Quantum Dense Coding with Atomic Qubits

We report the implementation of quantum dense coding on individual atomic qubits with the use of two trapped ${}^9\mathrm{B}\mathrm{e}^{+}$ ions. The protocol is implemented with a complete Bell measurement that distinguishes the four operations used to encode two bits of classical information. We measure an average transmission fidelity of 0.85(1) and determine a channel capacity of 1.16(1).

Figure 1

Schematic diagram of the dense-coding implementation using atomic qubits. In the top part of the figure, the trap zones used in the experiment (not to scale) are designated with numbers. The $z$ axis of the trap is parallel to the horizontal direction in the figure. The large arrows indicate schematically the laser beams used to implement the various operations. The dashed arrows indicate laser-induced fluorescence that is detected to determine the state of each ion ($|\downarrow \rangle$ fluoresces, $|\uparrow \rangle$ does not). To facilitate separate detection of qubits $A$ and $B$, qubit $B$ is measured first, then transferred to a nonfluorescing state, followed by detection of qubit $A$.