Implementing phase-covariant cloning in circuit quantum electrodynamics

Karol Bartkiewicz, Adam Miranowicz, Şahin Kaya Özdemir

We propose a quantum cloning machine, which clones a qubit into two clones assuming known modulus of expectation value of Pauli ${\ensuremath{\sigma}}_{z}$ matrix. The process is referred to as the mirror phase-covariant cloning for which the input state is a priori less known than that for the standard phase-covariant cloning. Analytical expressions describing the cloning transformation and fidelity of the clones are found. Extremal equations for the optimal cloning are derived and analytically solved by generalizing a method of Fiur\'a\ifmmode \check{s}\else \v{s}\fi{}ek [Phys. Rev. A 64, 062310 (2001)]. Quantum circuits implementing the optimal cloning transformation and their physical realization in a quantum-dot system are described.

Guillermo Romero, E. Solano, Lucas Lamata

Yuta Okubo, Francesco Buscemi, Akihisa Tomita +1 more

We propose an experimental implementation of a particular attack against the BB84 quantum key distribution protocol, utilizing a 1→3 asymmetric phase‐covariant quantum cloning. Our eavesdropping strategy is numerically optimized to extract the information about the input state. The proposed cloning circuit is easily implementable with linear optics and its performances turn out to be well close to the optimum attack.

Maxime Boissonneault, Jay Gambetta, Alexandre Blais

L. DiCarlo

Karol Bartkiewicz, Adam Miranowicz

We find an optimal quantum cloning machine, which clones qubits of arbitrary symmetrical distribution around the Bloch vector with the highest fidelity. The process is referred to as phase-independent cloning in contrast to the standard phase-covariant cloning for which an input qubit state is a priori better known. We assume that the information about the input state is encoded in an arbitrary axisymmetric distribution (phase function) on the Bloch sphere of the cloned qubits. We find analytical expressions describing the optimal cloning transformation and fidelity of the clones. As an illustration, we analyze cloning of qubit state described by the von Mises--Fisher and Brosseau distributions. Moreover, we show that the optimal phase-independent cloning machine can be implemented by modifying the mirror phase-covariant cloning machine for which quantum circuits are known.