The quantum-safe future will require new technologies for secure communication and cryptography. Quantum key distribution has already left the laboratory. Commercial systems are on the market, and they generally encode information in very faint light pulses, near the single-photon level. Reading out such weak signals calls for high-sensitivity, extremely low-noise detectors, so the cost of a single link stays high. This is a reasonable trade for a handful of high-value connections, but it leaves a gap where low-cost quantum communication would matter most: denser, shorter-range networks, where many modest links count for more than a few premium ones.
DUO-QUANT tackles the problem from the detector side. Silicon photomultipliers are compact, room-temperature devices able to resolve how many photons arrive in a pulse. Our group has already used them to perform weak-field homodyne detection, in which a mesoscopic reference beam is made to interfere with the incoming signal. So far that scheme has measured a single quadrature of the optical field, which restricts the encoding to two distinguishable states — one bit per pulse. DUO-QUANT doubles the receiver: two interferometers held at a relative phase of 90 degrees measure both quadratures simultaneously, giving access to the full amplitude and phase of the light that arrives.
Once both quadratures are available, information can be carried by a constellation of states, following quadrature-amplitude modulation. This is the same principle behind classical fibre and wireless links, transposed to the quantum regime. Theory predicts a higher information transfer rate than binary encoding allows, but the prediction has never been tested with photon-number-resolving detectors. The interesting question is where the trade-off sits: denser constellations pack more bits into each pulse, yet real-world noise such as interferometer drift, dark counts, detector crosstalk eventually eats the advantage. Charting that boundary, and benchmarking the result against our binary baseline, is the core of the project. This is a first step towards integrated, low-cost receivers for distributed networks such as the nodes of a power distribution grid.
PhD opportunities
DUO-QUANT is funded by the University of Insubria through the Fondo per ricercatori a tempo determinato 2025. The project is led by Marco Lamperti (PI) with Alessia Allevi (co-PI), Department of Science and High Technology.
PhD positions related to this activity are expected to open in July 2027. Prospective candidates are welcome to get in touch between May and June 2027.
