Quantum Light Sources From Semiconductor Nanostructures
Better quantum light sources are possible with new developments. Vienna and Innsbruck Multi-Photon Emission Control improvements for quantum technologies.
Recent discoveries by Vienna and Innsbruck research teams have improved the quest for non-classical light, which is needed for quantum technologies like secure communication and quantum computing. Despite the goal of producing pure single photons, resonant excitation in atoms, whether natural or artificial, always emits multi-photons. To better handle these complex light-matter interactions and generate more reliable and effective quantum light sources, two complementary research efforts have yielded crucial insights and feasible solutions.
Artificial Atom Multi-Photon Dynamics Revealed
L. Jehle, L. Carosini, L. M. Hansen, J. C. Loredo, and P. Walther along with F. Giorgino and P. Zahálka at the University of Vienna have studied multi-photon processes in silicon quantum dots. Because of their strong optical properties and solid-state synthesis, silicon quantum dots are becoming more popular platforms for quantum light source creation.
In “Multi-photon emission from a resonantly pumped quantum dot,” upper-order auto-correlation functions (g(2), g(3), and g(4)) and high-resolution temporal measurements quantify these emissions, providing a nuanced understanding of light-matter interactions. They proved that a single excitation pulse can emit four photons.
Conclusions from their detailed analysis include:
With single-photon emission, two, three, or four photons can be detected from a single stimulation. By showing that a bunched source (g(2)(0) > 1) does not always have a better probability of emitting two photons than one, the work showed the importance of the vacuum probability (likely to produce no photons). Increased multi-photon contributions occur in even pulse areas (Θ = 2nπ) due to quantum dot re-excitation during the excitation pulse duration. Closely resolved temporal studies confirmed that these multi-photon events are caused by successive spontaneous emissions, meaning the photons do not share the same temporal mode. This study showed a time-gating approach to purify single-photon sources. By rejecting early-arriving photons and catching photons during a specified time frame after excitation, researchers can prevent multi-photon events while maintaining detection efficiency. This works better than lowering excitation power for purity.
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Enhancing Multi-Photon Generation with Passive Demultiplexing
Gregor Weihs and Vikas Remesh from the University of Innsbruck led a worldwide research team that developed a sophisticated approach to overcome the disadvantages of traditional multi-photon state creation. Researchers develop multi-photon states from a single quantum dot using fast electro-optic modulators (EOMs) to multiplex emission into spatial and temporal modes. EOMs require specialised engineering, are expensive, and reduce efficiency.
Using stimulated two-photon excitation (sTPE), the Innsbruck team produces photon streams in multiple polarisation states from a quantum dot without active switching elements. This method limits multi-photon rate by the quantum dot's intrinsic lifetime rather than an EOM's switching speed.
The procedure:
By carefully timing laser pulses to excite the quantum dot, a biexciton state is generated. After that, polarization-controlled stimulation pulses that deterministically emit photons in the horizontal (H) and vertical (V) polarisation states follow. When used with active demultiplexing technologies, passive demultiplexing is cost-effective and doubles multi-photon generation. With g(2)(0) values of 0.022(2) for V-polarized photons and 0.028(2) for H-polarized photons, the group produced high-quality two-photon states with excellent single-photon characteristics. They validated high indistinguishability with adjusted Hong-Ou-Mandel (HOM) visibilities of 90(1)% for V-polarized photons and 93.7(3)% for H-polarized photons.
This innovation enables safe multi-party communication and fits directly into quantum key distribution systems. It also shows potential for multi-photon interference studies, which test quantum mechanical ideas.
These advancements improve quantum dot sources' practicality and efficacy overall. Multi-photon dynamics and passive, high-rate generation technologies are helping researchers create scalable and dependable photonic quantum computing.
