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As promising quantum sensors, nitrogen-vacancy (NV) centers in diamond have been widely used in frontier studies in condensed matter physics, material sciences, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, for example, phototoxicity and heating. Here we report a combined theoretical and experimental study of optically detected magnetic resonance (ODMR) of NV-center ensembles under weak 532-nm laser excitation. In this regime, both the width and splitting of ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model considering laser-induced charge neutralization of NV--N+ pairs, which alters the local electric field environment. These results are important for understanding and designing NV-based quantum sensing in light-sensitive applications.

Megabar pressures are of crucial importance for cutting-edge studies of condensed matter physics and geophysics. With the development of diamond anvil cell, laboratory studies of high pressure have entered the megabar era for decades. However, it is still challenging to implement in-situ magnetic sensing under ultrahigh pressures. Here, we demonstrate optically detected magnetic resonance of diamond nitrogen-vacancy (NV) centers, a promising quantum sensor of strain and magnetic fields, up to 1.4 Mbar. We quantify the reduction and blueshifts of NV fluorescence under high pressures. We demonstrate coherent manipulation of NV electron spins and extend its working pressure to the megabar region. These results shed new light on our understanding of diamond NV centers and will benefit quantum sensing under extreme conditions., Comment: 9 pages, 4 figures

The integration of NMR and high pressure technique brings unique opportunities to study electronic, structural and dynamical properties under extreme conditions. Despite a great degree of success has been achieved using coil-based schemes, the contradictory requirement on sample volume of these two techniques remains an outstanding challenge. In this letter, we introduce diamond nitrogen-vacancy (NV) centers, as the source and probe of in-situ nuclear spin polarization, to address the sample volume issue. We demonstrate hyperpolarization and coherent control of $^{14}$N nuclear spins under high pressures. NMR spectra of a micro-diamond are measured up to 16.6 GPa, and unexpected pressure shift of the $^{14}$N nuclear quadrupole and hyperfine coupling terms are observed. Our work contributes to quantum sensing enhanced spectrometry under extreme conditions., Comment: 6 pages, 4 figures

Quantum sensors have recently achieved to detect the magnetic moment of few or single nuclear spins and measure their magnetic resonance (NMR) signal. However, the spectral resolution, a key feature of NMR, has been limited by relaxation of the sensor to a few kHz at room temperature. The spectral resolution of NMR signals from single nuclear spins can be improved by, e.g., using quantum memories, however at the expense of sensitivity. Classical signals on the other hand can be measured with exceptional spectral resolution by using continuous measurement techniques, without compromising sensitivity. To apply these techniques to single-spin NMR, it is critical to overcome the impact of back action inherent of quantum measurements. Here we report sequential weak measurements on a single $^{13}$C nuclear spin. The back-action of repetitive weak measurements causes the spin to undergo a quantum dynamics phase transition from coherent trapping to coherent oscillation. Single-spin NMR at room-temperature with a spectral resolution of 3.8 Hz is achieved. These results enable the use of measurement-correlation schemes for the detection of very weakly coupled single spins., Comment: Article: 16 pages, 4 figures Supplementary: 23 pages, 9 figures

Single-shot readout of qubits is required for scalable quantum computing. Nuclear spins are superb quantum memories due to their long coherence times but are difficult to be read out in single shot due to their weak interaction with probes. Here we demonstrate single-shot readout of a weakly coupled $^{13}$C nuclear spin, which is unresolvable in traditional protocols. We use dynamical decoupling pulse sequences to selectively enhance the entanglement between the nuclear spin and a nitrogen-vacancy center electron spin, tuning the weak measurement of the nuclear spin to a strong, projective one. A nuclear spin coupled to the NV center with strength 330 kHz is read out in 200 ms with fidelity 95.5\%. This work provides a general protocol for single-shot readout of weakly coupled qubits and therefore largely extends the range of physical systems for scalable quantum computing., Comment: 5 pages, 4 figures

One unique feature of quantum mechanics is the Heisenberg uncertainty principle, which states that the outcomes of two incompatible measurements cannot simultaneously achieve arbitrary precision. In an information-theoretic context of quantum information, the uncertainty principle can be formulated as entropic uncertainty relations with two measurements for a quantum bit (qubit) in two-dimensional system. New entropic uncertainty relations are studied for a higher-dimensional quantum state with multiple measurements, the uncertainty bounds can be tighter than that expected from two measurements settings and cannot result from qubits system with or without a quantum memory. Here we report the first room-temperature experimental testing of the entropic uncertainty relations with three measurements in a natural three-dimensional solid-state system: the nitrogen-vacancy center in pure diamond. The experimental results confirm the entropic uncertainty relations for multiple measurements. Our result represents a more precise demonstrating of the fundamental uncertainty principle of quantum mechanics., Comment: 8 pages, 5 figures, 2 tables

