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Machine learning (ML) is an effective tool to interrogate complex systems to find optimal parameters more efficiently than through manual methods. This efficiency is particularly important for systems with complex dynamics between multiple parameters and a subsequent high number of parameter configurations, where an exhaustive optimisation search would be impractical. Here we present a number of automated machine learning strategies utilised for optimisation of a single-beam caesium (Cs) spin exchange relaxation free (SERF) optically pumped magnetometer (OPM). The sensitivity of the OPM (T/Hz), is optimised through direct measurement of the noise floor, and indirectly through measurement of the on-resonance demodulated gradient (mV/nT) of the zero-field resonance. Both methods provide a viable strategy for the optimisation of sensitivity through effective control of the OPM’s operational parameters. Ultimately, this machine learning approach increased the optimal sensitivity from 500 fT/Hz to

International audience; Recent developments in quantum technology have resulted in a new generation of sensors for measuring inertial quantities, such as acceleration and rotation. These sensors can exhibit unprecedented sensitivity and accuracy when operated in space, where the free-fall interrogation time can be extended at will and where the environment noise is minimal. European laboratories have played a leading role in this field by developing concepts and tools to operate these quantum sensors in relevant environment, such as parabolic flights, free-fall towers, or sounding rockets. With the recent achievement of Bose–Einstein condensation on the International Space Station, the challenge is now to reach a technology readiness level sufficiently high at both component and system levels to provide “off the shelf” payload for future generations of space missions in geodesy or fundamental physics. In this roadmap, we provide an extensive review on the status of all common parts, needs, and subsystems for the application of atom-based interferometers in space, in order to push for the development of generic technology components.

We demonstrate the integration of micro-electro-mechanical-systems (MEMS) scanning mirrors as active elements for the local optical pumping of ultra-cold atoms in a magneto-optical trap. A pair of MEMS mirrors steer a focused resonant beam through a cloud of trapped atoms shelved in the \textit{F}=1 ground-state of \textsuperscript{87}Rb for spatially-selective fluorescence of the atom cloud. Two-dimensional control is demonstrated by forming geometrical patterns along the imaging axis of the cold atom ensemble. Such control of the atomic ensemble with a microfabricated mirror pair could find applications in single atom selection, local optical pumping and arbitrary cloud shaping. This approach has significant potential for miniaturisation and in creating portable control systems for quantum optic experiments., 4 pages, 3 figures

A low-cost, mass-producible laser-cooling platform would have a transformative effect in the burgeoning field of quantum technologies and the wider research of atomic sensors. Recent advancements in the micro-fabrication of diffractive optics and vacuum apparatus have paved the way for a simple, stackable solution to the laser cooling of alkali atoms. In this paper we will highlight our recent investigations into a chip-scale, cold-atom platform, outlining our approach for on-chip wavelength referencing, examining a solution for imaging atoms in a planar stacked device, and finally discussing the limitations to passively pumped vacuum longevity. These results will be discussed in the context of an outlined road-map for the production and commercialisation of chip-scale, cold-atom sensors.

Cold atom fountain clocks provide exceptional long term stability as they increase interrogation time at the expense of a larger size. We present a compact cold atom fountain using a grating magneto-optical trap (GMOT) to laser cool and launch the atoms in a simplified optical setup. The fountain is evaluated using coherent population trapping and demonstrates improved single-shot stability from the launch. Ramsey times up to 100 ms were measured with a corresponding fringe linewidth of 5 Hz. This technique could improve both short- and long-term stability of cold atom clocks whilst remaining compact for portable applications., 5 pages, 4 figures

Here we present a cold atom system, small enough to be handheld, and suitable for use in a cold atom sensor. This has been achieved by developing compact and robust components for laser cooling. Notably integrating a centilitre scale vacuum system and diffractive grating magneto-optical trap optics has been a key step in creating this compact cold atom system. The performance of these components is discussed, including the validation of vacuum by using the cold atoms to measure the vacuum pressure. Future improvements are outlined.

We optically assess Fresnel zone plates (FZPs) that are designed to guide cold atoms. Imaging of various ring patterns produced by the FZPs gives an average RMS error in the brightest part of the ring of 3% with respect to trap depth. This residue will be due to the imaging system, incident beam shape and FZP manufacturing tolerances. Axial propagation of the potentials is presented experimentally and through numerical simulations, illustrating prospects for atom guiding without requiring light sheets., Comment: 10 pages, 5 figures

We present a small-scale piezoelectric MEMS micromirror, with resonant frequencies above 300 kHz for 1D scanning. The device is intended for higher frequency operation by reducing the scale of existing designs, and was fabricated using a multi-user silicon-on-insulator process. The performance of the mirror for addressing points along one axis was demonstrated using a free-space optics experimental setup.

