Since the emergence of quantum theory just over a century ago, quantum phenomena have become part of our daily lives—from transistors to lasers, which are ubiquitous in our mobile phones, communications, and the internet. Yet, fundamental concepts such as wave-particle duality, non-classical states of light and matter, and entanglement continue to drive major advancements in both fundamental and applied research. Today, the landscape of quantum technologies is structured around four main pillars: quantum metrology, quantum communications, quantum simulation, and quantum computing.

The first pillar, quantum metrology, harnesses the quantum properties of matter and light to achieve measurements of unparalleled precision, pushing the boundaries of classical physics. Exquisitely sensitive probes are designed using individual quantum objects, interferometers exploit the wave-like nature of matter, and entanglement properties such as "squeezing" provide metrological advantages. The intricate nature of quantum states is revealed through correlations between individual particles. These distinctive features of quantum physics have revolutionized diverse fields, including atomic physics, optics, and condensed matter. Moreover, the boundary between the quantum and classical worlds continues to shift as precision measurements extend to increasingly large and massive objects.

This special issue on quantum measurements brings together a selection of research works that illustrate these fascinating characteristics. Through a variety of platforms and themes—from microwave field detection to super-radiant lasers, from measuring correlations between individual atoms to applications in gravitational wave detection and astrophysics—these contributions highlight the vitality and diversity of this research field.