Atom interferometric sensors and quantum information processors must maintain coherence while the evolving quantum wave function is split, transformed, and recombined, but suffer from experimental inhomogeneities and uncertainties in the speeds and paths of these operations. Several error-correction techniques have been proposed to isolate the variable of interest. Here we apply composite pulse methods to velocity-sensitive Raman state manipulation in a freely expanding thermal atom cloud. We compare several established pulse sequences, and follow the state evolution within them. The agreement between measurements and simple predictions shows the underlying coherence of the atom ensemble, and the inversion infidelity in a $\ensuremath{\sim}80\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ atom cloud is halved. Composite pulse techniques, especially if tailored for atom interferometric applications, should allow greater interferometer areas, larger atomic samples, and longer interaction times, and hence improve the sensitivity of quantum technologies from inertial sensing and clocks to quantum information processors and tests of fundamental physics.
Atom interferometric sensors and quantum information processors must maintain coherence while the evolving quantum wave function is split, transformed, and recombined, but suffer from experimental inhomogeneities and uncertainties in the speeds and paths of these operations. Several error-correction techniques have been proposed to isolate the variable of interest. Here we apply composite pulse methods to velocity-sensitive Raman state manipulation in a freely expanding thermal atom cloud. We compare several established pulse sequences, and follow the state evolution within them. The agreement between measurements and simple predictions shows the underlying coherence of the atom ensemble, and the inversion infidelity in a \ensuremath∼80\ensuremathμK atom cloud is halved. Composite pulse techniques, especially if tailored for atom interferometric applications, should allow greater interferometer areas, larger atomic samples, and longer interaction times, and hence improve the sensitivity of quantum technologies from inertial sensing and clocks to quantum information processors and tests of fundamental physics.
Developments in the precise control of quantum systems have led to advances in both fundamental research and emerging technologies in areas including quantum sensing and computation. One particularly useful application is atom interferometry, which uses laser pulses to split, redirect, and recombine atomic wavepackets, enabling precision measurements of inertial effects. This thesis presents the construction and characterisation of a compact cold atom interferometer, to meet the longer-term aim of developing an atomic gyroscope. We detail the setup of a rubidium-85 Magneto-Optical Trap (MOT) and the development of a laser to perform Raman transitions across the hyperfine Rb-85 levels. We then demonstrate and characterise the coherent manipulation of the atomic state of the rubidium as well as atom-interferometric fringes, in a Doppler-insensitive configuration. Systematic errors can degrade the performance of laser pulse interactions with cold atoms by reducing the fidelity of the atom–light interactions. In this thesis, we investigate the implementation of composite pulses and optimal control pulses, which use pulses with varying phase sequences and durations to mitigate against experimental inhomogeneities. We also show that phase modulators can introduce distortions in both the applied laser phase and the Raman modulation amplitude, which can in turn degrade pulse performance. To address this, we develop a compensation protocol that corrects for these non-linearities and demonstrate that it leads to a further improvement in pulse fidelity, for the Waltz and Knill composite pulses. Applying this protocol to the rotary echo sequence led to Rabi oscillations that could last 4 times longer, compared to the uncalibrated IQ modulator. Although an optimal control inversion pulse showed more resilience to the IQ modulator distortions, the Mach-Zehnder interferometer sequence was more susceptible to them. It was demonstrated that compensating for IQ modulator non-linearities improved the optimised fringe signal-to-noise ratio, and reduced the phase-offset error by a factor of 2.4.
Atom interferometry is a highly versatile experimental technique for precision measurement, with applications in inertial sensing and tests of fundamental physics. Such interferometers rely on high fidelity transfer of atoms between internal states, which can be challenging when working with thermal atom clouds, where a range of resonance frequencies exists. Inhomogeneities in the spatial intensity distribution of the manipulation lasers can also give rise to a range of coupling strengths; both of these effects serve to reduce the fidelity of coherent manipulation operations. Composite pulses offer one route to high fidelity coherent manipulations. In this thesis we investigate the performance of composite pulses in a thermal cloud of Rubidium 85. We find remarkable agreement between theory and experimental investigation of composite inversion pulses, and build on this to model the performance of such pulses in atom interferometers. We also find excellent agreement between our model and the first experimental demonstration of an application of interferometry in the cooling of atoms; this is the first step towards implementing this cooling method in complex structures like molecules.
We report a demonstration of composite Raman pulses that achieve broadband population inversion and are used to increase the momentum splitting of an atom interferometer up to 18ℏk (corresponding to an increase in the inertial signal by a factor of nine). Composite Raman pulses suppress the effects of pulse length and detuning errors, providing higher transfer efficiency and velocity acceptance than single square pulses. We implement two composite pulse sequences, π/20°−π90°−π/20° and π/20°−π180°−3π/20°, and use the latter composite pulse to demonstrate large-area atom interferometry with stimulated Raman transitions. In addition to enabling larger momentum transfer and higher sensitivity, we argue that composite pulses can improve the robustness of atom interferometers operating in dynamic environments.
