Quantum entanglement between spatially separated objects is one of the most intriguing phenomena in physics. The outcomes of independent measurements on entangled objects show correlations that cannot be explained by classical physics. Besides being of fundamental interest, entanglement is a unique resource for quantum information processing and communication. Entangled qubits can be used to establish private information or implement quantum logical gates [1,2]. Such capabilities are particularly useful when the entangled qubits are spatially separated [3][4][5], opening the opportunity to create highly connected quantum networks [6] or extend quantum cryptography to long distances [7,8]. Here we present a key experiment towards the realization of long-distance quantum networks with solid-state quantum registers. We have entangled two electron spin qubits in diamond that are separated by a three-meter distance. We establish this entanglement using a robust protocol based on local creation of spin-photon entanglement and a subsequent joint measurement of the photons. Detection of the photons heralds the projection of the spin qubits onto an entangled state. We verify the resulting non-local quantum correlations by performing single-shot readout [9] on the qubits in different bases. The long-distance entanglement reported here can be combined with recently achieved initialization, readout and entanglement operations [9-13] on local long-lived nuclear spin registers, enabling deterministic long-distance teleportation, quantum repeaters and extended quantum networks.A quantum network can be constructed by using entanglement to connect local processing nodes, each containing a register of well-controlled and long-lived qubits [6]. Solids are an attractive platform for such registers, as the use of nanofabrication and material design may enable well-controlled and scalable qubit systems [14]. The potential impact of quantum networks on science and technology has recently spurred research efforts towards generating entangled states of distant solid-state qubits [15][16][17][18][19][20][21].A prime candidate for a solid-state quantum register is the nitrogen-vacancy (NV) defect centre in diamond. The NV centre combines a long-lived electronic spin (S=1) with a robust optical interface, enabling measurement and high-fidelity control of the spin qubit [15,[22][23][24]. Furthermore, the NV electron spin can be used to access and manipulate nearby nuclear spins [9-13], thereby forming a multi-qubit register. To use such registers in a quantum network requires a mechanism to coherently connect remote NV centres.Here we demonstrate the generation of entanglement between NV centre spin qubits in distant setups. We achieve this breakthrough by combining recently established spin initialization and single-shot readout techniques [9] with efficient resonant optical detection and feedback-based control over the optical transitions, all in a single experiment and executed with high fidelity. These results put solid-state qubits on ...

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...

Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson junction-based artificial atoms, while maintaining superior coherence. We demonstrate a novel superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in Josephson junction-based quantum circuits.

Projective measurements are a powerful tool for manipulating quantum states 1-13 . In particular, a set of qubits can be entangled by measuring a joint property 3-13 such as qubit parity. These joint measurements do not require a direct interaction between qubits and therefore provide a unique resource for quantum information processing with wellisolated qubits. Numerous schemes for entanglement-bymeasurement of solid-state qubits have been proposed [8][9][10][11][12][13] , but the demanding experimental requirements have so far hindered implementations. Here we realize a two-qubit parity measurement on nuclear spins localized near a nitrogenvacancy centre in diamond by exploiting an electron spin as a readout ancilla. The measurement enables us to project the initially uncorrelated nuclear spins into maximally entangled states. By combining this entanglement with single-shot readout we demonstrate the first violation of Bell's inequality with solid-state spins. These results introduce a new class of experiments in which projective measurements create, protect and manipulate entanglement between solid-state qubits.A quantum measurement not only extracts information from a system but also modifies its state: the system is projected into an eigenstate of the measurement operator. Such projective measurements can be used to control and entangle qubits [1][2][3][4][5][6][7][8][9] . Of particular importance is the qubit parity measurement that can create maximally entangled states 10-13 , plays a central role in quantum error correction 14 and enables deterministic two-qubit gates 8,10 .Qubit parity is a joint property that indicates whether an even or odd number of qubits is in a particular eigenstate. For two qubits, an ideal parity measurement projects the qubits either into the subspace in which the qubits have the same value (even parity) or into the subspace in which they have opposite values (odd parity). Crucially, only information on the parity is extracted: the measurement must not reveal any other information about the state of the qubits or otherwise disturb it.To realize such a parity measurement the use of an ancillary system is required, as illustrated in Fig. 1a. First, both qubits are made to interact with this system such that its state becomes correlated with the parity of the joint two-qubit state. A subsequent high-fidelity readout of the ancillary system then projects the two qubits into the even or odd parity subspace. For suitably chosen initial states, the parity measurement projects the qubits into a maximally entangled state. Implementations have been proposed for various qubit systems 8,10-13 , but owing to the high demands on qubit control and ancilla readout experimental realization has remained elusive.We achieve a heralded qubit parity measurement on two nuclear spins in diamond by using the electron spin of a nitrogen-vacancy 1 Kavli Institute of Nanoscience Delft, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands, 2 Element Six, Ltd, Kings Ride Park, Ascot S...

Entangling two remote quantum systems that never interact directly is an essential primitive in quantum information science and forms the basis for the modular architecture of quantum computing. When protocols to generate these remote entangled pairs rely on using traveling single-photon states as carriers of quantum information, they can be made robust to photon losses, unlike schemes that rely on continuous variable states. However, efficiently detecting single photons is challenging in the domain of superconducting quantum circuits because of the low energy of microwave quanta. Here, we report the realization of a robust form of concurrent remote entanglement based on a novel microwave photon detector implemented in the superconducting circuit quantum electrodynamics platform of quantum information. Remote entangled pairs with a fidelity of 0.57 AE 0.01 are generated at 200 Hz. Our experiment opens the way for the implementation of the modular architecture of quantum computation with superconducting qubits.