Dr. Cariñe-Catrileo, Jaime

In the device-independent quantum-information approach, the implementation of a given task can be self-tested solely from the recorded statistics and without detailed models for the employed devices. Even though experimentally demanding, it provides appealing verification schemes for advanced quantum technologies that naturally fulfil the associated requirements. In this work, we experimentally study whether self-testing protocols can be adopted to certify the proper functioning of quantum devices built with modern space-division multiplexing optical fiber technology. Specifically, we consider the prepare-and-measure protocol of Farkas and Kaniewski [Phys. Rev. A 99, 032316 (2019)] for self-testing measurements corresponding to mutually unbiased bases (MUBs) in a dimension 𝑑>2. In our scheme, the state preparation and measurement stages are implemented using a multiarm interferometer built with multicore optical fibers and related components. Due to the high overlap of the interferometer’s optical modes achieved with this technology, we are able to reach the required visibilities for self-testing the implementation of two four-dimensional MUBs. We also quantify two operational quantities of the measurements: (i) the incompatibility robustness, connected to Bell violations, and (ii) the randomness extractable from the outcomes. Since MUBs lie at the core of several quantum-information protocols, our results are of practical interest for future quantum works relying on space-division multiplexing optical fibers.

Device and semi-device-independent private quantum randomness generators are crucial for applications requiring private randomness. However, they are vulnerable to detection inefficiency attacks and this limits severely their usage for practical purposes. Here, we present a method for protecting semi-device-independent private quantum randomness generators in prepare-and-measure scenarios against detection inefficiency attacks. The key idea is the introduction of a blocking device that adds failures in the communication between the preparation and measurement devices. We prove that, for any detection efficiency, there is a blocking rate that provides protection against these attacks. We experimentally demonstrate the generation of private randomness using weak coherent states and standard avalanche photo-detectors.

The orbital angular momentum (OAM) spatial degree of freedom of light has been widely explored in many applications, including telecommunications, quantum information, and light-based micromanipulation. The ability to separate and distinguish between the different transverse spatial modes is called mode sorting or mode demultiplexing, and it is essential to recover the encoded information in such applications. An ideal d mode sorter should be able to faithfully distinguish between the different d spatial modes, with minimal losses, and have d outputs and fast response times. All previous mode sorters rely on bulk optical elements, such as spatial light modulators, which cannot be quickly tuned and have additional losses if they are to be integrated with optical fiber systems. Here, we propose and experimentally demonstrate, to the best of our knowledge, the first all-in-fiber method for OAM mode sorting with ultrafast dynamic reconfigurability. Our scheme first decomposes the OAM mode in-fiber-optical linearly polarized (LP) modes and then interferometrically recombines them to determine the topological charge, thus correctly sorting the OAM mode. In addition, our setup can also be used to perform ultrafast routing of the OAM modes. These results show a novel and fiber-integrated form of optical spatial mode sorting that can be readily used for many new applications in classical and quantum information processing.