Researchers from the University of Denmark and Ruhr University Bochum have made a significant advancement by demonstrating coherent emission by two-photon emitters fabricated in a photonic system. This new development promises to create new opportunities to expedite the use of quantum technology for commercial purposes.
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The foundation of light-matter interaction and the cornerstone of photonic quantum physics is photon emission. Developing high-quality photon emission provides cutting-edge sources for quantum computing and communication.
Coupling Multiple Light Emitters
In quantum optics and atomic physics, the radiative coupling of many light emitters has long been a problem. If experimentally achieved, coupling more than one photon emitter provides a method for implementing quantum gates between emitters. This would serve as a key building component for processing quantum information.
In most experimental platforms, radiative dipole-dipole coupling rapidly decays with increasing distance, often within a small fraction of the optical wavelength. Through various research efforts, It has become clear that waveguide quantum electrodynamics (QED) is the field of study best suited for overcoming the fundamentally weak dipole-dipole coupling. This is due to the fact that the radiative coupling is greatly stretched in waveguides as compared to the sub-wavelength limit found in unstructured mediums.
In order to see collective emission, a highly coherent light-matter interface is necessary. In particular, long-lived subradiant features may be difficult to detect when there are experimental challenges like dephasing.
The optical and microwave realms have been researched on collective multi-emitter effects. Multi-qubit interactions have been made possible in microwave QED, and coherent control of subradiant collective states has also been demonstrated.
It is crucial to realize these features in the optical domain because optical photons have the ability to be highly integrated, processed on-chip quickly and sent across long distances.
Quantum Dots as Single Photon Emitters
In the current study, the observation of coherent dynamics of a group of connected quantum dot (QD) emitters are excited collectively. Direct experimental verification of coherent coupling was provided through dynamics with adjusted collective emission.
QDs are described as artificial atoms that display ground and excited state features like real atomic systems. QDs can be optically manipulated to extract to develop useful technologies.
In this work, InAs QDs are incorporated into a photonic crystal waveguide (PCW) constructed from a suspended GaAs membrane and coupled to grating couplers at both ends to couple light into and out of the PCW.
Using a closed-cycle cryostat with optical access, the sample is cooled to 4 K. An external magnetic field was applied using a vector magnet. The sample was imaged, the QD was excited in free space, and light was coupled into the waveguide using a confocal microscope.
The focus of the experiment was on a single excitation that is shared across two QDs.
Zeeman tuning using the magnetic field is used to bring the QDs into mutual resonance. Direct observation revealed that it is possible to control the coherent fluctuations of the collective state by spectrally adjusting the QDs and changing the excitation circumstances.
Optical stimulation of one QD is achieved by optical pumping, and the photon-mediated coupling of two QDs in a PCW and the kinetics of its collective emission are collected. Then, the coupled QD system's emission dynamics show super- and subradiance that results from either constructive or destructive interference of the field that each QD scattered into the PCW.
Finally, by precisely regulating the detuning between two emitters, the coherent evolution of collective states was demonstrated. To enable their use in quantum information processing, collective states must first be prepared in a deterministic manner. The results illustrate an exact "control knob" of coupled collective quantum states that emitter tuning offers as an experimentally accessible tool.
The experimental findings, which are well reproduced by theory, show the rich, coherent dynamics of the collectively coupled QD emitter system.
Discussion and Future Outlook
The PCW's broadband spectrum functioning and long-range photon-mediated coherent interaction between QDs make it easier to observe super- and subradiant emission dynamics utilizing pairs of QDs embedded in PCWs and separated by distances much greater than the wavelength. This could be the first step in the development of multi-emitter applications that are crucial for technology, such as quantum memory with exponentially increasing photon storage fidelity or quantum transduction between microwave qubits and the optical domain.
When implementing a coherent spin inside the QD, the ability to regulate the detuning and pumping conditions will open up a whole new set of opportunities to alter the dynamics of the super- and subradiant state. A universal resource for measurement-based photonic quantum computing, for instance, might be created by deterministically generating advanced photonic cluster states.
In a broader sense, the ability to deterministically connect many quantum emitters opens up a new field of study for strongly correlated light and matter non-equilibrium quantum many-body physics, which might be applied to quantum simulations of strongly correlated condensed matter systems.
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References and Further Reading
Alexey Tiranov, Vasiliki Angelopoulou, Cornelis Jacobus van Diepen, Björn Schrinski, Oliver August Dall’Alba Sandberg, Ying Wang, Leonardo Midolo, Sven Scholz, Andreas Dirk Wieck, Arne Ludwig, Anders Søndberg Sørensen, Peter Lodahl. Collective super- and subradiant dynamics between distant optical quantum emitters. Science, 2023; 379 (6630): 389 DOI: 10.1126/science.ade9324
University of Copenhagen - Faculty of Science. (26 January 2023) Quantum physicists make major nanoscopic advance. [Online] ScienceDaily. Available at: https://www.sciencedaily.com/releases/2023/01/230126161926.htm
J. Eschner, C. Raab, F. Schmidt-Kaler, and R. Blatt, Nature 413, 495 (2001).
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