Research Outputs
Self-testing mutually unbiased bases in higher dimensions with space-division multiplexing optical fiber technology
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.
Computational advantage from the quantum superposition of multiple temporal orders of photonic gates
Models for quantum computation with circuit connections subject to the quantum superposition principle have recently been proposed. In them, a control quantum system can coherently determine the order in which a target quantum system undergoes N gate operations. This process, known as the quantum N-switch, is a resource for several information-processing tasks. In particular, it provides a computational advantage—over fixed-gate-order quantum circuits—for phase-estimation problems involving N unknown unitary gates. However, the corresponding algorithm requires an experimentally unfeasible target-system dimension (super)exponential in N. Here, we introduce a promise problem for which the quantum N-switch gives an equivalent computational speedup with target-system dimension as small as 2 regardless of N. We use state-of-the-art multicore optical-fiber technology to experimentally demonstrate the quantum N-switch with N = 4 gates acting on a photonic-polarization qubit. This is the first observation of a quantum superposition of more than N = 2 temporal orders, demonstrating its usefulness for efficient phase estimation.