementations of the algorithm using quantum spin-$d$ systems, $d$-well systems, and their emulation with spin-1/2 or double-well systems. We establish the analogy between our counting algorithm and the phase estimation algorithm and make use of the latter's performance analysis in stabilizing our scheme. Applications embrace a quantum metrological scheme to measure voltage (an analog to digital converter) and a simple procedure to entangle multiparticle states.