Session Q03: Focus Session: Quantum Networks: Prospects and Challenges

Towards Realizing a Quantum Repeater based on a Spin-Photon Quantum Interface

In this talk, I will review recent experimental progresses in the development of a spin-photon quantum interface for the realization of a quantum repeater, including results from my previous group [1, 2]. I will discuss two different schemes that can be employed to realize a quantum repeater based on such a spin-photon quantum interface, one based on entanglement swapping and one based on photonic cluster states (also referred to as the one-way quantum repeater). I will highlight a recent proposal from our group on deterministic generation of photonic tree cluster states and repeater graph states by using only a single spin-tagged quantum emitter without any ancillary quantum memories [3], and discuss our vision in its experimental realizations.

[1] S. Sun et al., Nature Nanotechnology 11, 539–544 (2016)

[2] S. Sun et al., Science 361 (6397), 57-60 (2018)

[3] Y. Zhan and S. Sun, Physical Review Letters 125, 223601 (2020)   

Tools for designing quantum networks

Recent experimental progress has enabled demonstrations of quantum networks. However, many challenges remain to scale these first demonstrations. While quantum networks share similar high-level challenges with their classical counterparts, noticeable differences render many classical approaches of little use, e.g., the unclonability of quantum information or the need to synchronize the parties sharing an entangled state. Quantum networks require novel strategies to enable long-distance communications as well as tools to quantify achievable rates and place requirements on the quantum hardware. In this seminar, I will introduce recent tools for designing quantum networks and evaluating the feasibility of applications such as quantum key distribution.   

Determining optimal policies for quantum networks – A Reinforcement Learning approach

Near-term implementation of quantum networks must overcome current hardware limitations such as link losses, non-ideal measurements, and low decoherence-time quantum memories. In this context, it has been shown that optimizing figures of merit such as average connection time, largest connected cluster size, and average network fidelity, can be formulated as a decision process. An optimal protocol or policy (series of decisions) can therefore be determined using reinforcement learning (RL), which is well suited to handle decision processes. In the present work, we simulate a near-term quantum network based on entanglement distribution between nodes and optimize the figures of merit using RL. More specifically, we use a model-independent algorithm called Q-learning. Nodes act as agents and perform actions to establish entanglement with other nodes. Nodes can collaborate to achieve optimal values for the figures of merit. Previous theoretical work has suggested that policies must depend on hardware limitations. We parametrize these limitations in a platform-independent way and show that optimal policies that Q-learning yields depend on these parameters. We then compare our findings with the limitations posed by the theory. We also study the role of local versus global collaboration between nodes, and how this effects the policies and network outcomes. To explore the applicability of the RL framework for policy optimization, we implement different network architectures and multipartite entanglement generation between nodes.   

Long-lived entanglement memory in a trapped-ion quantum network node

Trapped ions that are connected over a photonic network can be useful for quantum computing, cryptography and metrology. Mapping the remote entanglement to a long-lived memory qubit is a prerequisite for entanglement distillation, interactive client-server protocols, and entanglement-enhanced remote sensing applications.

Here, we present progress towards extending ion-photon entanglement to a memory qubit with a coherence time of >1s. As described in previous works [1], the electronic state of ⁸⁸Sr⁺ is entangled with the polarisation of a spontaneously emitted photon. The qubit decoheres predominantly due to magnetic field noise, limiting the coherence time to <1ms. Using local mixed-species entangling operations we are able to map the state onto a long-lived magnetic field insensitive "clock" qubit in co-trapped ⁴³Ca⁺. We report the fidelity of the mixed-species iswap operation and characterise the joint ⁴³Ca⁺-photon state using tomography after a free evolution time. Compared to ⁸⁸Sr⁺, we show an enhancement of the entangled qubit memory lifetime. [1] PRL 124, 110501 (2020)   

Routing algebras for quantum network

 The practical implementation of quantum communication relies on having efficient quantum networks and robust communication algorithms. In order to overcome the distance limitations imposed by entanglement decay, networks of quantum repeaters are required to extend the range of short-range entangled states through entanglement swapping and to increase the fidelity of the distributed entangled state through entannglement distillation. An important challenge is to find the paths through a complex quantum network that optimize the entanglement distribution rate between any two arbitrary nodes. We introduce an extension to the standard formulation of routing algebras, and we use this to develop an algorithm to find the optimal path when considering quantum networks which allow both entanglement swapping and entanglement distillation as their fundamental operations.   

Nondestructive detection of photonic qubits

Qubits encoded in single photons are very useful to distribute quantum information over remote locations, but at the same time are also very fragile objects. The loss of photonic qubits (through absorption, diffraction or scattering) is actually the main limitation in the maximum reachable quantum communication distance. In this context, the nondestructive detection of photonic qubits is a great scientific challenge that can help tracking the qubit transmission and mitigate the loss problem. Such a detector is envisioned to improve loss-sensitive qubit measurements [1], facilitate protocols in which distributed tasks depend on the successful dissemination of photonic qubits [2], and also enable certain quantum key distribution attacks [3]. We recently implemented such a detector [4] with a single atom coupled to two crossed fiber-based optical resonators, one for qubit-insensitive atom–photon coupling and the other for atomic-state detection. We achieve a nondestructive detection efficiency of 79(3) % conditioned on the survival of the photonic qubit, a photon survival probability of 31(1) %, and we preserve the qubit information with a fidelity of 96.2(0.3) %. To illustrate the potential of our detector we show that it can provide an advantage for long-distance entanglement and quantum-state distribution, resource optimization via qubit amplification, and detection-loophole-free Bell tests.

 

[1] N. Gisin et al., Phys. Rev. Lett. 105, 070501 (2010)

[2] K. Boone et al., Phys. Rev. A 91, 052325 (2015)

[3] G. Brassard et al., Phys. Rev. Lett. 85, 1330–1333 (2000)

[4] D. Niemietz et al., Nature 591, 570–574 (2021)