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Launched 2 years ago, the Deep-Space Atomic Clock (DSAC) mission has exceeded expectations for the first mercury ion clock in space, demonstrating a long-term stability beyond the current performance of other space clocks.

Two articles in Nature Photonics demonstrate how Einstein–Podolsky–Rosen entanglement can reduce quantum noise in gravitational-wave interferometers.

Frequency combs measure optical frequencies with an unprecedented precision, allowing myriad applications in optical metrology, high-precision spectroscopy, optical atomic clocks, attosecond science, astronomy and, recently, quantum information processing.

A sub-cycle modulation in reflectivity is observed in bulk crystals subjected to intense laser fields. The effect provides a new way to probe attosecond dynamics in materials.

Frequency combs, optical clocks and quantum techniques that go beyond classical limits are all making photonics a powerful tool for understanding and defining our universe in ever-greater detail.

Designed with a laser metrology system, LISA Pathfinder is on track to demonstrate the first in-flight test of low-frequency gravitational wave detection metrology in space.

A chip-based optical frequency comb has enabled the realization of a 300 GHz signal with record low phase noise. The development could yield ultra-compact, ultra-low-noise sources for millimetre-wave applications in telecommunications, remote sensing and precision spectroscopy.

The ability to make measurements of time and fundamental physical constants with extreme precision makes it possible to test theories to ever greater levels of scrutiny. A workshop in Tokyo in January discussed the challenges involved and the progress being made.

Observing accreting black holes in the early Universe allows precise comparison of clocks over intercontinental distances on Earth. This is achieved with a novel observation strategy using the next generation of very long baseline interferometry systems.

Optical clocks held at slightly different heights provide a stringent test of general relativity comparable to space experiments and open new opportunities for clock-based geophysical sensing.

The Nobel Prize-winning observation of gravitational waves has required laser interferometry to be pushed to extreme limits of sensitivity.

The direct measurement of few-cycle optical waveforms with arbitrary polarization and weak intensity is now made possible thanks to extreme ultraviolet interferometry with isolated attosecond pulses.

Applications of the concept of structured light are not limited to optical communications, metrology, and probing and sensing, they can also go beyond optics.

Spectral purity can now be transferred from one laser to another with a very different wavelength at an order of magnitude better than previously achievable. Yann Le Coq spoke to Nature Photonics about the new development.

The web of optical fibre networks deployed across Europe is proving useful for experiments in optical metrology and sensing in addition to their primary use of carrying Internet data and telephone calls.

The quantum concepts of entanglement and interaction-free measurements are applied to spectroscopy to successfully sense carbon dioxide in air.

Frequency combs are created by stabilizing the evolution of the carrier–envelope phase of a mode-locked laser. Researchers have now demonstrated a simplified method of creating frequency combs that enables trains of identical pulses to be easily produced.

Using a real-time measurement technique to study the single-shot properties of modulation instability, scientists have shown that its initial stochastic nature in an optical system can lead to specific correlation properties in both the spectral and temporal domains.

By combining the output from two synchronized light sources, single-cycle laser pulses at the telecommunications wavelength of 1.5 μm have been successfully generated. The achievement is set to benefit ultrafast optical spectroscopy and attosecond science.

Squeezed light allows quantum limits to be overcome in precision metrology. A new way of producing this special form of light has now been demonstrated by engineering the vibrations of nanostructured optical cavities.

Light is an excellent tool for making precise measurements of objects, but can sometimes alter or damage a sensitive sample. Researchers have now shown that entanglement and quantum-correlated light can be used to help alleviate this problem.

The tool of choice to measure optical frequencies with extremely high precision is the optical frequency comb. Camille-Sophie Brès explains what makes this technique so powerful.

Optical-lattice clocks have pushed the limits of frequency measurement — to such an extent that a tiny difference in altitude affects the clock's tick rate, as Hidetoshi Katori elucidates.

Frequency combs — broadband phase-coherent optical sources — are finding an increasing number of new applications in the field of metrology.

A 'comb' of photons at evenly spaced frequencies in the extreme ultraviolet has been generated. It will allow a more precise search for variation in the fine-structure constant, which sets the strength of the electromagnetic force. See Letter p.68

The latest generation of optical atomic clocks has reached such a degree of accuracy that questions about the need to redefine the second are raised. But even without such a redefinition, these breakthroughs will enable unprecedented precision tests of fundamental physics.

This year celebrates the twentieth anniversary of frequency-resolved optical gating — the first and most general technique for measuring ultrashort laser pulses.