Understanding the human brain is one of the most significant challenges of the 21st century. As theoretical studies continue to improve the description of the complex mechanisms that regulate biological processes, in parallel numerous experiments are conducted to enrich or verify these theoretical predictions also with the aim of extrapolating more accurate models. In the fields of magnetometry and thermometry, among the various sensors proposed for biological application, nitrogen‐vacancy (NV) centers are emerging as a promising solution due to their perfect biocompatibility and the possibility of being positioned in close proximity to the cell membrane, thus allowing a nanometric spatial resolution down to the nano‐scale. Still many issues must be overcome to obtain either a sensitivity capable of revealing the very weak electromagnetic fields generated by neurons (or other excitable cells) during their firing activity or a spatial resolution sufficient to measure intracellular thermal gradient due to biological processes. However, over the last few years, significant improvements have been achieved in this direction, thanks to the use of innovative techniques. In this review, the new results regarding the application of NV centers will be analyzed and the main challenges that must be afforded for leading to practical applications will be discussed.

Nitrogen-vacancy centers in diamond allow measurement of environment properties such as temperature, magnetic and electric fields at the nanoscale level, of utmost relevance for several research fields, ranging from nanotechnologies to biosensing. The working principle is based on the measurement of the resonance frequency shift of a single nitrogen-vacancy center (or an ensemble of them), usually detected by monitoring the center photoluminescence emission intensity. Albeit several schemes have already been proposed, the search for the simplest and most effective one is of key relevance for real applications. Here we present a continuous-wave lock-in-based technique able to reach high sensitivity in temperature measurement at microscale or nanoscale volumes (4.8 ${\mathrm{mK}/\mathrm{Hz}}^{1/2}$ in $\mu {\mathrm{m}}^3$). Furthermore, the present method has the advantage of being insensitive to environmental magnetic noise that in general introduces a bias in the temperature measurement.

High-resolution superconducting quantum interference device magnetization measurements in lead nanoparticles with particle size $d$ less than the superconducting coherence length $\xi$ are used to study zero-dimensional fluctuating diamagnetism. The diamagnetic magnetization $M_{\mathrm{dia}}$ ($H$, $T=\mathrm{const}$) as a function of the applied magnetic field $H$ at constant temperature is reported in the critical region and compared with the observed behavior in the temperature range where the first-order fluctuation corrections are expected to hold. The magnetization curves are analyzed in the framework of exact fluctuation theories based on the Ginzburg-Landau functional for $\xi ⪢d$. The role of the upturn field $H_{\mathrm{up}}$, where the slope of $M_{\mathrm{dia}}$ $\left(H\right)$ changes sign, is discussed. The relevance of the magnetization curves over a wide range of magnetic fields and the role of $H_{\mathrm{up}}$ for the study of fluctuating diamagnetism, in particular, when the first-order fluctuation correction breaks down, is pointed out. The size and temperature dependences of $H_{\mathrm{up}}$ in the critical region are obtained from the experimental data and compared to the theoretical derivations for $M_{\mathrm{dia}}$.

How irreversibility arises in a universe with time-reversal symmetric laws is a central problem in physics. In this Letter, we discuss a radically different take on the emergence of irreversibility, adopting the recently proposed constructor theory framework. Irreversibility is expressed as the requirement that a task is possible, while its inverse is not. We prove the compatibility of such irreversibility with quantum theory’s time-reversal symmetric laws, using a dynamical model based on the universal quantum homogenizer. We also test the physical realizability of this model by means of an experimental demonstration with high-quality single-photon qubits.