The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.
The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.
The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here the theoretical and experimental status quo of this very active field are reviewed. The potentials of different approaches are compared quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.
In the last few years there has been a lot of interest in quantum repeater protocols using only atomic ensembles and linear optics. Here we show that the local generation of high-fidelity entangled pairs of atomic excitations, in combination with the use of two-photon detections for long-distance entanglement generation, permits the implementation of a very attractive quantum repeater protocol. Such a repeater is robust with respect to phase fluctuations in the transmission channels, and at the same time achieves higher entanglement generation rates than other protocols using the same ingredients. We propose an efficient method of generating high-fidelity entangled pairs locally, based on the partial readout of the ensemble-based memories. We also discuss the experimental implementation of the proposed protocol.
Quantum repeaters hold the promise to prevent the photon losses in communication channels. Most recently, the serious efforts have been applied to achieve scalable distribution of entanglement over long distances. However, the probabilistic nature of entanglement generation and realistic quantum memory storage times make the implementation of quantum repeaters an outstanding experimental challenge. We propose a quantum repeater protocol based on the deterministic storage of a single photon in atomic ensembles confined in distant high-finesse cavities and show that this system is capable of distributing the entanglement over long distances with a much higher rate as compared to previous protocols, thereby alleviating the limitations on the quantum memory lifetime by several orders of magnitude. Our scheme is robust with respect to phase fluctuations in the quantum channel, while the fidelity imperfection is fixed and negligibly small at each step of entanglement swapping.
Abstract The distribution of quantum entanglement is essential for quantum communication. Contrary to classical communication, the signal in quantum communication is impossible to copy due to the basic principles of quantum mechanics, i.e. , quantum state collapse and no-cloning theorem. However, the communication range is limited by inevitable photon loss. To counteract this issue, one can insert several repeater stations between two remote end nodes to divide the full distance into several shorter segments and successively operate entanglement swapping between adjacent repeater stations to enable long-distance quantum communication. Up to now, one can classify quantum repeaters (QRs) into three generations based on the methods of reducing errors resulted from photon loss and imperfect operations. In this paper, we will briefly review advances for each generation of QRs and discuss the advantages and challenges of each generation. Moreover, we also discuss some efficient proposals of building blocks to overcome the bottlenecks for QRs.
Most quantum communication tasks need to rely on the transmission of quantum signals over long distances. Unfortunately, transmission of such signals is most often limited by losses in the channel, the same issue that affects classical communication. Simple signal amplification provides an elegant solution for the classical world, but this is not possible in the quantum world, as the no-cloning theorem forbids such an operation and, thus, an alternative approach, a quantum repeater, is needed. Quantum repeaters enable one to create a known maximally entangled state between the end points of the network by first segmenting the network into pieces, creating entanglement between the segments, and then, connecting those entanglement to create the required long range entanglement. Quantum teleportation then allows an unknown quantum message to be transmitted between them using the long-range entangled state. This form of quantum communication will be at the heart of the future quantum Internet. In this review, we will detail various approaches to quantum repeaters, and discuss their expected performance and limitations.
As children, whispering into the ear of a friend in the presence of others allows us to pass a secret without interception, and forms one of the simplest attempts at secret communications we can employ. However, sending secret messages becomes deeply nontrivial over long distances. \n \nA solution for two parties to communicate securely is to encrypt and decrypt a message with two identical strings of bits, one for each party. In this case, the security of the encrypted message is provable and does not rely on assumptions on computational power. Quantum theory provides a clear solution for the initial distribution of these identical bit strings through Quantum Key Distribution. However, once long distances are involved, the corresponding loss involved in direct transmission ruins the effectiveness of quantum key distribution by reducing the effective rate exponentially with the distance. \n \nTo circumvent the losses involved in direct transmission, quantum repeater architectures have been proposed. We present our contributions towards three aspects of quantum repeater systems in this thesis. We ensure conditions for implementing quantum repeaters with atomic ensembles, explore the option of optomechanical systems for implementing quantum repeaters and verify the success of completed quantum repeater protocols. \n \nIn the first part of this thesis, we show how we can ensure conditions for the successful implementation of quantum repeater systems with atomic ensembles. These quantum repeater systems are formed with 1-dimensional networks, where the nodes are made up of quantum memories connected by means of single photons. This requires memories that are highly efficient. Also, if quantum repeater systems are implemented with hybrid resources, tunable photon waveforms will be desirable. We propose a protocol to implement quantum memories with atomic ensembles using a clear recipe to optimise the efficiency. We also demonstrate that a cold ensemble of Rubidium-87 can act as an efficient tunable source of single photons, along with flexibility in the produced temporal shapes. \n \nNext, we show how we can explore alternative options for the nodes of quantum repeater systems. We focus on optomechanical oscillators, and recognise that they can also be used as quantum memories. We present a witness to certify that this memory successfully operates in the quantum regime. \n \nFinally, we focus on the verification of successfully implemented quantum repeater protocols. This verification will be essential for certifying that quantum repeater systems operate as instructed. We use only local homodyne measurements to witness the success of the network, and find that the witness is robust to loss. \n \nWe thus present distinct contributions towards three important aspects of quantum repeater systems. As far as a full-fledged quantum repeater system might seem to be right now, we have faith that our work brings the field of quantum-enabled secure communications forward.