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Coherently coupled molecules at cryogenic temperatures : from super/sub radiance to optical selection rules

Quentin Deplano, Jean-Baptiste Trebbia, Philippe Tamarat, Brahim Lounis

Published Aug 28, 2022• 0 citations
OpenAlex

Abstract

Using quantum emitters (such as quantum dots, atoms, NV defects in diamond ...) in order to create quantum logic gates, is a widely studied topic in modern physics. Among all of those objects, organic molecules trapped in condensed matter are nearly ideal quantum emitters dispensing single indistinguishable photons. Moreover, at low temperature (≈ 2K), molecules are very well modelized by a two-level system. At nanometric distances, two molecules can be coupled by the dipole-dipole interaction leading to the apparition of two delocalized states respectively sub- and super-radiant. The observation of two coupled dipoles has been reported only once in the literature in 2002, despite of numerous interesting applications for quantum computing. The major limitation is the low probability to find an entangled pair in a sample. For this purpose, we lately developed an optical nanoscopy method based on the saturation of the excited state by a scanned donut shaped beam, and operating at cryogenic temperature, allowing us to find some pairs of coupled dipoles. We obtained a sub 10-nm resolution for a single dipole imaging. When the donut is scanned over an entangled pair, the Rabi frequencies seen by the two emitters varies which modifies the four energy levels of the pair by AC lightshift and leads to the apparition of fluorescence dips in the final image, when two levels become degenerated. With this technique we succeeded in doing specific images of an entangled system delocalized over 20 nm. The goal of the research activity presented here is to manipulate, on demand, the degree of entanglement of such a system. The latter could only be changed by applying an external electric field on the pair, which tunes the energy difference of the two emitters by Stark effect. We have been able to put at resonance the two emitters and observed a broadening and shrinking of the symmetric and antisymmetric transitions compatible with a modification of theirs relative lifetimes.

