A Hybrid Atom-Photon-Superconductor Quantum Interface

量子信息领域的兴起是因为人们希望通过利用量子力学提供高效和可扩展的算法来克服在经典计算机上解决复杂或棘手问题的挑战。虽然最近在实现由几个量子比特组成的小规模量子电路方面取得了巨大的进展，但研究表明，能够利用量子力学的力量的容错量子计算机将需要数千个量子比特的网络。混合量子信息处理是一种利用不同量子技术的独特优势的替代方法，它提供了一条克服单一量子位体系结构的缺陷的途径，直接类比于经典计算硬件的设计。这项提议旨在结合三种不同的技术：i)超导电路，具有非常快的栅极时间(10 Ns)用于快速处理；ii)中性原子，具有长时间(10 S)相干时间用于长寿命量子存储器；iii)光光子，用于长距离光纤通信，创建一种新型的混合量子接口，能够存储、处理和产生用于量子网络和密码应用的高度纠缠态的光子，克服可扩展超导电路系统的短相干时间。这也为量子计量学提供了从光学领域到微波域量子信息的转换应用，使其有可能扩展接口以包括广泛的替代的基于固态的量子比特。该接口依赖于使用高激发的里德堡态，这种状态在微波区域具有令人难以置信的大的偶极矩和跃迁，可以与嵌入在平面微波波导腔中的超导量子比特共振耦合。大的里德堡偶极子还导致原子之间强烈的、可控的相互作用，以提供对单个光子的集体增强耦合，从而有效地存储和恢复光。实验的第一阶段是在4K下工作的超导微波谐振器上方捕获可空间寻址的原子系综，以展示与波导模的强耦合，这是实现混合界面的关键里程碑。然后，这些系综将被用来执行光学光子的相干存储和恢复，以及使用四波混频产生单光子。第二个阶段是利用与腔体的非共振相互作用，在单个微波谐振器中捕获的一对系综之间实现可控的长距离(~1 cm)纠缠。然后，这将被用来产生纠缠光子对，探索在系综内集体编码的好处，以实现长距离密码量子密钥分发的偏振自由度的纠缠。由此产生的混合量子接口为建立量子网络提供了理想的构建块。从长远来看，这可以与现有的超导量子比特技术相结合，朝着实现可扩展的量子计算迈出重要的一步。

The field of quantum information arises from a desire to overcome the challenges of solving complex or intractable problems on classical computers by harnessing quantum mechanics to provide efficient and scalable algorithms. Whilst there has been tremendous recent progress in the realisation of small-scale quantum circuits comprising several quantum bits (``qubits''), research indicates that a fault-tolerant quantum computer capable of harnessing the power of quantum mechanics will require a network of thousands of qubits. This goal is presently beyond the reach of any existing implementation based on a single physical qubit type.Hybrid quantum information processing is an alternative approach that exploits the unique strengths of disparate quantum technologies, and offers a route to overcome the drawbacks associated with of a single-qubit architecture in direct analogy to the design of classical computing hardware. This proposal aims to combine three different technologies: i) Superconducting circuits, with very fast (10 ns) gate times for fast processing, ii) Neutral atoms, with long (10 s) coherence times for long lived quantum memory, iii) Optical photons, for long distance fibre communication,to create a novel hybrid quantum interface capable of storing, processing and generating highly entangled states of photons for quantum networking and cryptography applications, overcoming the short coherence time associated with the scalable superconducting circuit systems. This also offers applications in quantum metrology for conversion from optical to microwave domain quantum information, making it possible to extend the interface to incorporate a wide range of alternative solid-state based qubits.The interface relies on use of highly excited Rydberg states, which have incredibly large dipole moments and transitions in the microwave regime, which can resonantly couple to superconducting qubits embedded in planar microwave waveguide cavities. The large Rydberg dipole also leads to strong, controllable interactions between atoms to provide a collective enhancement in the coupling to single photons for efficient storage and retrieval of light.The first stage of the experiment is to trap spatially addressable atomic ensembles above a superconducting microwave resonator operating at 4 K to demonstrate strong coupling to the waveguide mode, a key milestone for implementing the hybrid interface. The ensembles will then be utilised to perform coherent storage and retrieval of optical photons, as well as generation of single photons using four-wave mixing. The second stage is to exploit the off-resonant interaction with the cavity to achieve controllable long distance (~1 cm) entanglement between a pair of ensembles trapped within a single microwave resonator. This will then be used to generate entangled photon pairs, exploring the benefits of collective encoding within the ensembles for achieving entanglement in the polarisation degrees of freedom for long-distance cryptographic quantum key distribution. The resulting hybrid quantum interface provides an ideal building block for establishing quantum networks. Long term this can be integrated with existing superconducting qubit technologies, making a significant step towards the realisation of scalable quantum computing.