Scalable on-chip quantum state tomography

Abstract

Quantum information systems are on a path to vastly exceed the complexity of any classical device. The number of entangled qubits in quantum devices is rapidly increasing, and the information required to fully describe these systems scales exponentially with qubit number. This scaling is the key benefit of quantum systems, however it also presents a severe challenge. To characterize such systems typically requires an exponentially long sequence of different measurements, becoming highly resource demanding for large numbers of qubits. Here we propose and demonstrate a novel and scalable method for characterizing quantum systems based on expanding a multi-photon state to larger dimensionality. We establish that the complexity of this new measurement technique only scales linearly with the number of qubits, while providing a tomographically complete set of data without a need for reconfigurability. We experimentally demonstrate an integrated photonic chip capable of measuring two- and three-photon quantum states with statistical reconstruction fidelity of 99.71%. A single chip can be used to fully characterize multi-photon quantum states, without incorporating any reconfigurable elements. This is a fundamental advance since finding a complete description of quantum system normally requires a number of different measurement configurations that grows exponentially with the system size. The group of Andrey Sukhorukov at the Australian National University developed a theoretical concept, and experimental device fabrication and two-photon measurements were realized by the group of Alexander Szameit at the Friedrich-Schiller-Universität Jena and University of Rostock in Germany. The light traversing the device undergoes an optical transformation and expands into a larger number of modes at the output. The authors then use a computationally efficient algorithm to deduce the initial quantum state from the correlations of output photons. The number of modes required only grows linearly with the photon number, demonstrating that the design provides a scalable approach to measuring larger quantum states despite their increasing complexity.