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arXiv:1612.01931 · Journal of Physics B Atomic Molecular and Optical Physics

Quantum decoherence of phonons in Bose–Einstein condensates

Richard Howl, Carlos Sabín, Lucia Hackermüller, Ivette Fuentes

Published Oct 26, 2017• 17 citations
arXivHTMLPDF

Abstract

We apply modern techniques from quantum optics and quantum information science to Bose-Einstein condensates (BECs) in order to study, for the first time, the quantum decoherence of phonons of isolated BECs.In the last few years, major advances in the manipulation and control of phonons have highlighted their potential as carriers of quantum information in quantum technologies, particularly in quantum processing and quantum communication.Although most of these studies have focused on trapped ion and crystalline systems, another promising system that has remained relatively unexplored is BECs.The potential benefits in using this system have been emphasized recently with proposals of relativistic quantum devices that exploit quantum states of phonons in BECs to achieve, in principle, superior performance over standard nonrelativistic devices.Quantum decoherence is often the limiting factor in the practical realization of quantum technologies, but here we show that quantum decoherence of phonons is not expected to heavily constrain the performance of these proposed relativistic quantum devices.

DOI
10.1088/1361-6455/aa9622

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We apply modern techniques from quantum optics and quantum information science to Bose-Einstein condensates (BECs) in order to study, for the first time, the quantum decoherence of phonons of isolated BECs.In the last few years, major advances in the manipulation and control of phonons have highlighted their potential as carriers of quantum information in quantum technologies, particularly in quantum processing and quantum communication.Although most of these studies have focused on trapped ion and crystalline systems, another promising system that has remained relatively unexplored is BECs.The potential benefits in using this system have been emphasized recently with proposals of relativistic quantum devices that exploit quantum states of phonons in BECs to achieve, in principle, superior performance over standard nonrelativistic devices.Quantum decoherence is often the limiting factor in the practical realization of quantum technologies, but here we show that quantum decoherence of phonons is not expected to heavily constrain the performance of these proposed relativistic quantum devices.
This work investigates the theoretical and experimental decoherence of phonons in weakly and strongly-interacting Bose-Einstein Condensates (BECs). The theoretical analysis treats phonons as open quantum systems where the environment comprises all other quasi-particle modes of the BEC. The phonons are assumed to be Gaussian states and the time in which they decohere is estimated from the evolution of their purity and nonclassical depth in the dissipative channel. The calculations are performed for various BEC systems and it is found that the excited phonon states will always decohere much more rapidly than the rate at which they relax back to equilibrium with the environment. Part II of this work considers how the decoherence of the phonons can be measured experimentally. The experiment that is currently being investigated uses a strongly-interacting [superscript]6Li BEC where the scattering length can be varied with an external magnetic field. In a strongly-interacting Bose gas the mutual interaction between the condensed and noncondensed components plays a greater role than in weakly-interacting gases and results in distinct absorption images. Understanding the effects from this mutual interaction is vital to model the in-situ absorption images of a strongly-interacting BEC in order to extract accurate information such as that which will facilitate the experimental measurement of decoherence. Three theoretical models that could be used to fit in-situ absorption images of a strongly-interacting gas are analysed. These are the bi-modal, semi-ideal and Hartree-Fock models, which will be fit against the absorption images of the [superscript]6Li BEC for various scattering lengths. The validity of these models is also investigated to determine when beyond mean-field effects may be observed in the Bose gas. Controlling decoherence is essential to the operation and physical realisation of many quantum information tasks and quantum technologies. Recently, new technologies have emerged from relativistic quantum information science that, in principle, are more precise than their non-relativistic counterparts. The practical setup of these devices utilizes phonons of BECs but the decoherence of the phonons has not been considered. In this work, one of the BEC systems used to estimate the decoherence time of the phonons has been chosen to be based on these devices. This calculation is expected to inform the practical realization of these devices and inspire future related studies. Analogue gravity investigations based on BECs also utilize phonons, for example, in the analogue of Hawking radiation. The quantum properties of these states is of particular interest in these studies and understanding how they decohere, and at what rate, could potentially inform the theory of black hole physics as well as dictate what is possible to measure experimentally.