The laser cooling of atoms to ultracold temperatures has propelled many groundbreaking advances in fundamental research and precision measurement, through such applications as quantum simulators and interferometric sensors. Laser cooling remains, however, highly species-selective, and techniques for its application to molecules are still in their infancy. This thesis broadly concerns the development of laser cooling schemes, based on sequences of coherent optical pulses, which can in principle be applied to a wide range of species. We describe a cooling scheme, in which a velocity-selective impulse analogous to that in Doppler cooling is generated by a light-pulse Ramsey interferometer, and present a proof-of-concept demonstration of the scheme using ultracold rubidium-85 atoms as a test-bed. We realise an interferometer for the atoms, as they are in free-fall after release from a magneto-optical trap, by inducing stimulated Raman transitions between their ground hyperfine states. We provide a comprehensive characterisation of these Raman light-pulse interferometer optics, where particular attention is paid to light shift effects. Raman pulses, and indeed coherent operations in any quantum control system, unavoidably suffer from systematic errors in the control field intensity and frequency, and these lead to reductions in pulse fidelity and readout contrast. In parallel to the work on interferometric cooling in this thesis, we report our preliminary investigations into composite pulses, whereby 'naive' single pulses are replaced by sequences of rotations with tailored durations and phases, for improving pulse fidelity in the presence of inhomogeneities. We find that composite pulses can indeed be highly effective in our cold atom system, and propose that their application in such devices as interferometric sensors is a promising prospect.
Atom interferometric sensors can enable extremely precise measurements of inertial motion and external fields by manipulating and interfering atomic states using pulses of laser light. However, like many experiments that require the coherent control of a quantum system, the interaction fidelity is limited by inhomogeneities in the control fields. Variations in atomic velocity and laser intensity lead different atoms to experience different interactions under the same pulse, reducing the interference fringe contrast, introducing bias, and limiting the sensitivity. We present the theoretical design and experimental demonstration of pulses for atom interferometry which compensate inhomogeneities in atomic velocity and laser intensity. By varying the laser phase throughout a pulse and choosing an appropriate fidelity measure to be maximised, pulses are optimised by adapting optimal control techniques originally designed for nuclear magnetic resonance applications. We show using simulations that optimised pulses significantly improve the fidelity of interferometer operations and verify this experimentally using Raman transitions within a cold sample of 85Rb atoms. We demonstrate a robust state-transfer pulse that achieves a fidelity of 99.8(3)% in a ∼ 35 µK sample and obtain a threefold increase in the fringe contrast using a full sequence of optimised pulses. Many of the pulse shapes found by optimal control are simple and symmetrical, and we show that certain symmetries are integral to error compensation. By systematically exploring the dependence of these solutions on the model and optimisation parameters, we demonstrate a stability which underlines the general applicability of optimised pulses to a range of interferometer configurations. Finally, we introduce and computationally analyse a novel theoretical approach to improve the sensitivity of large-momentum-transfer (LMT) interferometers, whereby “biselective” pulses are optimised to track the changing resonance conditions encountered in extended pulse sequences that are designed to increase the measurement sensitivity. When conventional pulses of steady phase are used, the interference contrast decays rapidly as extra pulses are added because of the change in resonance. Using numerical simulations, we show that bi-selective pulses maintain interaction fidelity throughout extended pulse sequences, allowing significant increases in the sensitivity that may be obtained using LMT.
Ultracold samples of laser-cooled atoms are quantum systems over which modern atomic physicists can exert exquisite control. Largely decoupled from their environment, they can act as near-ideal test masses for inertial sensors based on atom interferometry and are well suited to experiments in coherent control of quantum systems that probe the fundamental nature of quantum mechanics and pave the way for practical quantum simulation and computation. This thesis details a series of experimental results that arise from the coherent control of rubidium atoms with laser light, focusing on the interplay between these interactions and atomic velocities; the laser frequency an atom ‘sees’ is Doppler shifted according to its velocity, while conservation of momentum dictates that, in exchanging photons with a laser, an atom’s velocity is altered. Coherent light–atom interactions can thus be tailored either to measure or to narrow the spread of velocities in an ultracold atomic gas. Alternatively, it can be desirable to design interactions that are homogeneous across a large spread of atomic velocities. All of these aspects are explored in this thesis. The velocity-sensitive interactions that lie at the heart of atom-interferometric inertial sensors are reexamined in a manner that yields considerable insight into the underlying processes and culminates in a novel, precise and elegant technique for measuring the velocity of ultracold atoms that is used to reveal the Gaussian nature of the velocity spread in a cloud with an effective temperature of 18.7(6) µK, undistorted by artefacts that plague other methods. Furthermore, optimal control techniques are applied to the problem of coherently and uniformly manipulating the quantum states of atoms in an ensemble with a large spread of velocities, even subject to variations in laser intensity. A broadband inversion pulse is demonstrated to change the internal state of 99.8(3) % of atoms in a ∼35 µK ensemble and — for the first time — this technique is used to optimise an entire atom interferometry sequence, yielding a threefold enhancement in the measurement contrast. Finally, a version of grey molasses cooling — in which atoms accumulate in velocity-dependent ‘dark’ states, narrowing their momentum spread and increasing their phase space density — is demonstrated with phase-coherent cooling beams; dark states that exist in this system prove to be particularly resilient to the spatially varying light shifts that are present in an optical dipole trap, and this is used both to enhance the number of atoms loaded into such a trap — by a factor of 7 compared to loading from a conventional optical molasses — and to further cool them once they are loaded in a technique that has promising prospects for the rapid production of ultracold, trapped, atoms.