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arXiv:1502.01605

Long spatial coherence times a few micro-meters from a room temperature surface

Shuyu Zhou, David Groswasser, Mark Keil, Yonathan Japha, R. Folman

Published Feb 5, 2015• 0 citations
arXivHTMLPDF

Abstract

The search for quantum coherence based on isolated atoms integrated with a room temperature solid state device (so-called atomchip [1-3]) has been intensifying in the last decade, with advances being made towards applications such as clocks, quantum information processing, surface probing and acceleration and gravitational field sensors. Such a device will also enable (and to some extent has already enabled) novel experiments in fundamental physics (e.g., [4-7]). Here we report on the trapping and maintenance of spatial coherence of atoms (in a Bose-Einstein Condensate -- BEC) about 5$μ$m from a room temperature surface, reducing significantly the distance previously achieved between the spatially coherent atoms and their classical environment [8-12], and most importantly entering the regime where atomic circuits are enabled. In addition, we enter the interesting regime in which the distance to the surface is much smaller than the probed coherence length, a regime in which the spatial dephasing reaches its maximal rate.

DOI
10.48550/arxiv.1502.01605

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The search for quantum coherence based on isolated atoms integrated with a room temperature solid state device (so-called atomchip [1-3]) has been intensifying in the last decade, with advances being made towards applications such as clocks, quantum information processing, surface probing and acceleration and gravitational field sensors. Such a device will also enable (and to some extent has already enabled) novel experiments in fundamental physics (e.g., [4-7]). Here we report on the trapping and maintenance of spatial coherence of atoms (in a Bose-Einstein Condensate -- BEC) about 5μμμm from a room temperature surface, reducing significantly the distance previously achieved between the spatially coherent atoms and their classical environment [8-12], and most importantly entering the regime where atomic circuits are enabled. In addition, we enter the interesting regime in which the distance to the surface is much smaller than the probed coherence length, a regime in which the spatial dephasing reaches its maximal rate.
In quantum physics spatial coherence allows a massive object to be present in two locations at the same time. Such spatial coherence is easily lost in the presence of a classical environment, making it unobservable in our day-to-day experience. Here we report the persistence of spatial coherence for ultra-cold atoms held only 5 m from a room temperature surface, reducing substantially the distance previously achieved between trapped atoms exhibiting spatial coherence and their classical environment. At this distance, the environment would normally destroy spatial coherence over any length greater than a few micrometers, but we nevertheless observe coherence over a length of 30 m. We show that no observable dephasing is taking place, even on a time scale on the order of one second. From a technological point of view, this may enable quantum devices based on atomic circuits. A room temperature solid quantum circuit would have enormous potential and perhaps be as important a development as the long-sought room temperature superconductor. Atom chips [1{3] are a promising candidate but a true analogue of a solid atomic circuit remains beyond reach, although impressive progress towards clocks, acceleration sensors and quantum information processing on these chips is being made. Alongside this progress towards a technological breakthrough, numerous fundamental experiments have also been made with atom chips (e.g., [4{9]). To realize a complete \solid state device with isolated atoms, at least three milestones { adapted from electronic devices { need to be met: arbitrary (e.g., not periodic) guides and traps; single site addressability; and controlled interaction via tunneling barriers. The latter requirement demands that potentials must be sculptured with a resolution on the order of the de-Broglie wavelength of the atoms (about 1 m). To achieve these milestones in a scalable device, thereby forming circuits for matter-waves (e.g., [10, 11]), one must be able to trap the atoms and manipulate their external degree of freedom coherently a few micrometers or less from the surface used to generate the potential
We study spatial coherence near a classical environment by loading a Bose-Einstein condensate into a magnetic lattice potential and observing diffraction. Even very close to a surface ($5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$), and even when the surface is at room temperature, spatial coherence persists for a relatively long time ($\ensuremath{\ge}\phantom{\rule{0.16em}{0ex}}500\phantom{\rule{0.16em}{0ex}}\mathrm{ms}$). In addition, the observed spatial coherence extends over several lattice sites, a significantly greater distance than the atom-surface separation. This opens the door for atomic circuits, and may help elucidate the interplay between spatial dephasing, interatomic interactions, and external noise.
Atom chips provide a flexible and scalable platform for applications of cold atoms and Bose-Einstein condensates (BECs) in future quantum technologies including miniaturised atomic clocks, quantum information processing and quantum sensors. Our project studies the coherent properties of a two-component BEC prepared on a magnetic film atom chip using a two-photon microwave/radiofrequency transition. At a particular value 3.23 G of the magnetic field two hyperfine states |F =1, mF = −1 > and |F = 2, mF = 1 > in 87Rb exhibit a low differential Zeeman shift (431 Hz/G2) and can be trapped simultaneously with long dephasing times. The coherent superposition of these two states has a long coherence time and is attractive for applications in quantum atom devices.
Ultra-cold neutral atoms and Bose-Einstein condensates (BECs) offer fascinating possibilities for quantum information processing. Long decoherence times, easy qubit addressing and the potential for scalability are some advantages. Storing and manipulating atoms in magnetic microtraps or 'Atom Chips' provides a promising base on which quantum logic operations could be performed. A primary source of decoherence in atom chip experiments arises from the interaction of the micro-Kelvin atoms with the room temperature chip surface. We have experimentally investigated these interactions and observe two interesting effects. Firstly, ultra-cold clouds are seen to fragment into clumps when brought within 100 microns of the chip surface. Secondly, the lifetime of atom clouds is seen to drop off near the surface due to spin flip transitions. These are caused by radio-frequency thermal fluctuations of the magnetic field as predicted but not previously observed.
Abstract The limitations for the coherent manipulation of neutral atoms with fabricated solid state devices, so-called ‘atom chips’, are addressed. Specifically, we examine the dominant decoherence mechanism, which is due to the magnetic noise originating from the surface of the atom chip. It is shown that the contribution of fluctuations in the chip wires at the shot noise level is not negligible. We estimate the coherence times and discuss ways to increase them. Our main conclusion is that future advances should allow for coherence times as long as 1 second, a few µm away from the surface. PACS: 03.75.-b Matter waves – 32.80.Lg Mechanical effects of light on atoms and ions – 03.67.Lx Quantum computation – 05.40.-a Fluctuation phenomena and noise In the quest for physical implementations of quantum information processing, “atom chips ” are currently of great interest.
The manipulation of quantum degenerate gases has seen a flourishing of results in the last years. One of the most used tools to create and address a Bose-Einstein condensate (BEC) is the atomchip: A lithographycally patterned surface on which gold conductors are used to generate magnetic fields. To fully realize what the atomchip has to offer the miniaturization of structures and the integration of different devices (resonators, light fields, permanent magnets) are key factors. We have devised a method to micro-engineer the trapping potentials by sculpturing the chip, overcoming some limitations of the lithographic process. Numerical simulations are done on model cases to show the potentials they generate. Experimental data collected on one of these microstructured geometries confirms the expectations and gives indications on the quality of its magnetic potential. We propose also a periodic structure allowing to observe a quantum phase transition between a BEC and a Mott insulator. A further possibility for expanding the capabilities of the atomchip is integrating light fields close to its surface. We generate an optical lattice on our chip and use it to realize an atomic beamsplitter. Coherent splitting of a BEC is observed.