Two component Bose-Einstein condensates (BECs) have been recently shown to be viable systems for storing and manipulating quantum information. Unlike standard single-particle qubits, the quantum information is duplicated in a large number of identical bosonic particles, thus can be considered to be a macroscopic qubit. The duplication of the quantum information makes them potentially more robust than conventional qubits, where all the quantum information is lost with a single error. It has been shown theoretically and experimentally that such ensembles can be used in many ways the same way as a standard qubit: they can be visualized on the Bloch sphere, and can be manipulated analogously to standard qubits. On the other hand, the BEC qubits do not have genuine interaction between each other and one of the main difficulties with such a system is how to effectively interact them together in order to transfer quantum information and create entanglement. Furthermore, the larger Hilbert space of the macroscopic bosonic system does not allow for unique mapping of standard quantum algorithms. However, in a few past years the main building blocks of quantum information processing and several quantum algorithms were translated to the BEC qubits. In this paper we give a brief colloquium of the current achievements and outline new perspectives of the use of BEC qubits and spin-coherent ensembles for quantum technologies.
In view of the coherent properties of a large number of atoms, Bose-Einstein condensates (BECs) have a high potential for sensing applications. Several proposals have been put forward to use collective excitations such as phonons in BECs for quantum-enhanced sensing in quantum metrology. However, the associated highly nonclassical states tend to be very vulnerable to decoherence. In this article, we investigate the effect of decoherence due to the omnipresent process of three-body loss in BECs. We find strong restrictions for a wide range of parameters, and we discuss possibilities to limit these restrictions.
Since Feynman's realization of using quantum systems to investigate quantum dynamics, interest in creating controllable quantum systems to simulate condensed matter phenomenon has been high. With the realization of BECs in 1995, the realization of a relatively clean testbed for simulating some of these phenomenon became a reality. \n My PhD research has been an exploration of the production and use of Bose-Einstein Condensates for the study of superfluidity. The first 3 years have been spent in the actual building of a Na BEC apparatus. During this time, we’ve implemented a distinct technique to trap ultra cold Na atoms, i.e. the Optically Plugged Trap. In the process, we have shown how atoms in a linear trap can show spin metastability and thus maintain a nonequilibrium state for long periods of time.\n In studying the interaction of ultra-cold atoms with light, we have developed a technique to measure the velocity distribution of atoms using a standing optical wave (Bragg Spectroscopy). Alongside this, we have also created optical traps for atoms in which we can change to shape of the trap itself to probe different condensed matter systems. The eventual goal being the investigation of condensed matter physics, specifically superfluidity, using ultra-cold atoms.
When the atoms in a gas are cooled to extremely low temperatures, their quantum-mechanical nature starts to dominate the properties of the whole gas. Under the appropriate conditions, the atoms will 'condense' into a single quantum state of the system-a phenomenon known as Bose-Einstein condensation (BEC). The resulting 'condensate' behaves as a single, observable quantum-mechanical object. During the last decade, this new state of matter has displayed many remarkable properties. In this paper, we review some of the most recent experimental developments in the BEC field, highlighting the role of atomic interactions and the high degree of control with which condensates may be manipulated.
For sufficiently low temperatures, a Bose gas experiences a transition into a Bose-Einstein condensate, characterized by a macroscopic occupation of the ground state of the system. The development of efficient cooling and trapping techniques for neutral atoms led in 1995, to the first observation of BEC in dilute alkali gases in 87Rb [1]. This remarkable achievement has been followed by BEC experiments in various other elements. The BEC has become during the last years, one of the most active and fruitful research areas of modern atomic physics, constituting an interdisciplinary field, which links atomic physics with various other research areas, like quantum optics, condensed-matter physics, and very especially nonlinear physics.