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Telecom-wavelength quantum memories in rare earth ion-doped materials for quantum repeaters

Mohsen Falamarzi Askarani

Published Nov 5, 2019• 0 citations
Publisher

Abstract

The quantum internet, when finally deployed, will enable a plethora of new applications such as theoretically proven secure communication and networked quantum computing, much the same as its classical counterpart whose development began back in the 1990’s. The task of creating a globe-spanning quantum network, however, is proving a rather difficult task due to the detrimental effect of loss on photon transmission. This problem can in theory be solved using so-called quantum repeaters. In one of the most well-known configurations, the long-distance span is divided into smaller segments—so called elementary links—at whose ends pairs of entangled photons are generated. One photon per pair is stored in a quantum memory, and the second member is transmitted via optical fiber to a remote measurement stations positioned at the centre of the segment. There, a joint measurement of the two photons, one from each end, then heralds the distribution of entanglement between the two quantum memories, i.e. heralds entanglement over the elementary link. In the final step, all neighbouring links are combined via a second joint measurement, and end-to-end entanglement is created...

DOI
10.4233/uuid:d418a98b-f2aa-4af3-b0e0-864875fcad2b

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The quantum internet, when finally deployed, will enable a plethora of new applications such as theoretically proven secure communication and networked quantum computing, much the same as its classical counterpart whose development began back in the 1990’s. The task of creating a globe-spanning quantum network, however, is proving a rather difficult task due to the detrimental effect of loss on photon transmission. This problem can in theory be solved using so-called quantum repeaters. In one of the most well-known configurations, the long-distance span is divided into smaller segments—so called elementary links—at whose ends pairs of entangled photons are generated. One photon per pair is stored in a quantum memory, and the second member is transmitted via optical fiber to a remote measurement stations positioned at the centre of the segment. There, a joint measurement of the two photons, one from each end, then heralds the distribution of entanglement between the two quantum memories, i.e. heralds entanglement over the elementary link. In the final step, all neighbouring links are combined via a second joint measurement, and end-to-end entanglement is created...
Due to the inevitable loss of single photon transmission in optical fiber, a quantum repeater scheme is needed to establish large-scale quantum networks. The existing elementary quantum repeater links are all based on emissive quantum memories, with entangled photons emitted by the memory itself. This architecture is difficult to support deterministic photon emission and multiplexing storage simultaneously, which fundamentally limits the rate of entanglement distribution. In this talk, I will present our recent work about the realization of heralded quantum entanglement between two absorptive quantum memories based on rare-earth-ion-doped crystals. This work confirmed the feasibility of constructing quantum repeater based on absorptive quantum memories and demonstrated the acceleration effect of multiplexing in quantum repeater for the first time, which lays a solid foundation for the construction of practical high-speed quantum networks.
Quantum networks of the future will be required to span the globe and interconnect many different real world systems employed by different commercial, academic, and government users. Thus, an important measure of such a network's capacity will be its ability to interact with and distribute entanglement among different mediums and devices with potentially disparate properties. Few experiments have to date shown this sort of interconnectivity between different systems with different atomic species. Here I will present our demonstration of a system in which we store two correlated photons in solid state quantum memories implemented in different rare earth ion-doped materials using the atomic frequency comb protocol. The photon pairs are generated through a spontaneous parametric down-conversion process with their wavelengths chosen to be compatible with quantum repeater implementations. One member, with a wavelength at 795 nm, is stored in an Tm:LiNbO3 crystal while, simultaneously, the other member, generated at 1535 nm wavelength, is stored in an amorphous erbium doped optical fiber. The measured cross-correlation function between the recalled photons is g(2) = 16 +/- 2, which verifies that the non-classical correlations are preserved and were present during the storage of both photons. The two storage mediums are diverse not only in terms of their different photon acceptance wavelengths, but also with regards to the material properties as hosts for the dopant rare-earth ions. Our results show that quantum nodes and networks based on fundamentally different systems can be interfaced to distribute entanglement.
In this talk, we report on recent progress towards the realization of quantum repeaters based on rare-earth doped crystal quantum memories (QM) and cavity-enhanced SPDC photon-pair sources. We first present our demonstration of on-demand telecom-heralded entanglement between multimode solid-state quantum memories with high rate and fully adjustable recall time. Then, we show how we successfully established long-distance non-classical correlations between a telecom photon and a QM in a field-deployed experiment in the city of Barcelona.
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.
A quantum repeater node is presented based on trapped ions that act as single-photon emitters, quantum memories, and an elementary quantum processor. The node's ability to establish entanglement across two 25-km-long optical fibers independently, then to swap that entanglement efficiently to extend it over both fibers, is demonstrated. The resultant entanglement is established between telecom-wavelength photons at either end of the 50 km channel. Finally, the system improvements to allow for repeater-node chains to establish stored entanglement over 800 km at hertz rates are calculated, revealing a near-term path to distributed networks of entangled sensors, atomic clocks, and quantum processors.
The ultimate realization of a global quantum internet will require advances in scalable technologies capable of generating, storing, and manipulating quantum information. The essential devices that will perform these tasks in a quantum network are quantum repeaters, which will enable the long-range distribution of entanglement between distant network nodes. In this review, we provide an overview of the primary functions of a quantum repeater and discuss progress that has been made toward the development of repeaters with rare-earth ion doped materials while noting challenges that are being faced as the technologies mature. We give particular attention to erbium, which is well suited for networking applications. Finally, we provide a discussion of near-term benchmarks that can further guide rare-earth ion platforms for impact in near-term quantum networks.