Using a single neutral 87Rb atom held in an optical trap, and "quantum jump" detection of single-photon-initiated state changes, we demonstrate a single-photon quantum jump photodetector (QJPD) with intrinsically narrow bandwidth and strong rejection of out-of-band photons, of interest for detecting weak optical signals in the presence of a strong broadband background. By analyzing fluorescence photon count distributions for the bright and dark states with and without excitation, we measure quantum efficiency of 2.9(2) x 10(-3), a record for single-pass quantum jump production, and signal-photon-unprovoked "dark jump" rate-analogous to the dark count rate of other detectors -of 3(10) x 10(-3) jumps/s during passive accumulation plus 4.0(4) x 10(-3) jumps per readout, orders of magnitude below those of traditional single-photon detectors. Available methods can substantially improve QJPD quantum efficiency, dark jump rate, bandwidth, and tunability.

Background: Optical microtraps at the focus of high numerical aperture (high-NA) imaging systems enable efficient collection, trapping, detection and manipulation of individual neutral atoms for quantum technology and studies of optical physics associated with super- and sub-radiant states. The recently developed “Maltese cross” geometry (MCG) atom trap uses four in-vacuum lenses to achieve four-directional high-NA optical coupling to single trapped atoms and small atomic arrays. This article presents the first extensive characterisation of atomic behaviour in a MCG atom trap. Methods: We employ a MCG system optimised for high coupling efficiency and characterise the resulting properties of the trap and trapped atoms. Using current best practices, we measure occupancy, loading rate, lifetime, temperature, fluorescence anti-bunching and trap frequencies. We also use the four-directional access to implement a new method to map the spatial distribution of collection efficiency from high-NA optics: we use the two on-trap-axis lenses to produce a 1D optical lattice, the sites of which are stochastically filled and emptied by the trap loading process. The two off-trap-axis lenses are used for imaging and single-mode collection. Correlations of single-mode and imaging fluorescence signals are then used to map the single-mode collection efficiency. Results: We observe trap characteristics comparable to what has been reported for single-atom traps with one- or two-lens optical systems. The collection efficiency distribution in the axial and transverse directions is directly observed to be in agreement with expected collection efficiency distribution from Gaussian beam optics. Conclusions: The multi-directional high-NA access provided by the Maltese cross geometry enables complex manipulations and measurements not possible in geometries with fewer directions of access, and can be achieved while preserving other trap characteristics such as lifetime, temperature, and trap size.

We employ a single trapped $^{87}$Rb atom as a sub-wavelength optical intensity sensor, by spectroscopic measurement of the ac Stark shifts on the $F=1 \rightarrow F'=2$ hyperfine transition of the D$_{2}$ line. This transition experiences very small tensor light shifts, allowing a precise measurement of the scalar shift caused by a linearly-polarized field. A"quantum jump"technique, in which a single photon absorbed on the transition of interest induces the scattering of hundreds of photons on a bright, closed transition, is used to boost signal level and avoid systematic effects such as probe-induced optical pumping. The resulting line shape is used to extract the intensity at trap focus and the atom temperature. The analogous probing method on the $F=1 \rightarrow F'=1$ transition, in contrast, experiences strong tensor light shifts, and may be useful in Zeeman-state-resolving detection.

We present precise, sub-wavelength optical intensity measurement using a single trapped ^87Rb atom as a sensor. The intensity is measured by the scalar ac Stark shift it produces on the F=1 → F'=2 hyperfine transition of the D_2 line, chosen for its F' = F+1 structure and very small tensor polarizability. To boost signal and reduce measurement-induced perturbations, we use a quantum jump spectroscopy technique in which a single absorbed photon on a transition of interest induces the scattering of hundreds of photons on a bright closed transition. The method greatly reduces systematic effects associated with the atomic state, optical polarization, probe power, and atom heating, and gives the atomic temperature as a second spectroscopic observable. We demonstrate the method by measuring the intensity at the focus of an optical tweezer.

We present precise, subwavelength optical intensity measurements using a single trapped 87Rb atom as a sensor. The intensity is measured by the scalar ac Stark shift it produces on the F = 1-* F' = 2 hyperfine transition of the D2 line, chosen for its F' = F + 1 structure and very small tensor polarizability. To boost signal and reduce measurement-induced perturbations, we use a quantum jump spectroscopy technique in which a single absorbed photon on a transition of interest induces the scattering of hundreds of photons on a bright closed transition. The method greatly reduces systematic effects associated with the atomic state, optical polarization, probe power, and atom heating, and gives the atomic temperature as a second spectroscopic observable. We demonstrate the method by measuring the intensity at the focus of an optical tweezer.

