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Electric field imaging of single molecular charges by a quantum sensor

Periodic Reporting for period 3 - SMel (Electric field imaging of single molecular charges by a quantum sensor)

Reporting period: 2020-08-01 to 2022-01-31

The aim of SMeL is to exploit the outstanding nanoscale quantum sensing capabilities of spin defects in diamond. By specifically tailoring the spin states of the diamond defects, most notably that of the nitrogen vacancy (NV) center in diamond by coherent quantum control, the project aims to maximize the sensitivity of the spin quantum states to electric fields and achieve highly sensitive charge detection down to single elementary charges under ambient conditions. The scientific work in SMeL aims to combine quantum sensing with vibronic excitation of molecules for the detection and imaging of their polarization fields, and ultimately, individual chemical bonds. SMeL intends to develop a nanoscale imaging technique that achieves this goal to study molecular properties and structures. It will provide images of the permanent/ induced electric dipole moments that directly reflect a molecule’s chemical structure and the quantum chemistry of its bonds. It is the visionary aim of SMeL to exemplify sensitivity by detecting and imaging charge transfer inside a single biomolecular photosynthetic reaction centre.While the shifts can be maximized in a rather straightforward fashion, mitigating noise has proven to be a more complex challenge. The group of the applicants has adapted a technique known from high resolution magnetometry with NV spins. Essentially, the technique mimics a classical lock-in, eliminating noise over a wide band width except for a narrow frequency range around the lock-in frequency. When run in a non-linear fashion, overtones of the fundamental lock-in frequency can be used to even improve the noise rejection. With this technique, we were able to show an electric field sensitivity of 10-4 V/cm√Hz . The minimum electric field detected was 10-7 V/cm. The grantee was also taking a next step in using nanoscale NMR to identify the chemical composition of molecules in nanoscale volumes. To this end, we were developing a high field set up operating at 3T background field. This maximizes the chemical shift of the target molecule and hence facilitates its resolution. The grantee was for the first time able to demonstrate nanoscale NMR on minute amounts of molecules. Among others, this achievement has been enabled by mitigating the influence of diffusion on the spectral resolution, i.e. the linewidth of the NMR spectra. We achieved this by working with defects that are more than 50nm under the surface and hence sense a volume with a diameter of around 50nm at the surface. In this case, even in a solvent with the viscosity of water diffusion time is long enough not to broaden the resonance lines too much for chemical shift resolution.
The grantee was also investigating quantum systems in other host materials like silicon carbide that, similar to diamond defects, carries a spin and does show optically detected spin resonance, those spins can be used for quantum sensing, albeit with less sensitivity due to the lower signal contrast. This is in part, especially for electric field sensing, compensated by a larger susceptibility to electric fields. The grantee has investigated its quantum properties and derived optimum parameters for quantum sensing experiments. Also the possibility to address single spins in large field gradients has been investigated. This enables precise localization of spins and facilitates high resolution imaging of the quantities to detect. Defects could be localized with a precision of better that 20nm in magnetic field gradients. Temperature sensing is an attractive feature of the NV quantum sensor. Small temperature changes can be measured at very small length scales, e.g. a few nm. The sensitivity scales down to a few 10 mK/√Hz. The measurement relies on a precision measurement of the fine structure splitting D of the spin ground state. The value of D is determined by the average distance of the two electron spins forming the ground state. The ongoing, second reporting period of the project focuses on developing novel imaging techniques, as well as fabricating improved sample materials and investigating new defects. We also explored new host materials to improve imaging and detection of electric and magnetic fields. Specifically, we achieved high resolution imaging of nuclear spins using a technique called wide-field imaging. In this approach, we combine imaging of diamond defects by a wide field fluorescence microscope with the sensing capabilities of the nitrogen vacancy (NV) center. We were demonstrating a sub 200nm spatial resolution in the detection of a nuclear magnetic resonance (NMR) signal of a thin layer of CaF2. The experiment showed that this nanoscale technique is particularly valuable if one intends to measure sample material close to the sensor.
The PI and his group also strived to develop novel sensing material and dopants which allow for a better, i.e. more sensitive detection electric fields. To this end, we were measuring novel types of dopants in various materials, among them silicon carbide (SiC). Defects in this material prove to be of excellent photostability and time sensitive to electric field changes. We demonstrated this by measuring the charge state of silicon vacancies (VSi) in pn-layers of SiC material. We were able to precisely measure the electric field inside the SiC layer. We could attribute the changes in the local electric field to an electric field-induced change of the charge state of a dark defect in SiC. Based on the aforementioned experiments, we accomplished the electrical detection of single VSi spins. We were photo ionizing single defects by short wavelength excitation. Subsequently, we were measuring the resulting photo current as a function of its spin state. We were able to show that the resulting electrically detected electron spin resonance signal is identical to the optically detected signal. In addition, we achieved the detection of spin coherent signal with coherence times that are comparable to those measured optically. These results pave the way for an integrated quantum device based on SiC. So far, the grantee has published 43 papers on the project.
In the first part of the project, we were exploring methods to enhance the sensitivity of the NV center in diamond to electric fields. It should be noted that the spin levels of the NV center are not sensitive to electric fields in first order. Because of the low spin orbit coupling and the specific antisymmetric combination of E-type wave function in the NV electronic ground state, electric field-induced shifts are rather small (few 10kHz cm/V) as compared to any magnetic field-induced energy changes. To render the spin E-field sensitive, it needs to be brought into a new eigenbasis which is either done in a coherent superposition of magnetic field eigenstates or via a magnetic field at a specific (orthogonal to symmetry axis) direction. We have carried out systematic investigations of various experimental conditions to maximize electric field sensitivity.
We were able to increase the sensitivity in electric field measurements beyond the hitherto known values. We were demonstrating unprecendented resolution in nanoscale nuclear magnetic resonance and show that essentially all functionalities of the technique are compatible with our method. We were calculating the electric field signal expected from neurons on diamond surfaces and made first steps towards its detection.