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Experimental Quantum Memory Applications and Demonstration of an Elementary Quantum Repeater Link with Entangled Light-Matter Interfaces

M. Weber

Published May 31, 2012• 0 citations
Publisher

Abstract

Entanglement is the essential feature of quantum mechanics. Remarkably, observers of two or more entangled particles will find correlations in their measurement results, that can not be explained by classical statistics. To make it a useful resource, particularly for scalable long-distance quantum communication, the heralded generation of entanglement between distant massive quantum systems is necessary. We report on the creation and analysis of heralded entanglement between spins of two single Rb-87 atoms trapped independently 20 meters apart. Our results illustrate the viability of an integral resource for quantum information science, as well as for fundamental tests of quantum mechanics.

DOI
10.5281/zenodo.19742105

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Entanglement is the essential feature of quantum mechanics. Remarkably, observers of two or more entangled particles will find correlations in their measurement results, that can not be explained by classical statistics. To make it a useful resource, particularly for scalable long-distance quantum communication, the heralded generation of entanglement between distant massive quantum systems is necessary. We report on the creation and analysis of heralded entanglement between spins of two single Rb-87 atoms trapped independently 20 meters apart. Our results illustrate the viability of an integral resource for quantum information science, as well as for fundamental tests of quantum mechanics.
Entanglement is the essential feature of quantum mechanics. Notably, observers of two or more entangled particles will find correlations in their measurement results that cannot be explained by classical statistics. To make it a useful resource, particularly for scalable long-distance quantum communication, the heralded generation of entanglement between distant massive quantum systems is necessary. We report on the creation and analysis of heralded entanglement between spins of two single rubidium-87 atoms trapped independently 20 meters apart. Our results illustrate the viability of an integral resource for quantum information science, as well as for fundamental tests of quantum mechanics.
Entanglement plays a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, a major challenge is the efficient generation of entanglement between distant stationary (spin) qubits. Here, we report the realization of heralded quantum entanglement between two semiconductor quantum dot spins separated by more than five meters. Our results extend the previous demonstrations of distant spin entanglement in single trapped ions or neutral atoms, in atom ensembles and nitrogen vacancy centers to the domain of artificial atoms in semiconductor nanostructures that allow for onchip integration of electronic and photonic elements. Moreover the efficient spin-photon interface provided by self-assembled quantum dots[1] allows us to reach an unprecedented rate of 2300 entangled spin pairs per second, which represents an improvement of four orders of magnitude as compared to prior experiments[2]. The entanglement generation scheme relies on single photon interference of Raman scattered light from both dots[3]. A single photon detection projects the system into a maximally entangled state. We developed a delayed two-photon interference scheme that allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum repeaters and quantum networks on a chip. [1] W.-B. Gao et al., Nature 491, 426 (2012) [2] L. Slodicka et al., Phys. Rev. Lett. 110, 083603 (2013) [3] C. Cabrillo et al. Phys. Rev. A 59, 1025 (1999) -1 0 1 2 3 4 5 6 7 8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 data sine fit ra ti o pulse duration (arb. u.) F x = 29.8 ± 2.6 % 180 min 0.13 /s 0 19 37 56 74 93 112 0.0 0.1 0.2 0.3 0.4 0.5 0.6 total fidelity = 55.2 ± 3.5 % F Z = 80.6 ± 6.6 % c o in c id e n c e s / 1 0 6 .5 h measurement coincidences ideal case
Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing. Remarkably, it is possible to generate entanglement even in the absence of direct interactions: provided that which path information is erased, weak spin-state dependent light scattering can be used to project two distant spins onto a maximally entangled state upon detection of a single photon. Even though this approach is necessarily probabilistic, successful generation of entanglement is heralded by the photon detection event. Here, we demonstrate heralded quantum entanglement of two quantum dot heavy-hole spins separated by 5 meters using single-photon interference. Thanks to the long coherence time of hole spins and the efficient spin-photon interface provided by self-assembled quantum dots embedded in leaky microcavity structures, we generate 2300 entangled spin pairs per second, which represents an improvement approaching three orders of magnitude as compared to prior experiments. Delayed two-photon interference scheme we developed allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum networks in semiconductor nanostructures. Combined with schemes for transferring quantum information to a long-lived memory qubit, fast entanglement generation we demonstrate could also impact quantum repeater architectures.
Quantum correlations arising from measurements on pairs of entangled particles are often referred to as the most non-classical feature of quantum mechanics. As such, they have found a role in many fundamental tests of quantum mechanics and most emerging applications in the field of quantum communication. The most well-known fundamental test is the Bell inequality. Its repeated violation in countless experiments has convinced physicists that local hidden variables cannot describe the correlations arising from measurements on entangled particles. On the application side, quantum correlations have been used in quantum key distribution, which aims to provably secure messages during transmission, and quantum repeaters, which are essential for future long-distance quantum communication. The main goal of this thesis was to use quantum entanglement for new fundamental studies and quantum communication applications. Towards this end, we developed a high-fidelity source of entanglement and used it in a fundamental test where we bounded the predictive power any physical theory could have about the outcomes of measurements on entangled particles. Secondly, we made use of entangling measurements to develop a new quantum cryptography system based on the promising MDI-QKD protocol, which protects users from otherwise undetectable hacking attacks. We developed a detailed model of MDI-QKD systems with which one can optimize any implementation, deployed our MDI-QKD system across the city of Calgary, and demonstrated the feasibility of this protocol and of long-distance entangling measurements. Thirdly, we have built a source of entanglement compatible with quantum memories, which is an essential ingredient of quantum repeaters. We then demonstrated several crucial steps towards a functioning quantum repeater, including the preservation of entanglement during storage and, more generally, the entire photonic wavefunction. While further work is required to bring our demonstrations to real-world applications, we are confident that they will prove useful in guiding future developments.
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.