Extending coherence time of macro-scale diamond magnetometer by dynamical decoupling with coplanar waveguide resonator

Abstract

Ultimate sensitivity for quantum magnetometry using nitrogen-vacancy (NV) centers in diamond is limited by number of NV centers and coherence time. Microwave irradiation with a high and homogeneous power density for a large detection volume is necessary to achieve highly sensitive magnetometer. Here, we demonstrate a microwave resonator to enhance the power density of the microwave field and an optical system with a detection volume of 1.4×10 −3 mm 3 . The strong microwave field enables us to achieve 48 ns Rabi oscillation which is sufficiently faster than the phase relaxation time of NV centers. This system combined with a decoupling pulse sequence, XY16, extends the spin coherence time (T2) up to 27 times longer than that with a spin echo method. Consequently, we obtained an AC magnetic field sensitivity of 10.8 pt/√Hz using the dynamical decoupling pulse sequence. Ensemble nitrogen-vacancy (NV) centers in diamond are expected to reach a magnetic sensitivity in the femto-tesla range at room temperature, required for biological/medical applications including magnetoencephalography [1-4]. The sensitivity of diamond magnetometer is determined by number of the NV centers, spin coherence time, and detection efficiency [5, 6]. Pioneering work reported by Wolf et al. achieved the highest AC magnetic field sensitivity of 900 fT/√Hz by enlarging the detection volume to increase the number of the NV centers used for detection. Wolf et al. achieved a detection volume of 8.5× 10 −4 mm 3 , a high detection efficiency of 65 %, and a spin coherence time of 100s obtained by a spin echo sequence [7].Dynamical decoupling of the large detection volume for extension of the spin coherence time beyond spin echo limit is necessary to extend its sensitivity limit. As for a single NV center, the dynamical decoupling sequences have been utilized to improve the spin coherence time and thus a magnetic sensitivity [8]. For driving the large volume of the NV ensembles, spatial inhomogeneity of the microwave field creates different Rabi oscillations of each NV center, causing rapid decays of the average signal from the NV ensemble. Therefore, the spatial homogeneity of the microwave field in the decoupling sequence becomes important. Furthermore, the Rabi oscillation faster than phase coherence time T2* is critical to reduce the control error of the dynamical decoupling pulses. Thus, uniform and strong microwave driving is essential to achieve the dynamical decoupling for the large detection volume. In previous reports, the microwave irradiation of the large detection volume of the ensemble NV centers was performed by means of a spin echo sequence only [7,9].In this study, we constructed an efficient microwave and optical system and applied dynamical decoupling pulse sequences to an ensemble of NV centers in a large detection volume of 1.4×10 −3 mm 3 . We used a coplanar waveguide resonator (CWR) with a wide center electrode for strong and uniform microwave irradiation. CWR enhances the microwave field only at the targeted frequency bandwid...

Optimization of a Diamond Nitrogen Vacancy Centre Magnetometer for Sensing of Biological Signals

“…Few groups have yet to replicate this level of sensitivity, let alone on a biological specimen. We note a distinction between DC-low frequency sensitivity and the typically significantly better AC (>10 kHz) sensitivity achievable using pulsed dynamical decoupling techniques, where 10 pT/ √ Hz has been demonstrated [12][13][14]. These frequencies are however too high for detection of many biological signals in the sub-kHz range.…”

Thermal Stability of Perfectly Aligned Nitrogen‐Vacancy Centers for High Sensitive Magnetometers

“…Nanoscale magnetic detection has been performed by measuring the object near the surface of the diamond with an isolated single NV center or a thin NV ensemble . On the other hand, the macro‐scale NV magnetometer (≈10 6 µm 3 ) has potential in biological/medical applications. The magnetic sensitivity of NV centers, δB , is determined by the magnetic resonance signal contrast, the number of NV centers contributing to the magnetic detection, and the spin relaxation time, and is expressed by Equation : …”

Simultaneous wide-field imaging of phase and magnitude of AC magnetic signal using diamond quantum magnetometry

“…The MW control for the sample #2 experiment was applied via a planar resonator with the diamond substrate mounted. This antenna used is a modified version of that reported in our previous work [21]. The MW distribution is almost uniform, but the MW strength is slightly stronger at points close to the current path due to coupling between the current path and MW resonator.…”

“…Wide-field imaging using NV center paves the way for inductive inspection [17,18], which is still challenging with micron-scale resolution by established methods. Previous studies indicate that dynamical decoupling (DD) protocols like XY8 achieve magnetic field spectroscopy with high sensitivity for magnitude and phase of such a signal [19][20][21][22]. Despite its high sensitivity, DD can only measure the output of fluorescence intensity with an intricate dependence on a magnitude and a phase of a signal.…”