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•Jan 1, 2018•0 citations
Using quantum emitters (such as quantum dots, atoms, NV defects in diamond ...) in order to create quantum logic gates, is a widely studied topic in modern physics. Among all of those objects, organic molecules trapped in condensed matter are nearly ideal quantum emitters dispensing single indistinguishable photons. Moreover, at low temperature (≈ 2K), molecules are very well modelized by a two-level system. At nanometric distances, two molecules can be coupled by the dipole-dipole interaction leading to the apparition of two delocalized states respectively sub- and super-radiant. The observation of two coupled dipoles has been reported only once in the literature in 2002, despite of numerous interesting applications for quantum computing. The major limitation is the low probability to find an entangled pair in a sample. For this purpose, we lately developed an optical nanoscopy method based on the saturation of the excited state by a scanned donut shaped beam, and operating at cryogenic temperature, allowing us to find some pairs of coupled dipoles. We obtained a sub 10-nm resolution for a single dipole imaging. When the donut is scanned over an entangled pair, the Rabi frequencies seen by the two emitters varies which modifies the four energy levels of the pair by AC lightshift and leads to the apparition of fluorescence dips in the final image, when two levels become degenerated. With this technique we succeeded in doing specific images of an entangled system delocalized over 20 nm. The goal of the research activity presented here is to manipulate, on demand, the degree of entanglement of such a system. The latter could only be changed by applying an external electric field on the pair, which tunes the energy difference of the two emitters by Stark effect. We have been able to put at resonance the two emitters and observed a broadening and shrinking of the symmetric and antisymmetric transitions compatible with a modification of theirs relative lifetimes.
The control and manipulation of quantum-entangled states is crucial for the development of quantum technologies. A promising route is to couple solid-state quantum emitters through their optical dipole-dipole interactions. Entanglement in itself is challenging, as it requires both nanometric distances between emitters and nearly degenerate electronic transitions. Here we implement hyperspectral imaging to identify pairs of coupled dibenzanthanthrene molecules, and find distinctive spectral signatures of maximally entangled superradiant and subradiant electronic states by tuning the molecular optical resonances with Stark effect. We demonstrate far-field selective excitation of the long-lived subradiant delocalized state with a laser field tailored in amplitude and phase. Optical nanoscopy of the coupled molecules unveils spatial signatures that result from quantum interferences in their excitation pathways and reveal the location of each emitter. Controlled electronic-states superposition will help deciphering more complex physical or biological mechanisms governed by the coherent coupling and developing quantum information schemes.
Abstract The control and manipulation of quantum-entangled states is crucial for the development of quantum technologies. A promising route is to couple solid-state quantum emitters through their optical dipole-dipole interactions. Entanglement in itself is challenging, as it requires both nanometric distances between emitters and nearly degenerate electronic transitions. Here we implement hyperspectral imaging to identify pairs of coupled dibenzanthanthrene molecules, and find distinctive spectral signatures of maximally entangled superradiant and subradiant electronic states by tuning the molecular optical resonances with Stark effect. We demonstrate far-field selective excitation of the long-lived subradiant delocalized state with a laser field tailored in amplitude and phase. Optical nanoscopy of the coupled molecules unveils spatial signatures that result from quantum interferences in their excitation pathways and reveal the location of each emitter. Controlled electronic-states superposition will help deciphering more complex physical or biological mechanisms governed by the coherent coupling and developing quantum information schemes.
By performing cryogenic laser spectroscopy under a scanning probe electrode that induces a local electric field, we have resolved two individual fluorescent molecules separated by 12 nanometers in an organic crystal. The two molecules undergo a strong coherent dipole-dipole coupling that produces entangled sub- and superradiant states. Under intense laser illumination, both molecules are excited via a two-photon transition, and the fluorescence from this doubly excited system displays photon bunching. Our experimental scheme can be used to optically resolve molecules at the nanometer scale and to manipulate the degree of entanglement among them.
Single semiconductor quantum dots, which are considered as two-level systems in the artificial atom model, are promising structures for the realization of integrated devices such as single photon sources for quantum information applications. However, contrary to genuine atoms, they are condensed matter systems which suffer from the coupling to their solid environment, leading to a degradation of the coherence of the emitted photons. One possibility to avoid this coupling, thereby improving the coherence of the photons until the one imposed by the radiative limit T2 = 2T1, is to perform strictly resonant excitation of a quantum dot at low temperature. Using an experimental setup that spatially decouples the excitation and detection paths, we showed that a fine control of the electrostatic environment related to the nanostructures residual doping remains a major challenge to answer the widely encountered problem of the inhibition of the resonance fluorescence of a quantum dot. Meanwhile, thanks to quantum optics experiments and high resolution optical spectroscopy, we demonstrated that the resonant Rayleigh scattering regime where quantum dots emit single photons with the laser coherence time, paves the way to the development of "ultra-coherent" sources of single photons with high degrees of indistinguishability. In this regime where the laser tailors the coherence of the photons and where the radiative limit becomes a secondary requirement, two photons emitted successively by the same quantum dot are indistinguishable on time scales never obtained before for a solid nano-emitter (up to ten nanoseconds, comparing to the carriers lifetime and decoherence time of about a hundred of picoseconds).
Single molecules trapped in the solid state at liquid helium temperatures are promising quantum emitters for the development of quantum technologies owing to their remarkable photostability and their lifetime-limited optical coherence time of the order of 10 ns. The coherent preparation of their electronic state requires resonant excitation with a Rabi period much shorter than their optical coherence time. Sculpting the optical excitation with sharp edges and a high on–off intensity ratio (∼3 × 105) from a single-frequency laser beam, we demonstrate sub-nanosecond drive of a single dibenzanthanthrene molecule embedded in a naphthalene matrix at 3.2 K, over more than 17 Rabi periods. With pulses tailored for a half-Rabi period, the electronic excited state is prepared with fidelity as high as 0.97. Using single-molecule Ramsey spectroscopy, we prove up to 5 K that the optical coherence lifetime remains at its fundamental upper limit set by twice the excited-state lifetime, making single molecules suitable for quantum bit manipulations under standard cryogen-free cooling technologies.
A quantum system with discrete, resolvable energy levels is an ideal system for sensing applications, taking into account that it can strongly interact with its environment. Although the strong response with the target properties such as magnetic or electric field is ideal, it can be a disadvantage when the system is interacting with other physical changes (noise) at the same time. However, the benefits of the strong coupling as well as small size of the sensor has motivated many researchers to explore quantum systems for measuring very small physical quantities. For instance, Nitrogen vacancy center in diamond is a unique quantum system enabling sensing of magnetic field from single molecules or electrons at room temperature. This thesis has focused on two different quantum emitters: NV centers in nanodiamonds and quantum emitters in hexagonal Boron Nitride (hBN). In the first part a new assembly technique based on electron beam induced deposition and crosslinking chemistry is developed. It is demonstrated that fluorescent nanodiamonds containing NV centers can be assembled into arrays of various size and shapes on any substrate. The produced array is ideal for device fabrication due to its outstanding robustness. A potential application of such arrays as magnetic field sensor with high spatial resolution is shown. The superior properties of quantum emitters in hBN has been studied in the second part of this thesis, where is has been shown that these emitters are not only stable but also maintain their single photon purity at elevated temperature. The highest measured quantum emission at 800 K in this study is the highest reported temperature for a quantum emitter so far, makes them a suitable candidate for temperature sensing. In addition, the level structure of emitters in hBN has been investigated in detail which reveals the unique photophysical properties of a class of these emitters which result in a nonlinear increase in the emission upon co-excitation with two lasers of different wavelength. Finally the photophysic property of these emitters has been employed to introduce a new modality of super-resolution microscopy with resolution down to about 70 nm. These findings will extend our understanding of quantum emitters in hBN and introduce a new functionality for them which paves the way toward their application in biology and sensing.