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Electric field imaging of single molecular charges by a quantum sensor

Periodic Reporting for period 2 - SMel (Electric field imaging of single molecular charges by a quantum sensor)

Reporting period: 2019-02-01 to 2020-07-31

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.
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.
While the shifts can be maximized in a rather straightforward fashion, mitigating noise has proven to be a more complex challenge. To this end, 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, the group of the grantee was 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.
As an alternative, the grantee was also investigating quantum systems in other host materials. Specifically, this was silicon carbide that, similar to diamond defects, carries a spin and does show optically detected spin resonance. Similar to diamond, 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.
The applicant also investigated the possibility to address single spins in large field gradients. This enables precise localization of spins and facilitates high resolution imaging of the quantities to detect. More specifically, defects could be localized with a precision of better that 20nm in magnetic field gradients, which can be switched with frequencies of a few GHz.
Temperature sensing is an outstandingly attractive feature of the NV quantum sensor. The specifically attractive feature is that small temperature changes can be measured at very small length scales, e.g. a few nano meters. The sensitivity scales down to a few 10 mK/√Hz. Essentially, 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. In addition, phonons contribute to its value. To separate the two contributions conclusively, the grantee measured in an international cooperation the lattice expansion by measuring the hyperfine coupling between the electron spins and neighboring nuclear spins. Quite remarkably, this results in temperature measurements which are quite sensitive and yield temperature values in cases when the D value is hard to determine. Also, it confirms the precise understanding of the defect center ground state, essential for numerous other sensing applications.
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. In this work, we could show the F2 nuclear magnetic resonance signal of patches of this layer. We demonstrated that the technique is able to measure the NMR signal of a CaF2 as thin as 1.7nm. The experiment showed that this nanoscale technique is particularly valuable if one intends to measure sample material close to the sensor. This is the case in imaging cell membranes, which is one of the main targets of SMeL. In a further publication, we demonstrate that one can achieve electric field sensing of a few V/cm by using a diamond ensemble sensor. This has been achieved by developing a new technique which allows to measure an electric field over an extended period of time (>10ms), while at the same time achieving a scaling of the sensitivity following the standard quantum limit.
We were attempting to use the hyperfine coupling of the NV center in diamond to calibrate temperature measurements using the NV center. Thermometry using NV centers is an upcoming quantum nanosensing scheme. These thermometers, however, are not particularly sensitive (mK/√t), but they are very small. Thus, they are used to measure local temperature on length scales below 100nm. The conventional measurement techniques are based on shifts of the fine structure of the NV defect. However, that shift was shown to depend on phononic contributions. We have calibrated this contribution by measuring the hyperfine contribution as a function of temperature, which entirely depends on changes in the lattice constant.
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 at the same 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.
Quite intriguingly, 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. To this end, 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.
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.