We report the development of a fiber-based, tunable optical cavity with open access. The cavity is of the Fabry-Perot type and is formed with miniature spherical mirrors positioned on the end of single-or multi-mode optical fibers by a transfer technique which involves lifting a high-quality mirror from a smooth convex substrate, either a ball lens or micro-lens. The cavities typically have a finesse of ∼ 1, 000 and a mode volume of 600 µm 3 . We demonstrate the detection of small ensembles of cold Rb atoms guided through such a cavity on an atom chip.

We show how the mechanical rigidity of a slightly detuned miniature Fabry–Pérot cavity can be modified with light. We use a microcavity in which one of the mirrors is a soft compliant microlever optimized to detect bolometric forces. The static compliance can either be decreased to zero or increased considerably depending on the detuning of the light with respect to the cavity resonance.

Avron et al. Reply: In the preceding Comment [1], Pathak and Hughes claim that they correct an error in our Letter [2], and claim to perform an exact calculation instead of an approximation we make. Their claim is incorrect. Our Letter is correct and to the best of our knowledge, free of any errors.In our Letter we calculated the theoretical optimum for the entanglement that can be obtained by time reordering. Pathak and Hughes present results for the special case of a linear phase shifter. Pathak and Hughes do not correct our results, but rather discuss a special case which, of course, gives weaker results. Furthermore, the special case Pathak and Hughes discuss is not necessarily the only possible implementation.We also note that the results of Pathak and Hughes are not new and are contained in the results presented in Fig. 3(c) of Troiani and Tejedor [3] (Ref.[3] of Pathak and Hughes).Finally, Pathak and Hughes doubt the feasibility of any other scheme for time reordering other than the simple linear phase shifter that they analyze. To counter this viewpoint we would like to quote from Richard Feymann and Val Telegdi: ''Yesterday's discovery is today's calibration and tomorrow's background.''

Recording processes and events that occur on sub-nanosecond timescales poses a difficult challenge. Conventional ultrafast imaging techniques often rely on long data collection times, which can be due to limited device sensitivity and/or the requirement of scanning the detection system to form an image. In this work, we use a single-photon avalanche detector array camera with pico-second timing accuracy to detect photons scattered by the cladding in optical fibers. We use this method to film supercontinuum generation and track a GHz pulse train in optical fibers. We also show how the limited spatial resolution of the array can be improved with computational imaging. The single-photon sensitivity of the camera and the absence of scanning the detection system results in short total acquisition times, as low as a few seconds depending on light levels. Our results allow us to calculate the group index of different wavelength bands within the supercontinuum generation process. This technology can be applied to a range of applications, e.g., the characterization of ultrafast processes, time-resolved fluorescence imaging, three-dimensional depth imaging, and tracking hidden objects around a corner.One of the first slow motion events captured on film was a galloping horse. "Sallie Gardner at a Gallop" is a series of 24 photographs taken in rapid succession to analyse the gait of a horse 1 . It demonstrated that all four feet were simultaneously off the ground during the gallop. Ever since, there has been a fascination with slow-motion video, and by the 1930s, speeds of 1000 frames per second (fps) were achievable on 16 mm film with a built-in shutter/ correction plate 2 . By the early 1960s, a 10,000 fps rotating prism camera was demonstrated 3 . Technological developments have led to cameras capable of realtime monochromatic filming at several million fps 4 and the observation of femtosecond pulses of light using a streak camera combined with a stroboscopic approach 5 . Most recently, compressed single-shot photography at 100 billion frames per second was reported using a streak camera 6 without relying on stroboscopic illumination. The main limitations of this work were the limit of 350 frames per acquisition (3.5 ns) and the need for computational reconstruction techniques to realise a final video.In photon-starved applications, single-photon detection and, more specifically, time-correlated single-photon counting (TCSPC), offers extreme sensitivity and picosecond timing resolution 7 . For many applications, single-photon avalanche diodes (SPADs) are the favoured choice of detector due to their relatively compact nature, ease of integration, high efficiency and low noise characteristics, combining to give ultra-high sensitivity devices. It is common to use silicon SPADs for visible wavelengths 8 and InGaAs/InP SPADs for NIR and telecommunications wavelengths 9 . When coupled to an optical system, usually with optical fiber, SPADs offer single-point detection. While useful in many applications, such as time-resolved p...