and macroscopic diamond 10 . Here we experimentally demonstrate entanglement between two engineered single solid-state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground-state spin correlation. Entanglement (fidelity = 0.67 ± 0.04) is proved by quantum state tomography. Moreover, the lifetime of electron spin entanglement is extended to milliseconds by entanglement swapping to nuclear spins. The experiments mark an important step towards a scalable room-temperature quantum device being of potential use in quantum information processing as well as metrology.Engineering entangled quantum states is a decisive step in quantum technology. Although entanglement among weakly interacting systems such as photons has been demonstrated already in the early stages of quantum optics, deterministic generation of entanglement in more complex systems such atoms or ions, not to mention solids, is a relatively recent achievement 11 . Usually in solid-state systems rapid dephasing ceases any useful degree of quantum correlations. Either decoupling must be used to protect quantum states or careful materials engineering is required to prolong coherence. Most often however, and this is especially important for solid-state systems, one needs to resort to low (milliKelvin) temperatures to achieve sufficiently robust and longlasting quantum coherence. Spins are sufficiently weakly coupled to their environment to allow for the observation of coherence at room temperatures.Diamond defect spins are particularly interesting solid-state spin qubit systems. A number of hallmark demonstrations such as single-, two-and three-qubit operations, high-fidelity single-shot readout 12 , one-and two-qubit algorithms 13 , and entanglement between nuclear and electron and nuclear spin qubits have been achieved 6,14 . Different schemes to scale the system to a larger number of entangled electron spins have been proposed [15][16][17] . A path towards room-temperature entanglement is strong coupling among the ground-state spin magnetic dipole moment of adjacent defects centres. This mutual dipolar interaction scales as distance d −3 and should be larger than the interaction of each electron spin with the residual paramagnetic impurities or nuclear spin moments in the lattice (Fig. 1d). Typical cutoff distances for strong interaction are thus limited by the electron spin dephasing time (milliseconds) to be around 30 nm. Here we demonstrate entanglement between two electron and nuclear spins at a distance of approximately 25 nm. At these distances magnetic dipole coupling is strong enough to attain high-fidelity entanglement while being able to address the spins individually by super-resolution optical microscopy 18 . The optical as well as spin physics of nitrogen vacancy (NV) defects in diamond has been subject to numerous investigations 11,19 . The fluorescence intensity of the strongly allowed optical transition between ground and excited spin triplet states depends on the magnetic quantum number of the groun...

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication 1 . However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials 2,3 , particularly diamond 4-6 , however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin-orbit coupling. Here we show that quantum logic elements can be realized by exploring long-range magnetic dipolar coupling between individually addressable single electron spins associated with separate colour centres in diamond. The strong distance dependence of this coupling was used to characterize the separation of single qubits (98±3 Å) with an accuracy close to the value of the crystal-lattice spacing. Our demonstration of coherent control over both electron spins, conditional dynamics, selective readout as well as switchable interaction should open the way towards a viable room-temperature solid-state quantum register. As both electron spins are optically addressable, this solid-state quantum device operating at ambient conditions provides a degree of control that is at present available only for a few systems at low temperature.One of the greatest challenges in quantum information technology is to build a room-temperature scalable quantum processor 7 . Isolated electron and nuclear spins in solids are considered to be among the most promising candidates for qubits in that respect 3,8 . Several benchmark experiments including entanglement and elements of quantum memory 9 have been achieved with spin ensembles, but ultimate functionality requires encoding quantum information into single spins. This however creates serious challenges in readout, addressing and nano-engineering single-spin arrays. The availability of photon-assisted single-spin readout 10,11 and the possibility to create single defects by ion implantation 12,13 make nitrogen-vacancy defects in diamond one of the most promising candidates in this respect. Paramagnetic nuclei in the vicinity of the electron spin can be used as auxiliary qubits with even more favourable relaxation properties 14 . As a consequence, coherence between electron and nuclear spin qubits has been exploited for showing all basic elements of a room-temperature quantum register 5,[15][16][17] . The size of these registers however is limited to a few quantum bits owing to the limited number of nuclear spins that can be addressed in frequency space 15,18 . A critical step towards scalability is to develop a technique enabling mutual coupling of individual optically addressable quantum systems. The system used in this study is a pair of single electron spins associated with separate nitrogen-vacancy defects in diamond. A single defect consists of a substitutional nitrogen atom in the diamond lattice and an adjacent vacancy (Fig. 1a,b). The electron spin triplet ground state of the defect shows a spin-depen...

Precise timekeeping is critical to metrology, forming the basis by which standards of time, length, and fundamental constants are determined. Stable clocks are particularly valuable in spectroscopy because they define the ultimate frequency precision that can be reached. In quantum metrology, the qubit coherence time defines the clock stability, from which the spectral linewidth and frequency precision are determined. We demonstrate a quantum sensing protocol in which the spectral precision goes beyond the sensor coherence time and is limited by the stability of a classical clock. Using this technique, we observed a precision in frequency estimation scaling in time as for classical oscillating fields. The narrow linewidth magnetometer based on single spins in diamond is used to sense nanoscale magnetic fields with an intrinsic frequency resolution of 607 microhertz, which is eight orders of magnitude narrower than the qubit coherence time.

This article reports stable photoluminescence and high-contrast optically detected electron spin resonance (ODESR) from single nitrogen-vacancy (NV) defect centers created within ultrasmall, disperse nanodiamonds of radius less than 4 nm. Unexpectedly, the efficiency for the production of NV fluorescent defects by electron irradiation is found to be independent of the size of the nanocrystals. Fluorescence lifetime imaging shows lifetimes with a mean value of around 17 ns, only slightly longer than the bulk value of the defects. After proper surface cleaning, the dephasing times of the electron spin resonance in the nanocrystals approach values of some microseconds, which is typical for the type Ib diamond from which the nanoparticle is made. We conclude that despite the tiny size of these nanodiamonds the photoactive nitrogen-vacancy color centers retain their bulk properties to the benefit of numerous exciting potential applications in photonics, biomedical labeling, and imaging.