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Robust quantum spatial coherence near a classical environment

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

Published May 11, 2015• 1 citation
OpenAlex

Abstract

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

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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
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.
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.
Quantum tunneling is a quantum phenomenon in which a microscopic object crosses through a potential barrier even if its energy cannot overcome the barrier. A general belief is that tunneling occurs only when the barrier width is comparable to, or smaller than the de Broglie’s wavelength of the object. Here, we study the tunneling of an ultracold atom among N far-separated trapping potentials in a state-selective optical lattice and present a mechanism to realize long-range tunneling. We find that, mediated by the propagating matter wave emitted from the atom, coherent tunneling of the atom to the remote lattices occurs as long as bound states are present in the energy spectrum of the system formed by the atom and its matter-wave. Going beyond the Markovian approximation, and breaking through the conventional distance constraint, our result opens another avenue to realizing tunneling and gives a guideline for developing tunneling devices. It is generally believed that quantum tunneling cannot occur when the distance is larger than the particle’s wavelength. This paper reveals that a long-range quantum tunneling of the atom to the remote trapping potentials mediated by its emitted matter wave occurs as long as bound states are present in the energy spectrum of the total system formed by the atom and its matter-wave.
The entry into the new millennium was marked in the quantum community by the first experimental realizations of the Bose-Einstein condensation of atoms in a vapor, achieved in different research groups. Twenty-five years later, such systems are 'routinely' produced and are the core of many experiments. At the frontier between condensed matter their y, quantum optics, and atomic physics, ultra-cold atoms are subjects of high scientific interest.Ultra-cold gas can be trapped in optical lattices, giving a realistic way of experimentally realizing a new range of lattice Hamiltonians, that are used to simulate condensed matter systems. In these simulations, ultra-cold atoms are used to mimic the behavior of electrons in a solid-state material and could be used to simulate a wide range of complex condensed matter phenomena, such as the fractional quantum Hall effect, topological insulators, and high-temperature superconductivity.In order to study such systems, diffraction-limited quantum gas microscopes have emerged as a way to manipulate and detect atoms trapped in lattice potentials. Using far-field interference, experiments have mainly been restricted to a lattice spacing of 500 nm. This limits the speed of dynamics of all physical processes.Higher energy regimes could be accessed by reducing the lattice spacing. For this purpose, one of the long-term goals of the team is to trap atoms in the near-field of a surface and to make use of the Casimir-Polder forces to shape sub-wavelength potentials. Thanks to nano-fabrication capabilities, the spacing can be reduced to a few tens of nanometers. However, this requires to adapt the imaging techniques to observe cold-atoms with sub-wavelength spatial resolutions.In this PhD thesis, I report first on the advances toward implementing an atom chip in our experiment, which will serve as an integrated platform to manipulate the quantum gas. I present simulations and study an experimental solution allowing the cloud to be transported to the near-field. The method is based on the interference of a laser radiation in the near field of a rotating surface. It has been fully simulated, and the limitations have been numerically and experimentally assessed. I conclude this part by presenting the first model of chip that has been fabricated.In a second part, I report on the development of a sub-wavelength imaging method of cold Rubidium 87 atoms which is based on excited state dressing. I detail the concept of this nano-metric spatial resolution method and apply it to super-resolve a single longitudinal wave function trapped in a tight lattice that exhibits a width below 30 nm.
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.
Abstract The past few decades have seen dramatic progress in our ability to manipulate and coherently control matter-waves. Although the duality between particles and waves has been well tested since de Broglie introduced the matter-wave analog of the optical wavelength in 1924, manipulating atoms with a level of coherence that enables one to use these properties for precision measurements has only become possible with our ability to produce atomic samples exhibiting temperatures of only a few millionths of a degree above absolute zero. Since the initial experiments a few decades ago, the field of atom optics has developed in many ways, with both fundamental and applied significance. The exquisite control of matter waves offers the prospect of a new generation of force sensors exhibiting unprecedented sensitivity and accuracy, for applications from navigation and geophysics to tests of general relativity. Thanks to the latest developments in this field, the first commercial products using this quantum technology are now available. In the future, our ability to create large coherent ensembles of atoms will allow us an even more precise control of the matter-wave and the ability to create highly entangled states for non-classical atom interferometry.