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Entanglement enhanced magnetic field measurement with two "hot" atoms

Poramaporn Ruksasakchai

Published Jan 1, 2024• 0 citations
OpenAlex

Abstract

A significant challenge in atomic physics is the precise preparation, manipulation, and measurement of the quantum-mechanical states of physical systems. One intriguing aspect of multi-particle quantum states is entanglement. Entanglement is of high interest for conducting non-classical computations, particularly in the application of quantum information, and the enhancement of measurements through entanglement. Spin-entangled states in many-body systems have been engineered and verified via spin-changing collisions. Using optical tweezers to isolate individual atomic pairs provides a unique avenue for investigating spin-changing collisions and the entangled states at the particle level. Up until now, the entanglement for an atomic pair has been achieved with groundstate-cooled atoms. The ability to generate entanglement at a higher temperature would be a step forward to robust practical applications in real world scenarios.

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A significant challenge in atomic physics is the precise preparation, manipulation, and measurement of the quantum-mechanical states of physical systems. One intriguing aspect of multi-particle quantum states is entanglement. Entanglement is of high interest for conducting non-classical computations, particularly in the application of quantum information, and the enhancement of measurements through entanglement. Spin-entangled states in many-body systems have been engineered and verified via spin-changing collisions. Using optical tweezers to isolate individual atomic pairs provides a unique avenue for investigating spin-changing collisions and the entangled states at the particle level. Up until now, the entanglement for an atomic pair has been achieved with groundstate-cooled atoms. The ability to generate entanglement at a higher temperature would be a step forward to robust practical applications in real world scenarios.
The predictions of quantum mechanics often differ from our everyday experiences. In the classical case, the state of a system can be predicted precisely, assuming the exact knowledge of all parameters. This is not possible in quantum mechanics. For example, the spin of one particle can be simultaneously in two different states, until a measurement defines the actual state. If multiple particles are in such superposition states, their spins can be coupled with each other such that the measurement of one particle changes the physical state of the other particles. This coupling effect is a fundamental part of quantum mechanics and is called entanglement. It is a central requirement for applications in the fields of quantum communication, quantum cryptography and quantum computing. One possibility to create entanglement are spin-changing collisions in a spinor Bose-Einstein condensate. These collisions are described by a nonlinear process, which creates non-classical correlations between all atoms of the Bose-Einstein condensate. Within this thesis, 87Rb spinor Bose-Einstein condensates are used to demonstrate Einstein-Podolsky-Rosen entanglement by the generation and analysis of a two-mode entangled state in spin space [A1]. These states are applied in an interferometric measurement with a resolution of 2.05+0.34 -0.37 dB beyond the standard quantum limit [A2]. Depending on the dynamics, the process can also be analogous to the dynamical Casimir effect [A3]. The entanglement between the particles in these publications is strongly connected with their indistinguishability. It is thus possible to make the particles distinguishable again and recover the entanglement between the now separated particles? This question is approached by cutting an entangled state of indistinguishable particles into two spatially separated atomic modes - creating a two-mode entangled state in real space. As shown in Ref. [A4], the entanglement between the two separated clouds of a Bose-Einstein condensate can be measured directly. In the future, the created state can be employed for a test of quantum nonlocality.
In this chapter, we will discuss the central topic in the current quantum optics studies, that is, entanglement. We focus mostly on the problem of creation of entanglement between two atoms and address the question how one could create multi-atom entangled states. The term entanglement, one of themost intriguing properties of multi-particle systems, was introduced by Schro¨dinger in his discussions of the foundations of quantum mechanics. It describes a multi-particle system that has the astonishing property that the results of a measurement on one particle cannot be specified independently of the results of measurements on the other particles. Although entangled systems can be physically separated, they can no longer be considered as independent, even when they are very far from one another. Entanglement is not only at the heart of the distinction between quantum and classical mechanics, but is now regarded to be a resource central to the development of quantum technologies ranging from quantum information, quantum cryptography, teleportation and quantum computation to atomic and molecular spectroscopy. These practical implementations all stem from the realization that we may control and manipulate quantum systems at the level of single atoms and photons to store information and transfer the information between distant systems in a controlled way and with high fidelity. This field of study is a relatively new one, and considerable research activity is taking place at present. The most active ones are the studies of practical schemes for creation of entanglement between trapped atoms or ions [91, 94].
The spin dynamics of two alkali-metal atoms in an optical tweezer is driven by spin-changing collisions that couple the spin state of the atoms to their relative motion. This paper experimentally studies the resulting spin states when the relative motion is in a thermal state with kBT much larger than the energies of the spin states that take part in the dynamics. We find that an initially unentangled spin state can evolve into an entangled state. This is contrary to the common case when coupling a quantum system to hot degrees of freedom leads to loss of entanglement and not its generation. Moreover, we show that the generated entanglement is technologically useful as it, in principle, can enhance the sensitivity of measurements beyond the standard quantum limit. This may provide a promising avenue for robust entanglement generation for future technologies.
Quantum technologies exploit entanglement to revolutionize computing, measurement, and communication. This has stimulated the research in different areas of physics to engineer and manipulate fragile many-particle entangled states. Progress has been particularly rapid for atoms. Thanks to the large and tunable nonlinearities and the well developed techniques for trapping, controlling and counting, many groundbreaking experiments have demonstrated the generation of entangled states of trapped ions, cold and ultracold gases of neutral atoms. Moreover, atoms can couple strongly to external forces and light fields, which makes them ideal for ultra-precise sensing and time keeping. All these factors call for generating non-classical atomic states designed for phase estimation in atomic clocks and atom interferometers, exploiting many-body entanglement to increase the sensitivity of precision measurements. The goal of this article is to review and illustrate successful theoretical proposals and experiments with atomic ensembles that have demonstrated many-particle entanglement and quantum-enhanced metrology.
One of the most intriguing features of quantum mechanics is the existence of entangled states. These states result from the superposition of individual quantum states in composite systems, such as a collection of particles, and cannot be expressed as a product of the individual states. The generation and manipulation of entangled states is fundamental to studies of the most basic aspects of quantum mechanics, and provides the basis of applications such as quantum computing, quantum communications and high-precision spectroscopy.
The spin-dynamics of two alkali atoms in an optical tweezer is driven by spin-changing collisions that couple the spin-state of the atoms to their relative motion. This paper experimentally studies the resulting spin-states when the relative motion is in a thermal state with k B T much larger than the energies of the spin-states that take part in the dynamics. We find that an initially unentangled spin-state can evolve into an entangled state. This is contrary to the common case when coupling a quantum system to hot degrees of freedom leads to loss of entanglement and not its generation. Moreover, we show that the generated entanglement is technologically useful as it, in principle, can enhance the sensitivity of measurements beyond the standard quantum limit. This may provide a promising avenue for robust entanglement generation for future technologies.