1,721,065 research outputs found
Sympathetic cooling and self-oscillations in a hybrid atom-membrane system
Get PDFHybrid systems combining mechanical oscillators and ultracold atoms provide novel
opportunities for cooling, detection and quantum control of mechanical motion with
applications in precision sensing, quantum-level signal transduction and for fundamental
tests of quantum mechanics.
In this thesis I present experiments performed with a hybrid atom-membrane
system, in which the vibrations of a Si_3N_4 membrane in an optical cavity are coupled
to the motion of laser-cooled atoms in an optical lattice. The interactions are
mediated by the lattice light over a macroscopic distance and enhanced by the cavity.
Via the coupling to the cold atoms, the fundamental vibrational mode of the
membrane at 2π x 276 kHz is cooled sympathetically from room temperature to
0.4(2) K, even though the mass of the mechanical oscillator exceeds that of the
atomic ensemble by a factor of 4 x 10^10. In other systems, sympathetic cooling
of molecules with cold atoms or ions has been limited to mass ratios of up to 90.
Previous theoretical work has shown that our coupling mechanism is able to cool the
membrane vibration into the ground state and to perform coherent state transfers
between atomic and membrane motion.
Under certain experimental conditions, the atom-membrane system shows self-oscillations,
which arise from an effective delay in the backaction of the atoms onto
the light. This retardation drives the system into limit-cycle oscillations if the coupling
is large. I study the dependence of this instability on several system parameters
and find that a larger atom number and a smaller atom-light detuning make the system
less stable. Further, the stability of the coupled system in presence of a delay is
investigated theoretically and a modified expression for the sympathetic cooling rate
is derived. This model allows to fit the measured atom number dependence with a
delay of τ = 88(1) ns. Moreover, direct measurements of the atomic backaction onto
the lattice light are presented. These show phase lags exceeding 180° in parameter
regimes where the instability is observed, proving that the retardation arises within
the atomic ensemble. Finally, I present the results of numerical simulations, which
show that collective atomic effects within the atomic ensemble in an asymmetric
lattice are able to induce the observed phase lag in the atomic backaction
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Strong light-mediated coupling between a membrane oscillator and an atomic spin ensemble
Get PDFThis thesis presents theoretical and experimental work on light-mediated coupling between a collective atomic spin and a micromechanical membrane oscillator. With our work we address a fundamental question of quantum optics: Can a beam of light mediate coherent Hamiltonian interactions between two distant quantum systems? This is an intriguing question whose answer is not a priori clear, since the light carries away information about the systems and might be subject to losses, giving rise to intrinsic decoherence channels associated with the coupling. Our answer is affirmative and we derive a particularly simple sufficient condition for the interactions to be Hamiltonian: The light field needs to interact twice with the systems and the second interaction has to be the time reversal of the first. We demonstrate theoretically that, even in the presence of significant optical loss, coherent interactions can be realized and generate substantial amounts of entanglement between the systems.
In our experiments, we employ this approach to strongly couple a spin-polarized atomic ensemble and a micromechanical oscillator via a free-space laser beam across a distance of one meter in a room-temperature environment. The atomic ensemble consists of about ten million laser-cooled Rubidium atoms in an optical dipole trap that interact with the coupling laser via an off-resonant Faraday interaction. The mechanical oscillator is a silicon nitride membrane which is mounted in a single-sided optical cavity and couples to the laser field via radiation-pressure forces. In order to mediate a bidirectional Hamiltonian interaction between spin and membrane, the coupling beam is arranged in a loop such that it couples twice to the spin. This looped geometry enables destructive interference of quantum back-action by the light field on the spin.
Using this setup, we experimentally demonstrate for the first time strong Hamiltonian coupling between remote quantum systems and explore different dynamical regimes of cascaded light-mediated interactions: With the spin initialized in its ground state we observe normal-mode splitting and coherent energy exchange oscillations, both hallmarks of strong coupling. If we invert the spin to its highest energy state, we observe parametric-gain interactions, resulting in two-mode thermal noise squeezing. Furthermore, by shifting the phase of the light field between spin and membrane we can switch to non-Hamiltonian coupled dynamics, allowing us to observe level attraction and exceptional points. This high level of control in a strongly coupled modular system gives access to a unique toolbox for designing hybrid quantum systems and coherent optical feedback loops. Our approach to engineer coherent long-distance interactions with light makes it possible to couple very different systems in a modular way, opening up a range of new opportunities for quantum control
Many-particle entanglement, Einstein-Podolsky-Rosen steering and Bell correlations in Bose-Einstein condensates
Get PDFAccording to quantum mechanics, the results of measurements performed on different systems can show correlations that are stronger than what is classically possible. Three important types of nonclassical correlations that have been identified are entanglement (nonseparability), Einstein-Podolsky-Rosen correlations (steering) and Bell correlations (nonlocality). Apart from shedding light on the foundations of quantum theory and on how nature behaves, they represent different resources for applications that are inaccessible by classical means. While such correlations have been extensively investigated in systems composed of a few particles, their role in many-body systems is much less explored.
