Final Report Summary - Q-SENS2 (Quantum-Enhanced Sensors with Single Spins) Among emerging quantum technologies, quantum sensing capitalizes on the quantum properties of a system to perform high-precision measurements of a physical quantity, and it is thus one of the most promising and closest to practical applications.The ERC project Q-SEnS2 has focused on using a solid-state qubit — that is, a bit working at the quantum level — formed by the electronic spin of single Nitrogen Vacancy (NV) center in diamond, for quantum sensing of weak magnetic fields at ambient temperatures and with nanoscale spatial resolution.The NV center is a fluorescent and paramagnetic subnanometer-sized defect in diamond, and has emerged in the last decade as a unique qubit thanks to its high brightness and photostability, long spin coherence times, and spin polarizability at room temperature. These features offer several advantages compared to other state-of-the-art sensors, which often require cryogenics, relatively large experimental setups that impose a standoff from nanoscale samples of interest, and considerable sensing volumes. NV centers in diamond can thus be used as fluorescent bio-markers in-vivo, as magnetic scanning tips, and even as electric field sensors, thermometers, and clocks.Despite these advantages and potential, real-world applications of solid-state quantum sensors has been so far limited by the fragile nature of quantum superposition states, as well as by difficulties in preparation, control and readout of useful quantum states. The Q-SEnS2 project has explored quantum schemes for initializing and controlling the NV electronic spin sensor and its core environment in order to boost the sensor’s performances.To push forward the NV center capabilities in magnetometry beyond current limitations, Q-SEnS2 has investigated enhanced metrology by coupling the NV sensor with an ancillary system, in order to increase the interrogation time and the precision of sensor readout. Specifically, the ancillary system is composed by nuclear spins in the core environment of the NV sensor. While the NV electronic spin can be optically addressed, the nuclear spins in its environment are not as easily accessible. Still, a fundamental requirement for ancillary-assisted sensing schemes is the capability of controlling and polarizing the auxiliary nuclear spins. We have thus designed a strategy to fully characterize the ancillary spin and measure its coupling with the NV electronic spin, which governs the nuclear spin dynamics and provides a tool for its initialization and control.Secondly, Q-SEnS2 has applied quantum optimal control methods for metrology, to design versatile sensing schemes for the measurement of weak magnetic fields in the presence of magnetic noise, achieving optimal compromises between signal acquisition and protection from deleterious noise. While control methods (dynamical decoupling) inherited from the long tradition of nuclear magnetic resonance have been recently exploited to extend the coherence time of the qubit sensor and selectively detect monochromatic AC signals, for more complex time-varying signals these schemes intrinsically lead to partial attenuation of the target signal. Q-SEnS2 has demonstrated an enhancement of the sensor performances with respect to those obtained via standard periodical dynamical decoupling, in the case of both multifrequency and multipulse signals, improving the sensor’s sensitivity — i.e. the minimum measurable signal per unit time, while effectively speeding up the measurement process.The demonstrated sensor is a prototypical device for high precision measurements of magnetic forces, with enhanced sensitivity and sub-micron spatial resolution, with potential applications both in physics and in life sciences, ranging from materials science to molecular scale bioimaging, and neuroscience.