We have produced a sample of laser-cooled atoms in a micro-fabricated alkali vapor cell using a grating MOT to direct the beams. We show that by detecting the blue fluorescence resulting from a two-photon cascade transition, we improve the rejection of cooling light scattered from the grating.

Laser-cooled atoms and coherent population trapping (CPT) are promising tools for realizing a compact microwave frequency reference with excellent stability. To realize a high performance device, it is necessary to understand and minimize all sources of technical noise. Here, we investigate the role of laser frequency noise in cold-atom CPT with an apparatus based on the grating magneto-optical trap (GMOT). We compare the performance of our setup with an external cavity diode laser (ECDL) and a distributed feedback diode laser (DFB). With the DFB, laser frequency noise is one of the dominant noise sources in our system. With the ECDL, it is significantly reduced. We also report frequency stability measurements of our apparatus with a short-term Allan deviation $\sigma_{y}(\tau) \approx 3 \times 10^{-11} / \sqrt{\tau}$ up to $\tau =10\mathrm{s}$.

Double-resonance optically pumped magnetometers are an attractive instrument for unshielded magnetic field measurements due to their wide dynamic range and high sensitivity. Use of linearly polarised pump light creates alignment in the atomic sample, which evolves in the local static magnetic field, and is driven by a resonant applied field perturbation, modulating the polarisation of transmitted light. We show for the first time that the amplitude and phase of observed first- and second-harmonic components in the transmitted polarisation signal contain sufficient information to measure static magnetic field magnitude and orientation. We describe a laboratory system for experimental measurements of these effects and verify a theoretical derivation of the observed signal. We demonstrate vector field tracking under varying static field orientations and show that the static field magnitude and orientation may be observed simultaneously, with experimentally realised resolution of 1.7 pT and 0.63 mrad in the most sensitive field orientation., Comment: 8 pages, 7 figures

An atom interferometer using a Bose-Einstein condensate of $^{87}$Rb atoms is utilized for the measurement of magnetic field gradients. Composite optical pulses are used to construct a spatially symmetric Mach-Zehnder geometry. Using a biased interferometer we demonstrate the ability to measure small residual forces in our system at the position of the atoms. These are a residual magnetic field gradient of 15$\pm$2 mG/cm and and an inertial acceleration of 0.08$\pm$0.02 m/s$^2$. Our method has important applications in the calibration of precision measurement devices and the reduction of systematic errors., 7 pages, 6 figures

Double resonance optically pumped magnetometry can be used to measure static magnetic fields with high sensitivity by detecting a resonant atomic spin response to a small oscillating field perturbation. Determination of the resonant frequency yields a scalar measurement of static field ($B_0$) magnitude. We present calculations and experimental data showing that the on-resonance polarimeter signal of light transmitted through an atomic vapour in arbitrarily oriented $B_0$ may be modelled by considering the evolution of alignment terms in atomic polarisation. We observe that the amplitude and phase of the magnetometer signal are highly dependent upon $B_0$ orientation, and present precise measurements of the distribution of these parameters over the full 4{\pi} solid angle.

An integrated system of hardware and software allowing precise definition of arbitrarily oriented magnetic fields up to |B| = 1 {\mu}T within a five-layer mumetal shield is described. The system is calibrated with reference to magnetic resonance observed between Zeeman states of the 6S$_{1/2}$ F = 4 $^{133}$Cs ground state. Magnetic field definition over the full 4{\pi} solid angle is demonstrated, with one-sigma tolerances in magnitude, orientation and gradient of {\delta}|B| = 0.94 nT, {\delta}{\theta} = 5.9 mrad and {\delta}$\nabla$ B = 13.0 pT/mm, respectively. This field control is used to empirically map Mx magnetometer signal amplitude as a function of the static field (B0) orientation., Comment: 14 pages, 10 figures

