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Quantum Information and Measurement (QIM)

Certification and quantification of the entanglement stored in a quantum memory using incomplete data

Alexey Tiranov, Marcus Huber, Jonathan Lavoie, Peter C. Strassmann, Emmanuel Zambrini Cruzeiro, Sébastien Designolle, Nicolas Brunner, Félix Bussières, Mikael Afzelius, Nicolas Gisin

Published Jan 1, 2017• 0 citations
Publisher

Abstract

Entanglement is a main resource for various applications in quantum information processing and quantum communication. Increasing complexity of entanglement structure paves the way for both deeper fundamental tests of nature and utterly new methods for quantum information tasks. Quantum memory which is capable to store complex entanglement is essential tool for this direction since it allows to use increased quantum information capacity. Particularly, it is necessary to realize quantum repeater scheme to perform long-distance quantum communication [1]. In this context, quantum memories with high temporal multimode capacity offer a tool to realize efficient multiplexing and open a way for various applications in quantum communication studies.

DOI
10.1364/qim.2017.qw2b.2

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Entanglement is a main resource for various applications in quantum information processing and quantum communication. Increasing complexity of entanglement structure paves the way for both deeper fundamental tests of nature and utterly new methods for quantum information tasks. Quantum memory which is capable to store complex entanglement is essential tool for this direction since it allows to use increased quantum information capacity. Particularly, it is necessary to realize quantum repeater scheme to perform long-distance quantum communication [1]. In this context, quantum memories with high temporal multimode capacity offer a tool to realize efficient multiplexing and open a way for various applications in quantum communication studies.
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.
The use of multidimensional entanglement opens new perspectives for quantum information processing. However, an important challenge in practice is to certify and characterize multidimensional entanglement from measurement data that are typically limited. Here, we report the certification and quantification of two-photon multidimensional energy-time entanglement between many temporal modes, after one photon has been stored in a crystal. We develop a method for entanglement quantification which makes use of only sparse data obtained with limited resources. This allows us to efficiently certify an entanglement of formation of 1.18 ebits after performing quantum storage. The theoretical methods we develop can be readily extended to a wide range of experimental platforms, while our experimental results demonstrate the suitability of energy-time multidimensional entanglement for a quantum repeater architecture.
Single photons are the flying qubits of choice for distributing entanglement in a quantum internet. Quantum memories embedded in quantum repeaters are crucial to overcome transmission loss and enhance the rate of quantum communication. A multimode memory can further boost the channel capacity. However, benchmarking and building a practical quantum memory that simultaneously optimizes multiple performance metrics poses two key challenges. Here, we introduce quantum interconnect rate to comprehensively quantify quantum memories, and further demonstrate a high-performance quantum memory that simultaneously integrates three essential criteria at once: large multimode capacity, high efficiency, and high fidelity. Operating on 11-dimensional spatial modes, our memory achieves a uniform efficiency exceeding 80% and qubit storage fidelities above 99%, enabling the efficient storage of high-dimensional qudits. Based on these capabilities, we estimate a distribution of 3.56 bits of quantum information over a 1000-km repeater link in one minute, highlighting a practical pathway toward scalable quantum interconnects and quantum networks.
Quantum communication between two remote locations often involves remote parties sharing an entangled quantum state. At present, entanglement distribution is usually performed using photons transmitted through optical fibers. However, the absorption of light in the fiber limits the communication distances to less than 200 km, even for optimal photon telecom wavelengths. To increase this distance, the quantum repeater idea was proposed. In the quantum repeater architecture, one divides communication distance into segments of the order of the attenuation length of the photons and places quantum memory nodes at the intermediate locations. Since the photon loss between intermediate locations is low, it is possible then to establish entanglement between intermediate quantum memory nodes. Once entanglement between adjacent nodes is established, one can extend it over larger distances using entanglement swapping.\n\nThe long coherence time of a quantum memory is a crucial requirement for the quantum repeater protocol. It is obvious that the coherence time of a quantum memory should be much longer that the time it takes for light to travel between remote locations. For a communication distance l = 1000 km, the corresponding time is t = l/c = 3.3 ms. One can show that for a simple repeater protocol and realistic success probabilities of entanglement generation, the required coherence time should be on the order of many seconds, while for the more complicated protocols that involve multiplexing and several quantum memory cells per intermediate node, the required coherence time is on the order of milliseconds.\n\nIn this thesis, I describe a quantum memory based on an ensemble of rubidium atoms confined in a one-dimensional optical lattice. The use of the magnetically- insensitive clock transition and suppression of atomic motion allows us to increase coherence time of the quantum memory by two-orders of magnitude compared to previous work. I also propose a method for determining the Zeeman content of atomic samples. In addition, I demonstrate the observation of quantum evolution under continuous measurement. The long quantum memory lifetime demonstrated in this work opens the way for scalable processing of quantum information and long distance quantum communication.
The transfer of quantum states of light and their mapping into stationary quantum nodes is one of the building blocks for future quantum networks and envisionned applications [1] . Central to this endeavor is the distribution of entanglement between the material nodes, which opens a variety of major applications [2] . For instance, to realize long-distance quantum communications one needs to divide the distance into shorter quantum repeater links connecting entangled memories. Subsequent entanglement swapping operations enable an exponential improvement in distribution time. In this context, the efficiency of the entanglement mapping is a key parameter.