Paz London

Bulk Nuclear Polarization Enhanced at Room Temperature by Optical Pumping

Bulk (13)C polarization can be strongly enhanced in diamond at room temperature based on the optical pumping of nitrogen-vacancy color centers. This effect was confirmed by irradiating single crystals at a ~50 mT field promoting anticrossings between electronic excited-state levels, followed by shuttling of the sample into an NMR setup and by subsequent (13)C detection. A nuclear polarization of ~0.5%--equivalent to the (13)C polarization achievable by thermal polarization at room temperature at fields of ~2000 T--was measured, and its bulk nature determined based on line shape and relaxation measurements. Positive and negative enhanced polarizations were obtained, with a generally complex but predictable dependence on the magnetic field during optical pumping. Owing to its simplicity, this (13)C room temperature polarizing strategy provides a promising new addition to existing nuclear hyperpolarization techniques.

Elimination, reversal and directional bias of optical diffraction

Electromagnetically induced transparency in an atomic gas can slow the propagation of images. It is now shown that the diffraction of such images as they propagate can be controlled and even eliminated. This is achieved by using atomic diffusion to influence the spreading of the image. Any image, imprinted on a wave field and propagating in free space, undergoes a paraxial diffraction spreading. The reduction or manipulation of diffraction is desirable for many applications, such as imaging, wave-guiding, microlithography and optical data processing. As was recently demonstrated, arbitrary images imprinted on light pulses are dramatically slowed1,2 when traversing an atomic medium of electromagnetically induced transparency3,4 and undergo diffusion due to the thermal atomic motion5,6. Here we experimentally demonstrate a new technique to eliminate the paraxial diffraction and the diffusion of slow light, regardless of its position and shape7. Unlike former suggestions for diffraction manipulation8,9,10,11,12, our scheme is linear and operates in the wavevector space, eliminating the diffraction for arbitrary images throughout their propagation. By tuning the interaction, we further demonstrate acceleration of diffraction, biased diffraction and induced deflection, and reverse diffraction, implementing a negative-diffraction lens13. Alongside recent advances in slow-light amplification14 and image entanglement15, diffraction control opens various possibilities for classical and quantum image manipulation.

Local and bulk 13 C hyperpolarization in nitrogen-vacancy-centred diamonds at variable fields and orientations

Polarizing nuclear spins is of fundamental importance in biology, chemistry and physics. Methods for hyperpolarizing 13C nuclei from free electrons in bulk usually demand operation at cryogenic temperatures. Room temperature approaches targeting diamonds with nitrogen-vacancy centres could alleviate this need; however, hitherto proposed strategies lack generality as they demand stringent conditions on the strength and/or alignment of the magnetic field. We report here an approach for achieving efficient electron-13C spin-alignment transfers, compatible with a broad range of magnetic field strengths and field orientations with respect to the diamond crystal. This versatility results from combining coherent microwave- and incoherent laser-induced transitions between selected energy states of the coupled electron–nuclear spin manifold. 13C-detected nuclear magnetic resonance experiments demonstrate that this hyperpolarization can be transferred via first-shell or via distant 13Cs throughout the nuclear bulk ensemble. This method opens new perspectives for applications of diamond nitrogen-vacancy centres in nuclear magnetic resonance, and in quantum information processing.

Strong driving of a single spin using arbitrarily polarized fields

The strong driving regime occurs when a quantum two-level system is driven with an external field whose amplitude is greater or equal to the energy splitting between the systems states, and is typically identified with the breaking of the rotating wave approximation (RWA). We report an experimental study, in which the spin of a single nitrogen-vacancy (NV) center in diamond is strongly driven with microwave (MW) fields of arbitrary polarization. We measure the NV center spin dynamics beyond the RWA, and characterize the limitations of this technique for generating high-fidelity quantum gates. Using circularly polarized MW fields, the NV spin can be harmonically driven in its rotating frame regardless of the field amplitude, thus allowing rotations around arbitrary axes. Our approach can effectively remove the RWA limit in quantum-sensing schemes, and assist in increasing the number of operations in QIP protocols.