Laser cooled atoms are central to modern precision measurements. They are also increasingly important as an enabling technology for experimental cavity quantum electrodynamics, quantum information processing and matter wave interferometry. Although significant progress has been made in miniaturising atomic metrological devices, these are limited in accuracy by their use of hot atomic ensembles and buffer gases. Advances have also been made in producing portable apparatus that benefit from the advantages of atoms in the microKelvin regime. However, simplifying atomic cooling and loading using microfabrication technology has proved difficult. In this letter we address this problem, realising an atom chip that enables the integration of laser cooling and trapping into a compact apparatus. Our source delivers ten thousand times more atoms than previous magneto-optical traps with microfabricated optics and, for the first time, can reach sub-Doppler temperatures. Moreover, the same chip design offers a simple way to form stable optical lattices. These features, combined with the simplicity of fabrication and the ease of operation, make these new traps a key advance in the development of cold-atom technology for high-accuracy, portable measurement devices., Comment: 5 pages, 5 figures

The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.

We demonstrate an optically pumped magnetometer (OPM) operated in a free-induction-decay (FID) configuration that is capable of tracking oscillating magnetic signals in the presence of a 50 uT static field. Excellent waveform reconstruction is demonstrated for low frequency modulations with respect to the Nyquist limited bandwidth. A 100 pT oscillation was successfully reconstructed using signal averaging, and an optimum sensitivity of 3.9 pT/sqrt{Hz} was measured from the spectrum of the residuals relative to the sinusoidal fit. The impact of the pump-probe repetition rate and spin depolarization on the frequency response of the sensor is investigated in detail using miniaturized vapor cell technology, with the (-3 dB) bandwidths residing beyond the Nyquist limit in each case. We also discuss technical limitations associated with the magnetometer when exposed to oscillating fields of sufficiently high amplitude or frequency. This is discussed in the context of potential distortions arising in the reproduced signals, induced by frequency-modulation (FM) and aliasing artefacts.

It has recently been shown that optical reflection gratings fabricated directly into an atom chip provide a simple and effective way to trap and cool substantial clouds of atoms (Nshii et al. in Nat Nanotechnol 8:321–324, 2013; McGilligan et al. in Opt Express 23(7):8948–8959, 2015). In this article, we describe how the gratings are designed and microfabricated and we characterise their optical properties, which determine their effectiveness as a cold atom source. We use simple scalar diffraction theory to understand how the morphology of the gratings determines the power in the diffracted beams.

We report the first use of a ring cavity to both enhance the output power and dramatically narrow the linewidth ($, 5 pages, 3 figures

We report on a novel material developed as the gain medium for a vertical-external-cavity surface-emitting laser (VECSEL) operating around 850 nm. The new material departs from the conventional approach of using GaAs as the quantum-well (QW) material and expands the previously reported concept of using InAlGaAs QWs. The inclusion of indium pins dislocation propagation into the active region of the VECSEL. Crucial for the success of this design is also the development of indium and phosphorous containing quinternary strain-compensating layers. These surround the QWs and provide a more substantial resistance to defect propagation. Results are presented for stable high-power single spatial mode operation of a laser based on this material together with measurements of the unsaturated gain of the device and the characteristic temperature for the threshold power

The experimental parameters required in order to maximize the stability of a single-ion based optical clock are analysed. This stability is a trade-off between on the one hand the steepness of the discriminant signal derived from the laser excitation of the ion and on the other the binomial noise associated with the excitation of a single particle. Optimum conditions are found for both Rabi and Ramsey excitation of the clock transition.

We report on a novel and compact reliable laser source capable of short-wavelength two-photon laser scanning fluorescence microscopy based on soliton self-frequency shift effects in photonic crystal fibre. We demonstrate the function of the system by performing two-photon microscopy of smooth muscle cells and cardiac myocytes from the rat pulmonary vein and Chinese hamster ovary cells loaded with the fluorescent calcium indicator fura-2/AM.

A short section of photonic crystal fibre has been used for ultra-short pulse compression. The unique optical properties of this novel medium in terms of high non-linearity and relatively small group velocity dispersion are shown to provide an ideal platform for the standard fibre pulse compression technique used directly on the nano-Joule output pulses from a commercial laser system. We report an order of magnitude reduction of the pulse width to 25 fs FWHM but predict a substantially improved performance with a dedicated fibre design. Good agreement is obtained with a simple model for the spectral broadening in the fibre.

