Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 muT, and the corresponding field from a single proton is a few nanoteslas. A sensor able to detect such magnetic fields with nanometre spatial resolution would enable powerful applications, ranging from the detection of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. Here we experimentally demonstrate an approach to such nanoscale magnetic sensing, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature. Using an ultra-pure diamond sample, we achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging. In addition, we demonstrate a sensitivity of 0.5 muT Hz(-1/2) for a diamond nanocrystal with a diameter of 30 nm.

Understanding and controlling the complex environment of solid-state quantum bits is a central challenge in spintronics and quantum information science. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond was used to gain insight into its local environment. We show that this environment is effectively separated into a set of individual proximal 13C nuclear spins, which are coupled coherently to the electron spin, and the remainder of the 13C nuclear spins, which cause the loss of coherence. The proximal nuclear spins can be addressed and coupled individually because of quantum back-action from the electron, which modifies their energy levels and magnetic moments, effectively distinguishing them from the rest of the nuclei. These results open the door to coherent manipulation of individual isolated nuclear spins in a solid-state environment even at room temperature.

The key challenge in experimental quantum information science is to identify isolated quantum mechanical systems with long coherence times that can be manipulated and coupled together in a scalable fashion. We describe the coherent manipulation of an individual electron spin and nearby individual nuclear spins to create a controllable quantum register. Using optical and microwave radiation to control an electron spin associated with the nitrogen vacancy (NV) color center in diamond, we demonstrated robust initialization of electron and nuclear spin quantum bits (qubits) and transfer of arbitrary quantum states between them at room temperature. Moreover, nuclear spin qubits could be well isolated from the electron spin, even during optical polarization and measurement of the electronic state. Finally, coherent interactions between individual nuclear spin qubits were observed and their excellent coherence properties were demonstrated. These registers can be used as a basis for scalable, optically coupled quantum information systems.

Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ions, atoms and atomic ensembles, which are then used to connect remote long-term memory nodes in distributed quantum networks. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks.

We describe a technique that enables a strong, coherent coupling between a single electronic spin qubit associated with a nitrogen-vacancy impurity in diamond and the quantized motion of a magnetized nano-mechanical resonator tip. This coupling is achieved via careful preparation of dressed spin states which are highly sensitive to the motion of the resonator but insensitive to perturbations from the nuclear spin bath. In combination with optical pumping techniques, the coherent exchange between spin and motional excitations enables ground state cooling and the controlled generation of arbitrary quantum superpositions of resonator states. Optical spin readout techniques provide a general measurement toolbox for the resonator with quantum limited precision.PACS numbers: 07.10. Cm, 42.50.Pq, Techniques for cooling and quantum manipulation of motional states of nano-mechanical resonators are now actively explored. Work in this field is motivated by ideas from quantum information science [1,2], testing quantum mechanics for macroscopic objects [3,4] and potential applications in nano-scale sensing [5,6]. Approaches based on mechanical resonators coupled to optical cavities [7], superconducting devices [8,9] or cold atoms [10] are presently being investigated in experiments.In this paper we describe a technique that enables a coherent coupling between the quantized motion of a mechanical resonator and an isolated spin qubit. Specifically, we focus on the electronic spin associated with a nitrogen-vacancy (NV) impurity in diamond [11] which can be optically polarized and detected, and exhibits excellent coherence properties even at room temperature [12]. Since its precession frequency depends on external magnetic fields via the Zeeman effect, single spins can be used as magnetic sensors operating at nanometer scales [13,14].The essential idea of the present work can be understood by considering a prototype system shown in Fig. 1. Here a single spin is used to sense the motion of the magnetized resonator tip, that is separated from the spin by an average distance h and oscillates at frequency ω r . These oscillations produce a time-varying magnetic field that causes Zeeman shifts of the spin qubit. Specifically, the shift corresponding to a single quantum of motion is λ = g s µ B G m a 0 , where g s ≃ 2, µ B is the Bohr magneton, G m the magnetic field gradient and a 0 = /2mω r the amplitude of zero-point fluctuations for a resonator of mass m. For realistic conditions, h ≈ 25 nm, ω r /2π ≈ 5 MHz, a 0 ≈ 5 × 10 −13 m and G m ≈ 10 7 T/m we find that λ/2π can approach 100 kHz. Such a large shift can be easily measured within a fraction of a millisecond by detecting the electronic spin state [14]. More importantly, the coupling constant λ can considerably exceed both the electronic spin coherence time (T 2 ∼ 1 ms) and the intrinsic damping rate, κ = ω r /Q, of high-Q mechani- cal resonators. In this regime, the spin becomes strongly coupled to mechanical motion in direct analogy to strong coupling of cavity quantum electro...

