Langevin Regime Optical Parametric Down-Conversion Demonstrates Single-Photon Compression by Nearly One Order of Magnitude

Fluctuations, often considered noise, actually play a crucial role in a quantum process called parametric down-conversion, particularly when operating in a challenging regime known as the Langevin regime. Yen-Ju Chen, Chun-Yuan Cheng, and Tien-Dat Pham, along with colleagues, now demonstrate the first experimental observation of this process on a microchip. The team precisely controls inherent loss due to fluctuation and, in doing so, observes an asymmetric Hong-Ou-Mandel dip, a clear signature of fluctuation-driven parametric down-conversion, and achieves a nearly tenfold compression of single photons. This breakthrough establishes a new ability to manipulate fluctuation itself, opening exciting possibilities for controlling quantum states and the interaction between a quantum system and its environment.

Temporal Shaping of Quantum Photon Pairs

This research focuses on generating and manipulating single photons and pairs of photons, known as biphotons, which are fundamental to quantum information science. The team is developing techniques to precisely control the timing of these photons, uniquely exploring how to utilize random fluctuations, known as Langevin noise, in the process. This represents a departure from traditional approaches that aim to eliminate noise, instead embracing it as a tool for control. The researchers have demonstrated a novel method for shaping single photons and biphotons by intentionally utilizing inherent losses within the system.

They operate in a regime where Langevin noise significantly influences the generation and shaping of photon pairs, allowing for unique control over their quantum properties. This has led to the development of a compact and efficient source of narrowband biphotons, and the ability to control the temporal waveforms of these photons, potentially enabling the creation of ultra-short pulses for specific applications. The techniques employed involve parametric down-conversion, a process for generating photon pairs, using lithium niobate waveguides to confine and guide the light. These waveguides incorporate a periodically poled structure to enhance the efficiency of down-conversion, and utilize electro-optic modulation to control the properties of light. This research has significant potential for applications in quantum communication, computing, imaging, and metrology, as well as the development of reliable single-photon sources. By embracing and controlling noise, and utilizing unconventional techniques, this work opens up new avenues for manipulating quantum states of light and developing advanced quantum technologies.

Controlling Loss Enables Single-Photon Compression

Scientists have experimentally demonstrated parametric down-conversion on a chip, achieving precise control over inherent loss linked to quantum fluctuation. This work reveals asymmetric Hong-Ou-Mandel dips, a hallmark of fluctuation-driven parametric down-conversion, and achieves compression of single photons to approximately eleven percent of their initial size, representing nearly one order of magnitude reduction. The team fabricated a lithium niobate waveguide pumped by a laser, incorporating a specialized structure optimized for efficient photon pair generation. Experiments involved depositing an absorption layer onto the waveguide, separated by a thin layer of silicon dioxide, followed by titanium and gold layers, to introduce controllable loss.

Simulations predicted specific absorption coefficients for the signal and idler photons, and measurements of the Hong-Ou-Mandel interference pattern closely matched theoretical predictions based on Langevin theory. The observed asymmetric interference pattern confirms the influence of fluctuation on the quantum fields, deviating significantly from predictions based on traditional theories. The team notes a small deviation near the interference dip, likely due to imperfections in the gold film altering the absorption coefficient and causing waveform distortion. By carefully controlling loss linked to quantum fluctuation, the team observed an asymmetric Hong-Ou-Mandel dip, a clear signature of fluctuation-driven parametric down-conversion, and achieved compression of single photons to approximately eleven percent of their initial size, representing nearly one order of magnitude reduction. These results establish a new method for manipulating quantum fluctuation, quantum states, and the interaction between a system and its surrounding environment. The authors acknowledge that their current setup does not achieve single-cycle pulse compression, but suggest several avenues for future work, including increasing absorption layer coverage, reducing buffer layer thickness, or exploring alternative materials to enhance absorption. Compared to existing techniques for pulse compression, this approach avoids the need for complex fabrication of specialized crystals or structures, offering a potentially simpler route to achieving these effects. The team highlights a distinction between the Langevin noise considered in cavity-based squeezed light generation and the single-pass parametric down-conversion studied here, emphasizing the unique insights gained at the single-photon level.