In this thesis I present both experimental and theoretical results on quantum correlations in many-body systems. Specifically, I report experiments where we prepare a Bose-Einstein condensate of approximately 600 Rubidium-87 atoms on an atom chip in a spin squeezed state, and analyze the correlations between the constituent atoms. First, I show state-of-the-art detection of entanglement, and of its depth, using collective measurements. For a state with a Wineland spin squeezing parameter of -6.8 dB we conclude an entanglement depth of approximately 56 particles. Then, I describe the first detection of Bell correlations in a many-body system. This result was enabled by deriving a witness for Bell correlations which involves only collective measurements on the atomic ensemble. Moreover, we present a sufficient criterion to close the statistics loop-hole, derive additional multi-partite inequalities and witnesses detecting Bell correlations in a larger class of states, and report ways to quantify their depth. Applying these to our experimental data, we conclude the presence of at least 6-partite Bell correlations. As a new tool, we provide a method to detect Bell correlations in experimental data by running a computer algorithm consisting in a hierarchy of semi-definite programs bounding the set of classical correlations.
Finally, I present the first observation of Einstein-Podolsky-Rosen steering between spatially separated regions in an ensemble of massive particles. This result was obtained by high resolution imaging of an expanded spin-squeezed Bose-Einstein condensate in order to measure spin correlations between spatially separated parts.
Our experimental and theoretical studies of quantum correlations in many-body systems are an important step towards exploring the predictions of quantum mechanics in macroscopic systems, and could enable a variety of quantum information tasks. Our experiments show that Bose-Einstein condensates on atom chips are an ideal platform for the implementation and investigation of such many-body quantum correlations
A miniaturised hybrid ion-atom chip trap and the non-equilibrium statistical mechanics of trapped ions
Get PDFExperiments involving trapped ultracold matter are of great interest to a diverse range of fields, from spectroscopy and quantum computing to ultracold chemistry. Hybrid traps allowing for the simultaneous confinement of charged and uncharged matter extend the scope of these experiments, but have not yet benefited from the miniaturisation of the trapping architectures demonstrated for traps which only confine either ions or neutral particles. This miniaturisation greatly enhances the spatial resolution of the forces with which the trapped particles are manipulated, and this thesis details the design and fabrication of a prototype miniaturised hybrid trap to take advantage of this increased precision. The co-trapping of ions and neutral particles leads to multiple mechanisms by which the energy distributions of the trapped ions may deviate from thermal statistics, which have previously been treated largely empirically. In this thesis, these effects are explored numerically and analytically to provide a theoretical framework for this behaviour through the formalism of superstatistics. The results derived here explain the deviations from thermal statistics observed in precision spectroscopy experiments and resolve outstanding questions about both the mechanism by which ions acquire a non-thermal energy distribution during buffer gas cooling with neutral atoms and the analytical form of this distribution. This significantly improves the ability to correctly interpret the results of experiments, and is applicable not only to the hybrid chip trap developed here, but to hybrid ion-neutral traps in general
High resolution field imaging with atomic vapor cells
Get PDFIn this thesis, I report on the development of imaging techniques in atomic vapor cells. This is a relatively unexplored area, despite the ubiquitous use of imaging in experiments with ultracold atoms. Our main focus is in high resolution imaging of microwave near fields, for which there is currently no satisfactory established technique. We detect microwave fields through Rabi oscillations driven by the microwave on atomic hyperfine transitions. The technique can be easily modified to also image dc magnetic fields. In addition, we have developed techniques to image vapor cell processes such as atomic T1 and T2 relaxation. These provide a new window into vapor cell physics, which we have used to obtain spatially resolved information on Rb interactions with the cell walls, and to estimate the Rb relaxation probability in a collision with the cell wall.
As a first application of our imaging techniques, we imaged the dc and microwave magnetic fields inside a state-of-the-art vapor cell atomic clock. This new clock characterisation technique should lead to real improvements in clock performance, and is in the process of being adopted by the atomic clock community.
We have developed a widefield, high resolution imaging setup using a microfabricated vapor cell, which we have used to image microwave and dc magnetic vector fields. With the addition of a 480 nm laser, the setup can be configured to image microwave electric fields. Our camera-based imaging system records 2D images with a 6x6 mm2 field of view at a rate of 10 Hz. It provides up to 50 um spatial resolution, and allows imaging of fields as close as 150 um above structures, through the use of extremely thin external cell walls. This is crucial in allowing us to take practical advantage of the high spatial resolution, as feature sizes in near-fields are on the order of the distance from their source, and represents an order of magnitude improvement in surface-feature resolution compared to previous vapor cell experiments. We demonstrate a microwave magnetic field sensitivity of 1.4 uT/sqrt-Hz per 50x50x140 um3 voxel, at present limited by the speed of our imaging system. Since we image 120x120 voxels in parallel, a single scanned sensor would require a sensitivity of at least 12 nT/sqrt-Hz to produce images with the same sensitivity.
The spatial resolution, distance of approach, and sensitivity of our high resolution setup are sufficient for characterising 6.8 GHz microwave fields above a range of real world devices. However, frequency tunability is essential for wider applications of our imaging technique. Industry is particularly interested in techniques for imaging high frequency microwaves, above 18 GHz, where simulations become increasingly unreliable. I have shown that our technique can be extended to image microwaves of any frequency, in principle from dc to 100s of GHz, by using a large dc magnetic field to Zeeman shift the hyperfine ground state transitions to the desired frequency. I present results from a proof-of-principle setup, where we have used a 0.8 T solenoid to detect and image microwaves from 2.3 GHz to 26.4 GHz
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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