Experimental quantum state transfer of an arbitrary single-qubit state on a cycle with four vertices using a coined quantum random walk

Abstract

We experimentally demonstrate the transfer of an unknown single-qubit state from Alice to Bob via a two-step discrete-time quantum random walk on a cycle with four vertices on a four-qubit nuclear magnetic resonance quantum processor. The qubits with Alice and Bob are used as coin qubits and the walk is carried out on in a two-qubit ‘Gaming Arena’. In this scheme, the required entangled state is generated naturally via conditional shift operators during the quantum walk, instead of being prepared in advance. We implement controlled operators at Bob’s end, which are controlled by Alice’s coin qubit and arena qubits, in order to reconstruct Alice’s randomly generated state at Bob’s end. To characterize the state transfer process, we perform quantum process tomography by repeating the experiment for a set of input states $$\{ \vert 0\rangle , \vert 1\rangle , \vert +\rangle , \vert -\rangle \}$$ { | 0 ⟩ , | 1 ⟩ , | + ⟩ , | - ⟩ } . Using an entanglement witness, we certify that the quantum walk generates a genuine quadripartite entangled state of all four qubits. To evaluate the efficacy of the transfer scheme, we use quantum state tomography to reconstruct the transferred state by calculating the projection of the experimentally reconstructed four-qubit density matrix onto three-qubit basis states. Our results demonstrate that the quantum circuit is able to perform quantum state transfer via the two-step quantum random walk with high fidelity.