Time-optimal universal control of two-level systems under strong driving

We report on a theoretical and experimental study of time-optimal construction of arbitrary single-qubit rotations under a single strong driving field of finite amplitude. Using radiation-dressed states of nitrogen vacancy centers in diamond we realize a strongly driven two-level system, with driving frequencies four times larger than its precession frequency. We implement time-optimal universal rotations on this system, characterize their performance using quantum process tomography, and demonstrate a dual-axis multiple-pulse control sequence where the qubit is rotated on time scales faster than its precession period. Our results pave the way for applying fast qubit control and high-density pulse schemes in the fields of quantum information processing and quantum metrology.

Self-Similar Modes of Coherent Diffusion

Self-similar solutions of the coherent diffusion equation are derived and measured. The set of real similarity solutions is generalized by the introduction of a nonuniform phase, based on the elegant Gaussian modes of optical diffraction. In a light-storage experiment, the complex solutions are imprinted on a gas of diffusing atoms, and the self-similar evolution of both their amplitude and phase pattern is demonstrated. An algebraic decay depending on the mode order is measured. Notably, as opposed to the regular diffusion spreading, a subset of the solutions exhibits a self-similar contraction.

Coherent control of solid state nuclear spin nano-ensembles

Detecting and controlling nuclear spin nano-ensembles is crucial for the further development of nuclear magnetic resonance (NMR) spectroscopy and for the emerging solid state quantum technology. Here we present the fabrication of a ≈1 nanometre thick diamond layer consisting of 13C nuclear spins doped with nitrogen-vacancy centres (NV) embedded in a spin-free 12C crystal matrix. A single NV in the vicinity of the layer is used for polarization of the 13C spins and the readout of their magnetization. We demonstrate a method for coherent control of few tens of nuclear spins by using radio frequency pulses, and show the basic coherent control experiments, Rabi oscillations and Ramsey spectroscopy, though any NMR pulse sequence can be implemented. The results shown here present an important step towards the realization of a nuclear spin based quantum simulator.Nitrogen-vacancy centres: nuclear spin ensembles for quantum simulationNitrogen-vacancy centres and carbon-13 atoms in carefully grown diamonds can be driven coherently, providing a potential quantum simulator. Carbon-13 nuclear spins in diamond have long coherence times, making them attractive candidates for use as qubits. However, to utilise their advantages they must be incorporated in sufficient numbers into externally-controllable devices. Boris Naydenov of Ulm University with colleagues from Germany, Israel, Switzerland and Japan have fabricated diamond layers made almost entirely from carbon-13 atoms with a high density of nitrogen-vacancy defects. Each NV centre strongly interacts with nearby carbon-13 nuclear spins, allowing the latter to be indirectly controlled using newly developed methods for manipulating small nuclear spin ensembles. The large number of closely spaced qubits could be exploited to simulate models with many interacting two-level systems that are difficult to solve with classical computers.

Local probing of nuclear bath polarization with a single electronic spin

We demonstrate experimentally that a polarized nuclear spin modifies the dynamic behavior of a neighboring electronic spin. Specifically, an out-of-phase component appears in the electronic spin-echo signal. This component is proportional to the nuclear spin degree of polarization and strongly depends on the nuclear polarization direction. When the electronic spin is surrounded by a polarized nuclear spin bath, the spin-echo quadrature manifests a characteristic frequency related only to the nuclear spins abundance and their collective polarization. We use this analysis to propose a novel measurement method for the local nuclear spin bath of a single electronic spin. We quantify the realistic experimental regimes at which the scheme is efficient. Our proposal has potential applications for quantum sensing schemes, and opens a route for a systematic study of polarized mesoscopical-systems.

All-optical reconstruction of atomic ground-state population

The population distribution within the ground state of an atomic ensemble is of great significance in a variety of quantum-optics processes. We present a method to reconstruct the detailed population distribution from a set of absorption measurements with various frequencies and polarizations, by utilizing the differences between the dipole matrix elements of the probed transitions. The technique is experimentally implemented on a thermal rubidium vapor, demonstrating a population-based analysis in two optical-pumping examples. The results are used to verify and calibrate an elaborated numerical model, and the limitations of the reconstruction scheme, which result from the symmetry properties of the dipole matrix elements, are discussed.

Self-Similar Structured Fields in Coherent Diffusing Media

We derive and measure self-similarly evolving fields under coherent diffusion, analogous to Gaussian modes of optical diffraction. We obtain a quasi-eigenmodes description of polariton dynamics in thermal vapor in the limit of dominating diffusion.