One of the main ingredients in most quantum information protocols is a reliable source of two entangled systems. Such systems have been generated experimentally several years ago for light but has only in the past few years been demonstrated for atomic systems. None of these approaches however involve two atomic systems situated in separate environments. This is necessary for the creation of entanglement over arbitrary distances which is required for many quantum information protocols such as atomic teleportation. We present an experimental realization of such distant entanglement based on an adaptation of the entanglement of macroscopic gas samples containing about 10^11 cesium atoms shown previously by our group. The entanglement is generated via the off-resonant Kerr interaction between the atomic samples and a pulse of light. The achieved entanglement distance is 0.35m but can be scaled arbitrarily. The feasibility of an implementation of various quantum information protocols using macroscopic samples of atoms has therefore been greatly increased. We also present a theoretical modeling in terms of canonical position and momentum operators X and P describing the entanglement generation and verification in presence of decoherence mechanisms.
One of the main ingredients in most quantum information protocols is a reliable source of two entangled systems. Such systems have been generated experimentally several years ago for light but has only in the past few years been demonstrated for atomic systems. None of these approaches however involve two atomic systems situated in separate environments. This is necessary for the creation of entanglement over arbitrary distances which is required for many quantum information protocols such as atomic teleportation. We present an experimental realization of such distant entanglement based on an adaptation of the entanglement of macroscopic gas samples containing about 10^11 cesium atoms shown previously by our group. The entanglement is generated via the off-resonant Kerr interaction between the atomic samples and a pulse of light. The achieved entanglement distance is 0.35m but can be scaled arbitrarily. The feasibility of an implementation of various quantum information protocols using macroscopic samples of atoms has therefore been greatly increased. We also present a theoretical modeling in terms of canonical position and momentum operators X and P describing the entanglement generation and verification in presence of decoherence mechanisms.
Atomic ensembles containing a large number of atoms have been proved to be an effective medium for quantum-state (quantum information) engineering and processing via their coupling with multi-photon light pulses. The general mechanism of this coupling, which involves continuous quantum variables for light and atoms, is described. The efficient quantum interface between light and atoms has led to the recent demonstration of an entangled state of two macroscopic atomic objects, more precisely two caesium gas samples. Based on this result, a proposal for teleportation of an entangled state of two atomic samples (entanglement swapping) is presented.
Abstract Entanglement is considered to be one of the most profound features of quantum mechanics1, 2. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems3, 4, 5, 6. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 1012 atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples7. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation8, 9, 10 of quantum states of matter and quantum memory.
Entanglement between two macroscopic atomic ensembles induced by measurement on an ancillary light system has proven to be a powerful method for engineering quantum memories and quantum state transfer. Here we investigate the feasibility of such methods for generation, manipulation, and detection of genuine multipartite entanglement (Greenberger-Horne-Zeilinger and clusterlike states) between mesoscopic atomic ensembles without the need of individual addressing of the samples. Our results extend in a nontrivial way the Einstein-Podolsky-Rosen entanglement between two macroscopic gas samples reported experimentally in [B. Julsgaard, A. Kozhekin, and E. Polzik, Nature (London) 413, 400 (2001)]. We find that under realistic conditions, a second orthogonal light pulse interacting with the atomic samples, can modify and even reverse the entangling action of the first one leaving the samples in a separable state.
Entanglement, or quantum inseparability, is a crucial resource in quantum information applications, and therefore the experimental generation of separated yet entangled systems is of paramount importance. Experimental demonstrations of inseparability with light are not uncommon, but such demonstrations in physically well-separated massive systems, such as distinct gases of atoms, are new and present significant challenges and opportunities. Rigorous theoretical criteria are needed for demonstrating that given data are sufficient to confirm entanglement. Such criteria for experimental data have been derived for the case of continuous-variable systems obeying the Heisenberg-Weyl (position-momentum) commutator. To address the question of experimental verification more generally, we develop a sufficiency criterion for arbitrary states of two arbitrary systems. When applied to the recent study by Julsgaard, Kozhekin, and Polzik [Nature 413, 400 (2001)] of spin-state entanglement of two separate, macroscopic samples of atoms, our criterion confirms the presence of spin entanglement.
Since the development of quantum mechanics almost a century ago, there has been considerable controversy over the interpretations and predictions of quantum theory. Owing to the counterintuitive predictions of quantum mechanics, Einstein himself even wondered if this theory should be considered complete. While these questions have troubled many physicists over the past century, the development of the new field of quantum information science and the applications that may result from large scale quantum systems have brought many of these fundamental questions of quantum mechanics to the mainstream of not only theoretical but also experimental physics. This thesis deals with a system at the heart of these questions - the first demonstration of quantum entanglement of two individual massive particles at a distance. I describe a theoretical and experimental framework for entanglement of two particles using trapped atomic ions. Trapped ions are among the most attractive systems for scalable quantum information protocols because they can be well isolated from the environment and manipulated easily with lasers. Using our trapped ion system, I show the first explicit demonstration of quantum entanglement between matter and light using a single ion and its single emitted photon. Further, by combining two such ion-photon entangled systems, I demonstrate the entanglement of two remotely located ions. These entanglement protocols, together with the recent developments of trapped ion quantum computing, can be used to create a platform for a scalable quantum information network, and perhaps confront some of the strangest features of quantum mechanics.
Quantum teleportation is one of the most enigmatic protocols realized so far in the area of quantum information, allowing for the transfer of quantum information assisted by an entangled state. It was a subject of science fiction 20 years ago, but now is a fundamental experimental protocol and quantum primitive for quantum information processing tasks. There have been many successful realizations of teleportation ranging from those using photons, atoms, and hybrid systems and long-range teleportation beyond 100km has already been achieved multiple times. These however typically involve explicitly single particle systems or effective few-particle systems. For larger (visible to the naked eye) objects involving many particles, teleportation is typically more difficult due to the fast decoherence such macroscopic objects suffer. In this case, the entanglement disappears almost as soon as it is created, making it useless for such tasks. More recently, teleportation was accomplished in the continuous variables (CV) framework between two atomic ensembles [1]. As remarkable as these achievements are, teleportation schemes that are known today (qubit/CV) do not allow us to teleport more complex quantum states at the macroscopic level. For example, while it is possible in principle to extend teleportation to multiple qubits, on a macroscopic level this is prohibitively complicated and highly susceptible to decoherence. For atomic ensemble CV based schemes, the teleported states are restricted to small displacements from the completely Sx-polarized state on the Bloch sphere.