#photonnumberresolvingdetector

Photon number resolving detector

Researchers have made major advances in photon-number-resolving (PNR) detection, a key technology for quantum computing and secure communications. These groups have overcome traditional detectors to count light particles with unprecedented accuracy, providing a robust barrier against modern cyberattacks.

The Light-Counting Challenge

High-performance quantum technologies like quantum imaging and sensing require the detection and characterization of many-photon states. Conventional single-photon detectors can detect light, but they often have problems distinguishing between one, two, or three photons. This restriction is hazardous because an eavesdropper can employ “multi-photon pairs” to steal information unnoticed in Quantum Key Distribution (QKD).

Emitter Cascade: New Architecture

Researchers from the University of Waterloo have devised a PNR detector system using a cascade of waveguide-coupled Λ-type emitters. Instead of using beamsplitters to distribute photons among detectors, this innovative method uses a chain of atoms or quantum dots coupled to a chiral (one-way) waveguide.

This system employs Single-Photon Raman Interaction. This arrangement uses a Λ-type atom as a “photon-activated switch”. The first photon of a light pulse is coherently redirected to a different output port and detected after interacting with the atom. Importantly, after “capturing” one photon, the atom becomes transparent to the pulse's other photons. Researchers can cascade emitters to deterministically “peel off” and count photons.

Guide to Nonlinear Frontier

A major focus at Waterloo was the shift from linear to nonlinear regimes. Photons are far apart in the linear regime, making them independent. As quantum networks accelerate and pulses compress, photons overlap during the “emitter’s lifetime”.

Multiple photons arriving within the atomic response time generate nonlinear interactions. The researchers found that these interactions cause “saturation effects” and photon correlations like bunching and anti-bunching. The researchers used Green's function formalisms and the quantum trajectory technique to generate complex “scattering matrices” to predict how these interactions affect detector accuracy. They found that increasing the number of emitters in the cascade can compensate for nonlinearity issues and outperform beamsplitter-based detectors in practice.

Protection against “Photon Number Splitting”

At the same time, an Indian Institute of Technology Delhi team proved that multi-photon emissions degrade quantum entanglement. Entanglement, or “spooky action at a distance,” underpins safe quantum communication, but it is not perfect.

Single, double, and triple photon pair events produced by spontaneous parametric down-conversion (SPDC) were separated and measured using parallel superconducting nanowire single-photon detectors at IIT Delhi. Increased multi-photon pair generation causes a “marked reduction” in the Bell parameter (S parameter), a standard indicator of entanglement quality.

Security and technical issues arise from this degradation. In a Photon Number Splitting (PNS) attack, Eve, an eavesdropper, intercepts one photon from a multi-photon state while the rest pass to the permitted recipient. Eve can steal the cryptographic key without alarming a typical detector since the whole entanglement appears intact.

The researchers showed that PNR detectors solve this problem. Accurate photon counts in real time allow genuine parties to discover “anomalies in their expected correlations” and eliminate compromised multi-photon states. This allows Alice and Bob, quantum communicators, to detect eavesdropping and maintain their secure connection.

Technical Specifications and Performance

Using different voltage thresholds, the IIT Delhi P-SNSPDs can resolve up to four photons, proving their remarkable PNR capabilities. These detectors have fast recovery times (80 ps) and high efficiency (up to 99.5%, according to studies).

The Waterloo team's emitter-based device offers high performance using quantum dots in photonic crystal waveguides. Experimental platforms have achieved coupling efficiencies of 0.98 and directionality close to unity. When coupling rates reach GHz, photon absorption and re-emission become fast and efficient, making these devices ideal for high-speed quantum processors.

Road Ahead

Both research say effective PNR detection is “essential for achieving robust, secure quantum communication”. Traditional spatial demultiplexing algorithms have been used, but the deterministic emitter cascade allows new quantum tomography forms and higher precision.