Robust entanglement at room temperature is a necessary requirement for practical applications in quantum technology. We demonstrate
the creation of bipartite- and tripartite-entangled quantum states in a small quantum register consisting of individual 13C nuclei in a diamond lattice. Individual nuclear spins are controlled via their hyperfine coupling to a single electron at
a nitrogen-vacancy defect center. Quantum correlations are of high quality and persist on a millisecond time scale even at
room temperature, which is adequate for sophisticated quantum operations.
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"With electron spin states accessible by visible lasers and microwave radiation   , diamond nitrogenvacancy (NV) centers have broad applications in quantum information    and nanoscale sensing of magnetic field   , temperature  , and beyond . Combining such NV spin systems with mechanical resonators   will provide a hybrid system with versatile functions . "
[Show abstract][Hide abstract] ABSTRACT: Electron spins of diamond nitrogen-vacancy (NV) centers are important quantum
resources for nanoscale sensing and quantum information. Combining such NV spin
systems with levitated optomechanical resonators will provide a hybrid quantum
system for many novel applications. Here we optically levitate a nanodiamond
and demonstrate electron spin control of its built-in NV centers in low vacuum.
We observe that the strength of electron spin resonance (ESR) is enhanced when
the air pressure is reduced. To better understand this novel system, we also
investigate the effects of trap power and measure the absolute internal
temperature of levitated nanodiamonds with ESR after calibration of the strain
effect. Our results show that optical levitation of nanodiamonds in vacuum not
only can improve the mechanical quality of its oscillation, but also enhance
the ESR contrast, which pave the way towards a novel levitated
spin-optomechanical system for studying macroscopic quantum mechanics. The
results also indicate potential applications of NV centers in gas sensing.
"arXiv:1303.2433v1 [quant-ph] 11 Mar 2013 Several architectures have realized multi-particle entanglement     but trapped ions have demonstrated particularly high fidelity in the preparation, manipulation and detection of quantum states. Similar to other systems, however, such ion trap architectures are affected by crosstalk. "
[Show abstract][Hide abstract] ABSTRACT: Entanglement in a quantum system can be demonstrated experimentally by
performing the measurements prescribed by an appropriate entanglement witness.
However, the unavoidable mismatch between the implementation of measurements in
practical devices and their precise theoretical modelling generally results in
the undesired possibility of false-positive entanglement detection. Such
scenarios can be avoided by using the recently developed device-independent
entanglement witnesses (DIEWs) for genuine multipartite entanglement. Similarly
to Bell inequalities, DIEWs only assume that consistent measurements are
performed locally on each subsystem. No precise description of the measurement
devices is required. Here we report an experimental test of DIEWs on up to six
entangled 40Ca+ ions. We also demonstrate genuine multipartite quantum
nonlocality between up to six parties with the detection loophole closed.
"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– 24]. Furthermore, the NV electron spin can be used to access and manipulate nearby nuclear spins     , thereby forming a multi-qubit register. To use such registers in a quantum network requires a mechanism to coherently connect remote NV centres. "
[Show abstract][Hide abstract] ABSTRACT: 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. As well as being of fundamental interest, entanglement is a unique resource for quantum information processing and communication. Entangled quantum bits (qubits) can be used to share private information or implement quantum logical gates. Such capabilities are particularly useful when the entangled qubits are spatially separated, providing the opportunity to create highly connected quantum networks or extend quantum cryptography to long distances. Here we report entanglement of two electron spin qubits in diamond with a spatial separation of three metres. We establish this entanglement using a robust protocol based on creation of spin-photon entanglement at each location 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 on the qubits in different bases. The long-distance entanglement reported here can be combined with recently achieved initialization, readout and entanglement operations on local long-lived nuclear spin registers, paving the way for deterministic long-distance teleportation, quantum repeaters and extended quantum networks.