Precise parameter estimation plays a central role in science and technology. The statistical error in estimation can be decreased by repeating measurement, leading to that the resultant uncertainty of the estimated parameter is proportional to the square root of the number of repetitions in accordance with the central limit theorem. Quantum parameter estimation, an emerging field of quantum technology, aims to use quantum resources to yield higher statistical precision than classical approaches. Here, we report the first room-temperature implementation of entanglement-enhanced phase estimation in a solid-state system: the nitrogen-vacancy centre in pure diamond. We demonstrate a super-resolving phase measurement with two entangled qubits of different physical realizations: an nitrogen-vacancy centre electron spin and a proximal ${}^{13}$C nuclear spin. The experimental data shows clearly the uncertainty reduction when entanglement resource is used, confirming the theoretical expectation. Our results represent an elemental demonstration of enhancement of quantum metrology against classical procedure., Comment: 9 pages including the supplementary material, 6 figures in main text plus 3 figures in supplementary material

High fidelity quantum control over qubits is of crucial importance for realistic quantum computing, and it turns to be more challenging when there are inevitable interactions among qubits. By employing a bandselective shaped pulse, we demonstrate a high fidelity flip over electron spin of nitrogen-vacancy (NV) centers in diamond. In contrast with traditional rectangular pulses, the shaped pulse has almost equal excitation effect among a sharply edged region (in frequency domain). So the three sub-levels of host $^{14}N$ nuclear spin can be flipped accurately at the same time, while the redundant flip of other sublevels (e. g. of a nearby $^{13}C$ nuclear spin ) is well suppressed. The shaped pulse can be applied to a large amount of quantum systems in which band-selective operation are required., Comment: 4 pages, 4 figures

Realistic quantum computing is subjected to noise. A most important frontier in research of quantum computing is to implement noise-resilient quantum control over qubits. Dynamical decoupling can protect coherence of qubits. Here we demonstrate non-trivial quantum evolution steered by dynamical decoupling control, which automatically suppresses the noise effect. We designed and implemented a self-protected controlled-NOT gate on the electron spin of a nitrogen-vacancy centre and a nearby carbon-13 nuclear spin in diamond at room temperature, by employing an engineered dynamical decoupling control on the electron spin. Final state fidelities of 0.91 and 0.88 were observed even with imperfect initial states. In the mean time, the qubit coherence time has been elongated by at least 30 folds. The design scheme does not require that the dynamical decoupling control commute with the qubit interaction and works for general systems. This work marks a step toward realistic quantum computing., Comment: Originally submitted version. Revised version and Supplementary information can be download at http://www.nature.com/ncomms/2013/130805/ncomms3254/full/ncomms3254.html

We experimentally investigate the protection of electron spin coherence of nitrogen vacancy (NV) center in diamond by dynamical nuclear polarization. The electron spin decoherence of an NV center is caused by the magnetic ield fluctuation of the $^{13}$C nuclear spin bath, which contributes large thermal fluctuation to the center electron spin when it is in equilibrium state at room temperature. To address this issue, we continuously transfer the angular momentum from electron spin to nuclear spins, and pump the nuclear spin bath to a polarized state under Hartman-Hahn condition. The bath polarization effect is verified by the observation of prolongation of the electron spin coherence time ($T_2^*$). Optimal conditions for the dynamical nuclear polarization (DNP) process, including the pumping pulse duration and depolarization effect of laser pulses, are studied. Our experimental results provide strong support for quantum information processing and quantum simulation using polarized nuclear spin bath in solid state systems., Comment: 4 pages, 4 figures

Quantum mechanics implies that not all physical properties can be simultaneously well defined, such as the momentum and position due to Heisenberg uncertainty principle. Some alternative theories have been explored, notably the non-contextual hidden variable theories in which the properties of a system have pre-defined values which are independent of the measurement contextual. However, the Kochen-Specker theorem showed that such non-contextual hidden variable theories are in conflict with quantum mechanics. Recently, a state-independent inequality satisfied by non-contextual hidden variable theories and violated by quantum mechanics is proposed in the simplest three-state system (a qutrit) by the least 13 projection measurement rays. Here, we report an experimental demonstration of the violation of this inequality. This provides a state-independent experimental test of quantum contextuality, for the first time in solid state system, by a natural qutrit of nitrogen-vacancy center in diamond at room temperature., Comment: 20 pages, 4 figures

Fluctuations of local fields cause decoherence of quantum objects. It is generally believed that at high temperatures, thermal noises are much stronger than quantum fluctuations unless the thermal effects are suppressed by certain techniques such as spin echo. Here we report the discovery of strong quantum-fluctuation effects of nuclear spin baths on free-induction decay of single electron spins in solids at room temperature. We find that the competition between the quantum and thermal fluctuations is controllable by an external magnetic field. These findings are based on Ramsey interference measurement of single nitrogen-vacancy center spins in diamond and numerical simulation of the decoherence, which are in excellent agreement., Comment: 4 pages 4 figures, experiment & theory

Here we report an experimental realization of optimal phase-covariant quantum cloning machine with a single electron spin in solid state system at room temperature. The involved three states of two logic qubits are encoded physically in three levels of a single electron spin with two Zeeman sub-levels at a nitrogen-vacancy defect center in diamond. The preparation of input state and the phase-covariant quantum cloning transformation are controlled by two independent microwave fields. The average experimental fidelity reaches 85.2% which is very close to theoretical optimal fidelity 85.4% and is beyond the bound 83.3% of universal cloning., Comment: 3 pages; 5 figures