We report on efficient second harmonic generation in periodically poled KTiOPO4 (ppKTP) at 423 nm pumped by a Ti:Sapphire solid state laser system. An overall optical conversion efficiency of over 60% was achieved at a moderate input power of 400 mW. The nonlinear crystal used in the experiments was a first-order quasi-phase-matched flux-grown KTP crystal (period 4 μm, length 10 mm). The crystal has a single-pass conversion efficiency of 1.38±0.08%/W cm and was placed in a ring cavity. We have demonstrated stable operation with a generated cw power at 423 nm of up to 225 mW. With a sweeping cavity we generated 425 mW peak power with an incident pump power of 680 mW. At this power level it was not possible to lock the cavity due to a bistability caused by localised heating of the crystal.

A 3D lattice based on a high-power CO2 laser is considered in the context of laser cooling and trapping of atomic calcium. We expect to be able to realize a system with }10\,000$>>10 000 lattice sites each with more than 100 atoms, and the facility to laser cool all the atoms into the vibrational ground state using the intercombination line. The configuration allows the production of an array of small Bose?Einstein condensates, enabling an investigation of the build-up of phase coherence as a function of atom number.

We investigate the properties of two separate dipole traps, realised using a cw mode-locked Ti:sapphire laser and a cw mode-locked Nd:YVO4 laser, red-detuned by 25 nm and 284 nm, respectively. Approximately 103 laser-cooled 85Rb atoms were confined in the traps at ≈50 μK, with no observable heating after initial loading. The lifetimes of the traps were consistent with limitations imposed by wavelength-dependent photoassociation losses and collisions with background vapour. Determination of the ac Stark shift of the 780 nm cooling transition using a weak probe beam showed no observable difference between using narrow-bandwidth or mode-locked trapping light. Techniques for trapping and focusing of atoms based on the dipole force of blue and uv light become much more accessible through efficient doubling, tripling and quadrupling of mode-locked sources. This opens up the possibility of manipulating more technologically interesting species.

We report an experimental demonstration of a continuous, slow and cold beam of rubidium atoms from a two-dimensional magneto-optic trap or atomic funnel. Typically 7.3(7)×108 atoms/s are ejected from the funnel with a variable velocity in the range 2–8 m/s and a temperature of 45–55 μK in the moving frame. This represents the first demonstration of sub-Doppler laser cooling in an atomic beam and temperatures as low as ≈25 μK have been observed.

A simple optical sideband injection locking technique is demonstrated to frequency offset-lock a 674 nm slave extended cavity diode laser to a single frequency source. By injecting light from a master laser into either sideband of the frequency modulated extended cavity slave laser, it is shown that the carrier of the slave is frequency locked to the master laser with a relative linewidth much less than 10 Hz. The locking bandwidth is investigated to determine the largest capture range. An observation of the 674 nm 2 S 1/2 – 2 D 5/2 transition in a single ion of 88 Sr + was performed using this laser system, yielding ≈1 kHz linewidth Zeeman structure, limited by the master laser linewidth. National Physical Laboratory

A novel variation of the technique of two-photon frequency modulation (FM) spectroscopy has been developed, which allows signals to be obtained from small (∼10 5 atoms) and dilute (∼10 10 cm −3 ) samples – in this case laser-cooled rubidium atoms. This was possible by detecting and demodulating the fluorescence signal rather than the transmission through the sample. Data are presented demonstrating the dependence of the signal on phase and frequency of the imposed modulation. Initially the atoms were held in a magneto-optic trap but, in order to eliminate line broadening due to the presence of the trapping fields, data were taken with the atoms in free fall. A similar signal, slightly shifted and broadened by the trapping fields, was obtained while the trap was on and used for long-term stabilisation of the probe laser.

We have produced squeezed states of light using a degenerate optical parametric amplifier pumped by an all-solid-state cw mode-locked laser source. The process resulted in squeezed light pulses of less than 3 ps in duration. Up to 0.7 dB of amplitude-squeezed light at 1047 nm and 0.5 dB of noise reduction in vacuum were observed directly. The inferred squeezing level in both cases, after all propagation losses in the detection system and the effects of spatial and temporal mode overlap had been taken into account, was 1.8 dB.

We present an experimental realization of discrimination between non-orthogonal polarization states at the Helstrom bound for minimum probability of error. The experiment was performed with highly attenuated laser light with a mean number of about 0·1 photons per pulse.

The ability to cool, manipulate, and trap atoms using laser light has allowed a new, rapidly expanding field to emerge. Current research focuses on improving existing cooling techniques, and the development of cold atoms as a source for applications ranging from atomic clocks to studies of quantum degeneracy. This review explains the basic mechanisms used in laser cooling and trapping, and illustrates the development of the field by describing a selection of key experiments.

