Chinmay Belthangady

Nanoscale NMR spectroscopy and imaging of multiple nuclear species

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) provide non-invasive information about multiple nuclear species in bulk matter, with wide-ranging applications from basic physics and chemistry to biomedical imaging. However, the spatial resolution of conventional NMR and MRI is limited to several micrometres even at large magnetic fields (>1 T), which is inadequate for many frontier scientific applications such as single-molecule NMR spectroscopy and in vivo MRI of individual biological cells. A promising approach for nanoscale NMR and MRI exploits optical measurements of nitrogen-vacancy (NV) colour centres in diamond, which provide a combination of magnetic field sensitivity and nanoscale spatial resolution unmatched by any existing technology, while operating under ambient conditions in a robust, solid-state system. Recently, single, shallow NV centres were used to demonstrate NMR of nanoscale ensembles of proton spins, consisting of a statistical polarization equivalent to ∼100-1,000 spins in uniform samples covering the surface of a bulk diamond chip. Here, we realize nanoscale NMR spectroscopy and MRI of multiple nuclear species ((1)H, (19)F, (31)P) in non-uniform (spatially structured) samples under ambient conditions and at moderate magnetic fields (∼20 mT) using two complementary sensor modalities.

Electro-optic Modulation of Single Photons

We use the Stokes photon of a biphoton pair to set the time origin for electro-optic modulation of the wave function of the anti-Stokes photon thereby allowing arbitrary phase and amplitude modulation. We demonstrate conditional single-photon wave functions composed of several pulses, or instead, having Gaussian or exponential shapes.

Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems

Multi-qubit systems are crucial for the advancement and application of quantum science. Such systems require maintaining long coherence times while increasing the number of qubits available for coherent manipulation. For solid-state spin systems, qubit coherence is closely related to fundamental questions of many-body spin dynamics. Here we apply a coherent spectroscopic technique to characterize the dynamics of the composite solid-state spin environment of nitrogen-vacancy colour centres in room temperature diamond. We identify a possible new mechanism in diamond for suppression of electronic spin-bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression enhances the efficacy of dynamical decoupling techniques, resulting in increased coherence times for multi-spin-qubit systems, thus paving the way for applications in quantum information, sensing and metrology.

Observation of optical precursors at the biphoton level

We describe the observation of a sharp leading-edge spike in a biphoton wave packet that is produced using slow light and measured by two-photon correlation. Using the stationary-phase approximation we characterize this spike as a Sommerfeld-Brillouin precursor resulting from the interference of low- and high-frequency spectral components.

Hiding Single Photons with Spread Spectrum Technology

We describe a proof-of-principal experiment demonstrating the use of spread spectrum technology at the single photon level. We show how single photons with a prescribed temporal shape, in the presence of interfering noise, may be hidden and recovered.

Modulation and Measurement of Time-Energy Entangled Photons

We describe a proof-of-principle experiment demonstrating a Fourier technique for measuring the shape of biphoton wavepackets that vary rapidly as compared to the speed of available photon detectors. The technique uses synchronously driven telecommunication modulators.

Selective addressing of solid-state spins at the nanoscale via magnetic resonance frequency encoding

The nitrogen vacancy centre in diamond is a leading platform for nanoscale sensing and imaging, as well as quantum information processing in the solid state. To date, individual control of two nitrogen vacancy electronic spins at the nanoscale has been demonstrated. However, a key challenge is to scale up such control to arrays of nitrogen vacancy spins. Here, we apply nanoscale magnetic resonance frequency encoding to realize site-selective addressing and coherent control of a four-site array of nitrogen vacancy spins. Sites in the array are separated by 100 nm, with each site containing multiple nitrogen vacancies separated by ~15 nm. Microcoils fabricated on the diamond chip provide electrically tuneable magnetic field gradients ~0.1 G/nm. Tailored application of gradient fields and resonant microwaves allow site-selective nitrogen vacancy spin manipulation and sensing applications, including Rabi oscillations, imaging, and nuclear magnetic resonance spectroscopy with nanoscale resolution. Microcoil-based magnetic resonance of solid-state spins provides a practical platform for quantum-assisted sensing, quantum information processing, and the study of nanoscale spin networks.Selective nanoscale addressing of solid-state spinsArrays of spins in solids are a promising modality for a wide range of quantum science applications—from sensing to information processing. A team led by Ronald Walsworth at Harvard University adapted methods from magnetic resonance imaging (MRI) to realize site-selective addressing and coherent control of small arrays of optically active electronic spins in diamond known as nitrogen vacancy (NV) colour centres. Microcoils fabricated on the diamond chip provide electrically tunable magnetic field gradients that allow selective NV spin addressing with 30 nm resolution. The team experimentally demonstrated site-selective NV electron spin resonance spectroscopy, Rabi oscillations, Fourier magnetic imaging, and nuclear magnetic resonance (NMR) spectroscopy. The approach should be scalable to selective coherent control of large-scale arrays of strongly interacting NVs, with a broad spectrum of high-impact quantum science applications.

Generation of biphotons with controllable temporal length

We report the generation of time-energy entangled paired photons with subnatural linewidths and controllable temporal lengths. The photons are generated using EIT and slow light in a high optical depth two-dimensional MOT.

Measurement of biphoton wavefunctions using fast amplitude modulators

We demonstrate a proof-of-principle experimental realization of a novel technique that uses fast amplitude modulators to measure biphoton waveforms whose temporal lengths are shorter than the resolution time of present single photon counting modules.

Generation of Subnatural Linewidth Biphotons

We describe the generation of time-energy entangled bipotons with a sub-natural linewidth and a correlation time of about 100 ns. We use electromagnetically-induced transparency in a85Rb two-dimensional magneto-optical trap.