Large-scale integrated quantum optics
The ability to pattern optical circuits on-chip, along with coupling in single and entangled photon sources, provides the basis for an integrated quantum optics platform. Wang et al. demonstrate how they can expand on that platform to fabricate very large quantum optical circuitry. They integrated more than 550 quantum optical components and 16 photon sources on a state-of-the-art single silicon chip, enabling universal generation, control, and analysis of multidimensional entanglement. The results illustrate the power of an integrated quantum optics approach for developing quantum technologies.
The ability to control multidimensional quantum systems is central to the development of advanced quantum technologies. We demonstrate a multidimensional integrated quantum photonic platform able to generate, control, and analyze high-dimensional entanglement. A programmable bipartite entangled system is realized with dimensions up to 15 × 15 on a large-scale silicon photonics quantum circuit. The device integrates more than 550 photonic components on a single chip, including 16 identical photon-pair sources. We verify the high precision, generality, and controllability of our multidimensional technology, and further exploit these abilities to demonstrate previously unexplored quantum applications, such as quantum randomness expansion and self-testing on multidimensional states. Our work provides an experimental platform for the development of multidimensional quantum technologies.
As a generalization of two-level quantum systems (qubits), multidimensional quantum systems (qudits) exhibit distinct quantum properties and can offer improvements in particular applications. For example, qudit systems allow higher capacity and noise robustness in quantum communications (1–3), can be used to strengthen the violations of generalized Bell and Einstein-Podolsky-Rosen (EPR) steering inequalities (4–6), provide richer resources for quantum simulation (7, 8), and offer higher efficiency and flexibility in quantum computing (9, 10). Moreover, encoding and processing qudits can represent a more viable route to larger Hilbert spaces. These advantages motivate the development of multidimensional quantum technologies in a variety of systems, such as photons (11, 12), superconductors (8, 13), and atomic systems (14, 15). Unlike superconducting and atomic qudits, which cannot be encoded and manipulated without complex interaction engineering and control sequences, photons represent a promising platform able to naturally encode and process qudits in various degrees of freedom, such as orbital angular momentum (OAM) (11, 12), temporal modes (3, 16), and frequency (17, 18). Previous work on qudits includes realizations of complex entanglement (19), entanglement in ultrahigh dimensions (20), and practical applications in quantum communication (1–3) and computing (7–9). However, these approaches incur limitations in terms of controllability, precision, and universality, which represent bottlenecks for further developments of multidimensional technologies. For example, the arbitrary generation of high-dimensional entanglement is a key experimental challenge, typically relying on complex bulk-optical networks and postselection schemes (12, 16–18). In general, these approaches lack the ability to perform arbitrary multidimensional unitary operations with high fidelity (16, 19), an important factor in quantum information tasks. Integrated microring resonators able to emit multidimensional OAM (21) and frequency (18) states have been reported, but these present limited fidelity and difficulties for on-chip state control and analysis, thus not fully exploiting the high precision, scalability, and programmability of integrated optics.
We report a multidimensional integrated quantum photonic device that is able to generate, manipulate, and measure multidimensional entanglement fully on-chip with unprecedented precision, controllability, and universality. Path-encoded qudits are obtained in which each photon exists over d spatial modes simultaneously, and entanglement is produced by a coherent and controllable excitation of an array of d identical photon-pair sources. Our device allows the generation of multidimensional entangled states with an arbitrary degree of entanglement. Universal operations on path-encoded qudits are possible in linear optics for any dimension (22, 23), and our device performs arbitrary multidimensional projective measurements with high fidelity. The capabilities achieved allow us to demonstrate high-quality multidimensional quantum correlations, verified by generalized Bell and EPR steering violations, and to implement unexplored multidimensional quantum information protocols.
Large-scale integrated quantum photonic circuit
Entangled path-encoded qubits can be generated by coherently pumping two photon-pair sources produced by spontaneous parametric downconversion (24) or by spontaneous four-wave mixing (SFWM) (25). The approach can be generalized to qudits via the generation of photons entangled over d spatial modes by coherently pumping d sources (24). However, scaling this approach to high numbers of dimensions has been challenging because of the requirement for a stable and scalable technology able to coherently embed large arrays of identical photon sources and to precisely control qudit states in large optical interferometers.