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

We investigate the coherence properties of individual nuclear spin quantum bits in diamond [Dutt et al., Science, 316, 1312] when a proximal electronic spin associated with a nitrogenvacancy (NV) center is being interrogated by optical radiation. The resulting nuclear spin dynamics are governed by time-dependent hyperfine interaction associated with rapid electronic transitions, which can be described by a spin-fluctuator model. We show that due to a process analogous to motional averaging in nuclear magnetic resonance, the nuclear spin coherence can be preserved after a large number of optical excitation cycles. Our theoretical analysis is in good agreement with experimental results. It indicates a novel approach that could potentially isolate the nuclear spin system completely from the electronic environment.Nuclear spins are of fundamental importance for storage and processing of quantum information. Their excellent coherence properties make them a superior qubit candidate even in room temperature solids. Unfortunately, their weak coupling to the environment also makes it difficult to isolate and manipulate individual nuclei. Recently, coherent preparation, manipulation and readout of individual 13 C nuclear spins in the diamond lattice were demonstrated [1,2]. These experiments make use of optical polarization and manipulation of the electronic spin associated with a nitrogen-vacancy (NV) color center in the diamond lattice [3,4,5,6]. This enables reliable control of the nuclear spin qubit via hyperfine interactions with the electronic spin.In order to be useful for applications in scalable quantum information processing [3], such as quantum communication [7] and quantum computation [8], the quantum coherence of the nuclear spins must be maintained even when the electronic state is undergoing fast transitions associated with optical measurement and with entanglement generation between electronic spins. In this Letter, we investigate coherence properties of such an optically illuminated nuclear spin-electron spin system. We show that these properties are well-described by a spin-fluctuator model [9,10,11,12], involving a single nuclear spin (system) coupled by the hyperfine interaction to an electron [13] (fluctuator) that undergoes rapid optical transitions and mediates the coupling between the nuclear spin and the environment. We generalize the spin-fluctuator model to a vector description, necessary for single NV centers in diamond [1], and make direct comparisons with experiments. Most importantly we demonstrate that the decoherence of the nuclear spin due to the rapidly fluctuating electron is greatly suppressed via a mechanism analogous to motional narrowing in nuclear magnetic resonance (NMR) [14,15], allowing the nuclear spin coherence to be preserved even after hundreds of optical excitation cycles. We further show that by proper tuning of experimental parameters it may be possible to completely decouple the nuclear spin system from the electronic environment. The spinfluctuator model discussed here ...

Magnetic sensors capable of detecting nanoscale volumes of spins allow for non-invasive, element-specific probing. The error in such measurements is usually reduced by increasing the measurement time, and noise averaging the signal. However, achieving the best precision requires restricting the maximum possible field strength to much less than the spectral linewidth of the sensor. Quantum entanglement and squeezing can then be used to improve precision (although they are difficult to implement in solid-state environments). When the field strength is comparable to or greater than the spectral linewidth, an undesirable trade-off between field strength and signal precision occurs. Here, we implement novel phase estimation algorithms on a single electronic spin associated with the nitrogen-vacancy defect centre in diamond to achieve an ∼8.5-fold improvement in the ratio of the maximum field strength to precision, for field magnitudes that are large (∼0.3 mT) compared to the spectral linewidth of the sensor (∼4.5 µT). The field uncertainty in our approach scales as 1/T(0.88), compared to 1/T(0.5) in the standard measurement approach, where T is the measurement time. Quantum phase estimation algorithms have also recently been implemented using a single nuclear spin in a nitrogen-vacancy centre. Besides their direct impact on applications in magnetic sensing and imaging at the nanoscale, these results may prove useful in improving a variety of high-precision spectroscopy techniques.