We report the transfer of phase structure, and in particular of orbital angular momentum (OAM), from near-infrared pump light to blue light generated in a four-wave-mixing process in 85Rb vapour. The intensity and phase profile of the two pump lasers at 780nm and 776nm, shaped by a spatial light modulator (SLM), influences the phase and intensity profile of light at 420nm which is generated in a subsequent coherent cascade. In particular we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.

We use magnetic levitation [1-3] and a variable-separation dual optical plug [4] to obtain clear spatial interference between two BECs axially separated by up to 0.25mm. Fringes are observed using standard (i.e. non-tomographic) resonant absorption imaging. The `magnifying' effect of a weak inverted parabola potential on fringe separation is observed and agrees well with theory [4]. With 160ms levitation we recently observed single-shot interference contrasts of 95% (see 1.5×0.75mm absorption image below), close to the theoretical limit due to pixellation of the sinusoidal fringes on our CCD camera. Interference patterns with fringe periods of 85microns (individual de Broglie wavelengths of 170microns) are possible with 200ms levitation.

Using laser cooled samples of 85Rb and 87Rb in a magneto-optical trap, high resolution spectroscopy of the two-photon transitions 5 S 1 2 −5 D 3 2 , 5 S 1 2 −5 D 5 2 and 5 S 1 2 −7 S 1 2 have been investigated using the output from a mode-locked titanium sapphire laser. The hyperfine constants for the 7S terms of both isotopes have been measured with improved accuracy.

A scheme is demonstrated for optical pumping of a vertical-external-cavity surface-emitting laser. The scheme is based on absorption of the pump light within the wells of the multiple-quantum-well gain structure rather than the conventional approach of absorption of a shorter wavelength in the barrier regions. The operation of a laser around 850 nm pumped by an 808 nm source demonstrates the potential of this technique for allowing operation at a significantly shorter range of wavelengths for these devices in general and specific application of high-brightness pump lasers for devices in this spectral region. A further advantage is the smaller quantum defect which results in reduced heating of the gain medium. These advantages are achieved while maintaining a slope efficiency of up to 18%, which is comparable to results obtained with a traditional pumping scheme with a similar gain medium.

We have measured the velocity dependence of the spatially averaged laser cooling force in the (110) direction of three-dimensional σ+−σ− optical molasses. The force is well described by the function F(v) = −αv[1 + (vvc)2] known from 1D lin ⊥ lin optical molasses. The dependence of the friction coefficient α and the critical velocity vc on laser detuning and intensity have been measured. We also present experimental results for the velocity dependence of the average diffusion of momentum.

We describe an experimental configuration suitable for observing the geometric phase of Aharonov and Casher in atomic systems. Using this we have been able to show experimentally that the Aharonov-Casher phase is both independent of velocity and proportional to an electric field and we have verified the predicted size of the effect with an accuracy of 2%.

We report the power scaling of a diode-pumped GaAs-based 850-nm vertical external-cavity surface-emitting laser, by use of an intracavity silicon carbide (SiC) heatspreader optically contacted to the semiconductor surface. To our knowledge, this is the first demonstration of bonding of SiC to a III-V semiconductor structure using the technique of liquid capillarity. High output power of >0.5 W in a circularly symmetric, TEM/sub 00/ output beam has been achieved with a spectral shift of only 0.6 nm/W of pump power. No thermal rollover was evident up to the highest pump power available, implying significant further output-power scaling potential using this approach.

We have developed a removable and bakeable viewport for UHV applications. The viewport consists of a modified blank Conflat flange; a ring of flux-free solder to form a vacuum seal; a viewport window; a clamping flange, and conical disk springs. We have cyclically baked viewports up to 240 °C and have achieved an ultimate pressure of 1.2×10−10 Torr (limited by our pumping station), with no leak detected at the 10−10 atm cm3/s level. Both BK7 and zinc selenide windows have been used successfully.

We report the first demonstration of an inductively coupled magnetic ring trap for cold atoms. A uniform, ac magnetic field is used to induce current in a copper ring, which creates an opposing magnetic field that is time-averaged to produce a smooth cylindrically symmetric ring trap of radius 5 mm. We use a laser-cooled atomic sample to characterise the loading efficiency and adiabaticity of the magnetic potential, achieving a vacuum-limited lifetime in the trap. This technique is suitable for creating scalable toroidal waveguides for applications in matterwave interferometry, offering long interaction times and large enclosed areas., Comment: 12 pages, 6 figures