Quantum Cloning using Protective Measurement

TL;DR

This paper demonstrates that protective measurement can, in principle, enable near-perfect cloning of an unknown quantum spin state by transferring information to spatial degrees of freedom and measuring without entanglement.

Contribution

It introduces a novel method for quantum cloning using protective measurement, combining state transfer, trapping, and non-entangling measurement techniques.

Findings

Cloning with arbitrary precision is theoretically possible.

Protective measurement allows extracting expectation values from single quantum systems.

The method can also be applied to nonorthogonal state discrimination.

Abstract

Here we show that, in principle it is possible to clone (measure) a single arbitrary unknown quantum state of a spin-$\frac{1}{2}$ particle (an electron) with arbitrary precision and with success probability tending to one, using protective measurement. We first transfer the information from spin to spatial degree of freedom (d.o.f) of system electron, then trap it in a double well potential, and finally measure it protectively using a probe electron (which donot get entangled with system electron, but still extracts expectation value of an observable from a single quantum system (system electron)) to obtain information about the unknown spin polarization. Nonorthogonal state discrimination being a subclass of cloning, part of the paper (till finding out $\theta_{m}$, polar angle corresponding to the unknown spin polarization) is sufficient for discrimination.