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.
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.
Organic chemistry offers the potential to synthesize individual molecules for specific tasks in quantum information processing. One of their striking properties is the emission of single photons with nearly ideal coherence. Here, we implement Ramsey spectroscopy to measure the electronic state coherence of a single molecule. Conventionally, the emitter’s coherence is characterized by saturation or intensity auto-correlation measurements. However, both methods are under the steady influence of continuous interaction with the excitation laser. This influence can be eliminated by using a pump–probe sequence of two optical pulses to observe the decoherence. We have measured a near-transform-limited decoherence time of 17.6 ns, while the radiative lifetime is 10.1 ns. We also perform frequency-detuned excitation, gaining richer insights into the dephasing behavior of the molecule. The experiments exhibit that optical Ramsey spectroscopy is a promising tool for measuring the emitter’s coherence properties.
Abstract The local interaction of charges and light in organic solids is the basis of distinct and fundamental effects. We here observe, at the single-molecule scale, how a focused laser beam can locally shift by hundreds of times their natural line width and, in a persistent way, the transition frequency of organic chromophores cooled at liquid helium temperature in different host matrices. Supported by quantum chemistry calculations, the results can be interpreted as effects of a photoionization cascade, leading to a stable electric field, which Stark-shifts the molecular electronic levels. The experimental observation is then applied to a common challenge in quantum photonics, i.e., the independent tuning and synchronization of close-by quantum emitters, which is desirable for multiphoton experiments. Five molecules that are spatially separated by about 50 μm and originally 20 GHz apart are brought into resonance within twice their line width. This tuning method, which does not require additional fabrication steps, is here independently applied to multiple emitters, with an emission line width that is only limited by the spontaneous decay and an inhomogeneous broadening limited to 1 nm. The system hence shows promise for photonic quantum technologies.
Summary form only given. Organic dye molecules have been used in great many scientific and technological applications. Among these, the polycyclic aromatic hydrocarbons (PAH) behave very favourably as quantum emitters when embedded in organic crystals at temperatures below 4K. PAHs can serve as solid-state emitters with remarkable optical properties such as near-unity quantum efficiency and high photostability. However, transitions to short-lived molecular vibrational levels reduce the scattering cross section by a branching ratio of about 30-50%. In order to overcome this shortcoming and render the molecule as a two-level system, one can enhance its zero-phonon emission by using the Purcell effect, e.g., via coupling to an optical cavity, to obtain a near-unity branching ratio. Here we report the first coherent coupling of a single molecule to a scannable, tunable and broadband microcavity. The cavity consists of a planar distributed Bragg reflector and a micromirror with a radius of curvature of 5 μm fabricated with focused ion-beam milling [1] and coated with silver. By integrating an organic matrix in the microcavity, we are able to coherently couple individual molecules to the cavity mode at liquid helium temperature.
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.
We present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a pi-pulse, we use this coherent control to make a high quality triggered source of single photons. We obtain an average single photon rate of 9600 s-1 at the detector. Measurements of the second-order temporal correlation function show almost perfect antibunching at zero delay. In addition, we present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sub-levels.
We present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a pi-pulse, we use this coherent control to make a high quality triggered source of single photons. We obtain an average single photon rate of 9600 s-1 at the detector. Measurements of the second-order temporal correlation function show almost perfect antibunching at zero delay. In addition, we present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sub-levels.