Optimal phase cloning machine in a three‐dimensional transmon qubit system

Abstract

AbstractIn quantum mechanics, although the no‐cloning theorem forbids us to clone an arbitrary quantum state ideally, it is still possible to partially reproduce an unknown quantum state by using a phase cloning machine. Since the no‐cloning theorem guarantees the safety of quantum key distribution (QKD), resulting in the famous Bennett–Brassard 1984 (BB84) protocol, phase cloning of BB84 states increases the probability to eavesdrop the transmitted information between correspondents, forcing us to recheck the security of the QKD protocol. In this work, we realize an optimal phase cloning machine for BB84 states in a superconducting quantum bit (qubit). The lowest three energy levels in this system are exploited as logical qubits to demonstrate this cloning procedure. Two microwaves are used to prepare and clone the states. In order to achieve the maximum fidelity, we use the state tomography to readout the experimental results. It is found that the state information is well preserved during this cloning process. The average fidelity between input and output states is 83.92%, making this a valid quantum cloning machine.