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The solid-state quantum repeater

C. Y. Hu, John G. Rarity

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

Over the last decade, the record distance for quantum communications remains in the range of 100 km due to losses and decoherence in the quantum channel. To achieve long distance, H.J. Briegel et al proposed a quantum repeater protocol, which combine quantum memory, entanglement purification and entanglement swapping [1]. Due to the lack of perfect quantum memory, the quantum repeater has not yet been implemented experimentally though much efforts have been made over many years. Here we present a solid-state repeater scheme using a quantum-dot spin in an optical microcavity [2,3]. This repeater has several advantages: (1) deterministic (if optimized); (2) complete and loss-resistant Bell-state analyzer with a built-in spin memory; (3) the storage time for spin memory could be significantly prolonged using proposed spin-echo techniques; (4) compatible with standard semiconductor processing techniques. [1] H. -J. Briegel et al, Phys. Rev. Lett. 81, 5932 (1998). [2] C.Y. Hu et al, Phys. Rev. B, 78, 085307 (2008); 78, 125316 (2008); 80, 205326 (2009). [3] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [4] S. Reitzenstein et al, Appl. Phys. Lett. 90, 251109 (2007).

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Over the last decade, the record distance for quantum communications remains in the range of 100 km due to losses and decoherence in the quantum channel. To achieve long distance, H.J. Briegel et al proposed a quantum repeater protocol, which combine quantum memory, entanglement purification and entanglement swapping [1]. Due to the lack of perfect quantum memory, the quantum repeater has not yet been implemented experimentally though much efforts have been made over many years. Here we present a solid-state repeater scheme using a quantum-dot spin in an optical microcavity [2,3]. This repeater has several advantages: (1) deterministic (if optimized); (2) complete and loss-resistant Bell-state analyzer with a built-in spin memory; (3) the storage time for spin memory could be significantly prolonged using proposed spin-echo techniques; (4) compatible with standard semiconductor processing techniques. [1] H. -J. Briegel et al, Phys. Rev. Lett. 81, 5932 (1998). [2] C.Y. Hu et al, Phys. Rev. B, 78, 085307 (2008); 78, 125316 (2008); 80, 205326 (2009). [3] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [4] S. Reitzenstein et al, Appl. Phys. Lett. 90, 251109 (2007).
A fascinating dream in quantum information science is to build quantum computers with super-performance and quantum networks with unconditional security [1]. Quantum networks utilize static (matter) quantum bits (qubits) to store and process quantum information at local nodes, and photons as flying qubits for long-distance quantum state transmission between different nodes. To realize a quantum network, it is crucial to achieve light-matter entanglement and reversible quantum-state transfer between light and matter, i.e., the light-matter quantum interface, and the quantum repeater for large-scale quantum communications [2]. Recent experiments have shown that both electrons and holes confined in semiconductor quantum dots (QDs) have long spin relaxation time (T1e, T1h ~ ms) and long spin coherence time (T2e ~ microseconds, T2h > 100 ns). Moreover, fast spin cooling and ultra-fast spin manipulation as well as spin echoes to preserve the spin coherence have also been demonstrated. Undoubtedly these rapid progresses imply that the QD spin is a good candidate for matter qubit in quantum information processing. Furthermore, QD-based single photon sources have been also developed. Therefore, semiconductor QDs offer a good platform for solid-state quantum networking and quantum computation. Here we present two types of conditional quantum gates, i.e., the photon-spin entangling gates [3-4] using a single QD spin in a single-sided or double-sided optical microcavity. Both gates are universal and deterministic (if they are optimized). The spin-selective coherent photon-spin interaction enhanced by the cavity QED lead to giant circular birefringence, which allows us to build these gates. We will show the versatile spin-cavity systems can be applied in all aspects of quantum information science and technology, not only for large-scale quantum communication networks (solid-state quantum repeaters) [5], but also for scalable quantum computing with either photons or spins as qubits. We will also discuss other applications using these gates, such as photon-number resolving detection, quantum feed-back control, loss-resistant quantum metrology, and loophole-free Bell test [6], and other devices based on optical nonlinearity at single-photon levels in these gate structures. All these schemes are feasible with current semiconductor technology [7], and we have recently seen conditional phase shifts in uncharged QD-cavity systems [8]. References [1] H.J. Kimble, Nature (London) 453, 1023 (2008). [2] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998). [3] C.Y. Hu et al., Phys. Rev. B 78, 085307 (2008); ibid. 78, 125318 (2008). [4] C.Y. Hu et al., Phys. Rev. B 80, 205326 (2009). [5] C.Y. Hu and J.G. Rarity, Phys. Rev. B 83, 115303 (2011). [6] N. Brunner et al., Arxiv: quant-phys 1303.6522 (2013). [7] S. Reitzenstein et al., Appl. Phys. Lett. 90, 251109 (2007). [8] A.B. Young et al., Phys. Rev. A 84, 011803 (2011).