We describe optical methods for trapping, cooling, and observing single $^{87}$Rb atoms in a four-lens "Maltese cross" geometry (MCG). The use of four high numerical-aperture lenses in the cardinal directions enables efficient collection of light from non-collinear directions, but also restricts the optical access for cooling and optical pumping tasks. We demonstrate three-dimensional atom localization with sub-wavelength precision, and present measurements of the trap lifetime, temperature and transverse trap frequency in this geometry. We observe a trap performance comparable to what has been reported for single-atom traps with one- or two-lens optical systems, and conclude that the additional coupling directions provided by the MCG come at little cost to other trap characteristics.

We demonstrate a novel geometry for strong coupling of light and matter in free space, i.e., without the use of optical cavities. Guided by optical metrology tools, we use a manual pick-and-place technique to precisely and stably position four high numerical aperture (NA=0.5) aspheric lenses along the four cardinal directions with their foci at a single central point. The geometry immediately doubles the available solid angle and thus the light-atom coupling relative to two-lens schemes, and will enable new trapping, excitation, and collection methods. We test the system by trapping a single 87Rb at the common focus and observing its fluorescence simultaneously from four directions. The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

We report on the simultaneous observation from four directions of the fluorescence of single 87 Rb atoms trapped at the common focus of four high numerical aperture (NA = 0.5) aspheric lenses.We use an interferometrically-guided pick-and-place technique to precisely and stably position the lenses along the four cardinal directions with their foci at a single central point.The geometry gives right angle access to a single quantum emitter, and will enable new trapping, excitation, and collection methods.The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

We describe a cavity-enhanced spontaneous parametric down-conversion (CE-SPDC) source for narrowband photon pairs with filters designed such that 97.7% of the correlated photons are in a single mode of 4.3(4) MHz bandwidth. Type-II phase matching, a tuneable-birefringence resonator, MHz-resolution pump tuning, and tuneable Fabry-Perot filters are used to achieve independent signal and idler tuning. We map the CE-SPDC spectrum using difference frequency generation to precisely locate the emission clusters, demonstrate CE-SPDC driven atomic spectroscopy, and measure a contribution from unwanted modes of 7.7%. The generated photon pairs efficiently interact with neutral rubidium, a well-developed system for quantum networking and quantum simulation. The techniques are readily extensible to other material systems.

Simultaneous precise measurement of the non-commuting observables spin angle and spin amplitude is achieved by directing the error due to quantum measurement back-action into an unmeasured spin component. Many quantum systems that are currently used to enhance metrological precision obey the regular Heisenberg uncertainty relations that apply to conjugate variables such as position and momentum. These systems can be 'squeezed' to reduce the uncertainty of one variable at the expense of greater uncertainty in another, and thereby to surpass the limits set by classical physics in metrology. However, spin systems and pseudo-spin systems obey different uncertainty relations because of their underlying symmetries. On the basis of these relations, the authors demonstrate simultaneous measurement of spin amplitude and spin angle beyond classical limits. This approach has potential applications in spin-based sensors and could increase the sensitivity for several applications, such as magnetic resonance measurements, in which spin relaxation rates could be correlated with precession frequency with higher precision than is currently possible. Measurement of spin precession is central to extreme sensing in physics1,2, geophysics3, chemistry4, nanotechnology5 and neuroscience6, and underlies magnetic resonance spectroscopy7. Because there is no spin-angle operator, any measurement of spin precession is necessarily indirect, for example, it may be inferred from spin projectors at different times. Such projectors do not commute, and so quantum measurement back-action—the random change in a quantum state due to measurement—necessarily enters the spin measurement record, introducing errors and limiting sensitivity. Here we show that this disturbance in the spin projector can be reduced below N1/2—the classical limit for N spins—by directing the quantum measurement back-action almost entirely into an unmeasured spin component. This generates a planar squeezed state8 that, because spins obey non-Heisenberg uncertainty relations9,10, enables simultaneous precise knowledge of spin angle and spin amplitude. We use high-dynamic-range optical quantum non-demolition measurements11,12,13 applied to a precessing magnetic spin ensemble to demonstrate spin tracking with steady-state angular sensitivity 2.9 decibels below the standard quantum limit, simultaneously with amplitude sensitivity 7.0 decibels below the Poissonian variance14. The standard quantum limit and Poissonian variance indicate the best possible sensitivity with independent particles. Our method surpasses these limits in non-commuting observables, enabling orders-of-magnitude improvements in sensitivity for state-of-the-art sensing15,16,17,18 and spectroscopy19,20.