Realizing robust quantum information transfer between long-lived qubit registers is a key challenge for quantum information science and technology. Here we demonstrate unconditional teleportation of arbitrary quantum states between diamond spin qubits separated by 3 meters. We prepare the teleporter through photon-mediated heralded entanglement between two distant electron spins and subsequently encode the source qubit in a single nuclear spin. By realizing a fully deterministic Bell-state measurement combined with real-time feed-forward we achieve teleportation in each attempt while obtaining an average state fidelity exceeding the classical limit. These results establish diamond spin qubits as a prime candidate for the realization of quantum networks for quantum communication and network-based quantum computing.The reliable transmission of quantum states between remote locations is a major open challenge in quantum science today. Quantum state transfer between nodes containing long-lived qubits [1][2][3] can extend quantum key distribution to long distances [4], enable blind quantum computing in the cloud [5] and serve as a critical primitive for a future quantum network [6]. When provided with a single copy of an unknown quantum state, directly sending the state in a carrier such as a photon is unreliable due to inevitable losses. Creating and sending several copies of the state to counteract such transmission losses is impossible by the no-cloning theorem [7]. Nevertheless, quantum information can be faithfully transmitted over arbitrary distances through quantum teleportation provided the network parties (named "Alice" and "Bob") have previously established a shared entangled state and can communicate classically [8][9][10][11].The teleportation protocol is sketched in Fig. 1A. At the start, Alice is in possession of the state to be teleported (qubit 1) which is most generally given by |ψ = α|0 + β|1 . Alice and Bob each have one qubit of an entangled pair (qubits 2 and 3) in the joint state |ΨThe combined state of all three qubits can be rewritten aswhere |Φ ± = (|00 ± |11 )/ √ 2 and |Ψ ± = (|01 ± |10 )/ √ 2 are the four Bell states. To teleport the quantum state Alice performs a joint measurement on her * Present address: Department of Applied Physics, Yale University, New Haven, CT 06511, USA † r.hanson@tudelft.nl qubits (qubits 1 and 2) in the Bell basis, projecting Bob's qubit into a state that is equal to |ψ up to a unitary operation that depends on the outcome of Alice's measurement. Alice sends the outcome via a classical communication channel to Bob, who can then recover the original state by applying the corresponding local transformation.Because the source qubit state always disappears on Alice's side, it is irrevocably lost whenever the protocol fails. Therefore, to ensure that each qubit state inserted into the teleporter unconditionally re-appears on Bob's side, Alice must be able to distinguish between all four Bell states in a single shot and Bob has to preserve the coherence of the target q...

The advancement of quantum optical science and technology with solid-state emitters such as nitrogen-vacancy (NV) centers in diamond critically relies on the coherence of the emitters' optical transitions. A widely employed strategy to create NV centers at precisely controlled locations is nitrogen ion implantation followed by a high-temperature annealing process. We report on experimental data directly correlating the NV center optical coherence to the origin of the nitrogen atom. These studies reveal low-strain, narrow-optical-linewidth (< 500 MHz) NV centers formed from naturally-occurring 14 N atoms. In contrast, NV centers formed from implanted 15 N atoms exhibit significantly broadened optical transitions (> 1 GHz) and higher strain. The data show that the poor optical coherence of the NV centers formed from implanted nitrogen is not due to an intrinsic effect related to the diamond or isotope. These results have immediate implications for the positioning accuracy of current NV center creation protocols and point to the need to further investigate the influence of lattice damage on the coherence of NV centers from implanted ions.

We report on the fabrication and characterization of a Fabry-Perot microcavity enclosing a thin diamond membrane at cryogenic temperatures. The cavity is designed to enhance resonant emission of single nitrogen-vacancy centers by allowing spectral and spatial tuning while preserving the optical properties observed in bulk diamond. We demonstrate cavity finesse at cryogenic temperatures within the range of F=4000–12 000 and find a sub-nanometer cavity stability. Modeling shows that coupling nitrogen-vacancy centers to these cavities could lead to an increase in remote entanglement success rates by three orders of magnitude.