As the reach of quantum technologies extends ever further in communication and information science, a reliable way of transferring quantum information between distant locations becomes ever more crucial. While photons are widely accepted as excellent carriers due to their speed and low decoherence, losses of transmission (in free space or fibre) and the impossibility of cloning quantum information still pose a great challenge. The quantum repeater architecture was suggested as a solution to both problems [1]. In a quantum repeater the information encoded in an input state is transferred to a new one through entanglement swapping, that is then sent on along the channel. In this work we present our advances towards the realisation of a quantum repeater. Our system of choice combines a solid-state quantum memory with a source of photon pairs. The memory is based on a Rare-Earth Doped crystal, where quantum information can be stored in Pr3+ ions as a collective excitation using the Atomic Frequency Comb technique. On demand retrieval of the information is realised by transferring the excitation to a long-lived spin state. Record values of storage times and retrieval efficiencies have been demonstrated in this system [2]. Entangled pairs of single photons are generated by parametric down conversion in a periodically poled crystal placed inside a bow-tie cavity. This allows us to generate narrow band photons pairs, where the signal is spectrally matched to the memory (606nm), while the idler is in the telecom band [3]. Such a configuration allows us to benefit from the high performance of the memory, that also allows for temporal [2] and frequency [4] multimodality, while at the same time overcoming the high optical losses of 606nm photons by pair generation of a telecom photon. The first stepping stone, progress towards which is presented in this work, is the successful demonstration of energy-time entanglement between the telecom idler photon and the signal photon, stored as spin-wave excitation. The entanglement of the original pair is maintained by the memory temporal multimodality. The entanglement analysis will be made through time-bin qubit analysers made of a fibre-based Mach-Zehnder interferometer, for the former, and a solid-state equivalent based on two AFC with different storage times, for the latter [5]. In this direction we have already doubled the efficiency of the AFC storage protocols, that will be beneficial to count rates and signal-to-noise ratio. With respect to [2], we also increased the spectral-matching between the source and the memory [4]. Our experiment will provide an increase in storage time of 3 orders of magnitude with respect to previous demonstrations, as well as introducing for the first time on-demand read-out in a highly multi-mode memory. Demonstration of the successful transfer of quantum information between the signal photon and the long-lived solid-state excitation will open the way to the demonstration of long-distance entanglement between individual nodes in a quantum network.
The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as among one of the most promising solutions. In this work, we focus on memory-based quantum-repeater schemes that rely on semiconductor quantum dots for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural-artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with quantum dots and highlight the next challenges ahead for the implementation of practical quantum networks.
Due to the inevitable loss in communication channels, the distance of entanglement distribution is limited to approximately 100 km on the ground. Quantum repeater can circumvent this problem by utilizing quantum memory and entanglement swapping. As the elementary functional nodes for quantum repeater, the heralded generation of two-party entanglement between two remote nodes has only been realized with built-in-type quantum memories. These schemes suffer from the trade-off between multiplexing capacity and deterministic property and hence hinder their way to efficient quantum repeaters. Here we present the first experimental demonstration of functional quantum repeater nodes using absorptive quantum memories. We build two nodes separated by 3.5 m, each contains a polarization-entangled photon-pair source and a solid-state quantum memory. A joint Bell-state measurement in the middle station heralds the successful generation of maximally-entangled states between the two quantum memories with a fidelity of (80.4$\pm$2.1)%. The quantum memories used here are compatible with deterministic entanglement sources and can support multiplexing simultaneously, which paves the way to the construction of solid-state quantum repeaters and high-speed quantum networks.