ingle luminescent molecules provide a unique approach in the development of quantum technologies utilizing single photon sources. This includes quantum metrology by using a molecule’s sensitivity of its emission and magnetic properties to the local environment. In this thesis, we present our investigation of the optical properties of several different classes of luminescent molecules. We begin by providing some theoretical background of single quantum emitters as well as a brief description of the experimental methodologies and equipment. Next, we present an optical investigation of an ensemble of iron phthalocyanines molecules. This molecule possesses a ground state triplet, which is a desirable property for optically active spin qubits, but has a very weak optical emission. Diffraction limited spots displayed photo-instability in the form of blinking and irreversible bleaching. In ensemble form however, their optical stability allowed us to identify a possible Raman peak where we calculated the associated phonon frequency. Next, we present our single molecule study of vanadium phthalocyanine. This molecule has been documented as displaying very long spin coherence times even at room temperatures. We confirmed the presence of a single molecule by measuring the second order correlation function. Additionally, we looked at the intensity and spectral response as a function of the excitation laser polarization. The spectrum was fitted to a two Gaussian function, which may correspond to the two dipole transitions as suggested by theoretical calculations. Lastly, we looked at the optical properties of rare earth europium complexes known for having very sharp optical transitions in the emission spectrum, with each having varying levels of sensitivity to the local environment. Motivated by techniques to investigate non-radiative decay channels, we looked at the optical response of four different europium complexes under two 1 µs pulses of 515 nm laser separated by 1 µs. Each displayed a very different results and allowed us to identify the best candidates for single molecule studies. Finally, we looked at the emission spectrum as well as the optical response under a 6 µs long pulse using time-correlation single photon spectroscopy.
ingle luminescent molecules provide a unique approach in the development of quantum technologies utilizing single photon sources. This includes quantum metrology by using a molecule’s sensitivity of its emission and magnetic properties to the local environment. In this thesis, we present our investigation of the optical properties of several different classes of luminescent molecules. We begin by providing some theoretical background of single quantum emitters as well as a brief description of the experimental methodologies and equipment. Next, we present an optical investigation of an ensemble of iron phthalocyanines molecules. This molecule possesses a ground state triplet, which is a desirable property for optically active spin qubits, but has a very weak optical emission. Diffraction limited spots displayed photo-instability in the form of blinking and irreversible bleaching. In ensemble form however, their optical stability allowed us to identify a possible Raman peak where we calculated the associated phonon frequency. Next, we present our single molecule study of vanadium phthalocyanine. This molecule has been documented as displaying very long spin coherence times even at room temperatures. We confirmed the presence of a single molecule by measuring the second order correlation function. Additionally, we looked at the intensity and spectral response as a function of the excitation laser polarization. The spectrum was fitted to a two Gaussian function, which may correspond to the two dipole transitions as suggested by theoretical calculations. Lastly, we looked at the optical properties of rare earth europium complexes known for having very sharp optical transitions in the emission spectrum, with each having varying levels of sensitivity to the local environment. Motivated by techniques to investigate non-radiative decay channels, we looked at the optical response of four different europium complexes under two 1 µs pulses of 515 nm laser separated by 1 µs. Each displayed a very different results and allowed us to identify the best candidates for single molecule studies. Finally, we looked at the emission spectrum as well as the optical response under a 6 µs long pulse using time-correlation single photon spectroscopy.
Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, especially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to the large coherence times of their ground state electronic spin, which are on the order of microseconds when measured in the ensemble. However, the optical properties of such VOPc molecules at the single emitter level have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarizations agrees well with a system having pyramidal C 4 v symmetry. Furthermore, we provide theoretical calculations that support our experimental findings and provide insight into the role of phonons and the internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.
Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, specially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to their large coherence times measured in ensemble. However, the optical properties of individual molecules have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarization agrees well with a system having pyramidal C4v symmetry. Furthermore, the molecule reveals a non-radiative transition rate that depends on the excitation wavelength when its lifetime is interrogated. We provide theoretical calculations that support our experimental findings and provide insight to the role of phonons and internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.
Light-matter interaction at single quantum object level is a long-lived challenge and dream of physicists. From the 1970s, the development of single ion spectroscopy and the observation of single photon states pave a way to a new era of detecting, and coherently manipulating quantum objects with electromagnetic field at single-ion levels. Later on, the invention of single molecular spectroscopy offers a new approach to investigate single quantum objects in condensed matter, where the sophisticated trapping systems like in ions and cold atoms are not needed, resulting in the discovery of various of single photon emitters embedded in solids. Among all, single quantum dots and single negatively charged nitrogen-vacancy centers in diamond attract most attentions, since their single spins can be coherently manipulated and entangled by the light field giving rise to numerous applications from nanoscale sensing to quantum computing. They are intensively studied as the most promising candidates for the realization of solid-state quantum bits. However, these systems coupled to a complex environment which can result in rapid loss of spin coherence or optical instability. Finding an alternative solid-state single system with remarkable optical and spin properties is the motivation of this dissertation. Rare earth ions doped in inorganic crystal hosts provide promises not only by the high quality factor 4f-4f optical transitions but also by the well preserved and long-lived spin states, with the applications from solid-state laser spectroscopy to quantum information processing. However, for almost three decades, not being able to identify single rare earth elements in solids is the obstacle to achieve their full potential functionality. We demonstrated the first direct optical detection of single rare earth ions (praseodymium) in solids in 2012. This research was covered in my Master Degree Thesis. This dissertation presents experimental progress in optically detecting other single rare earth species in crystals, understanding their fundamental spectroscopy properties and coherently controlling them, which is mainly focused on single trivalent cerium ions in different host materials. Following the optical detection of single cerium ions in yttrium aluminum garnet (YAG), the optical and spin properties of single Ce ions are investigated. Single Ce ions show good photostability under the pulsed laser excitation. Still, like in other solid emitters, photo-induced dynamics of single Ce ions is observed, however, in a counterintuitive way under the continuous laser irradiation. To understand this behavior, we proposed a charge dynamics model, and developed a novel method to suppress this undesired charge dynamics. With this approach, we obtain the photoluminescence excitation spectrum of single Ce ions in YAG under a CW laser excitation. Narrow (~80 MHz) and spectrally stable optical transitions between the lowest Kramers doublets of the ground and the excited states are observed. In comparison with the large inhomogeneous broadening (500 GHz), narrow resonant transitions provide the good selectivity in the spectral domain. Moreover, the single Ce ground spin state presents a spin 1/2 system with anisotropy g factor. It results in the optical selection rules dependent on the directions of the external magnetic field. Under parallel external magnetic field with respect to the quantization axis, only the spin-flip transitions are allowed with high transition contrast. It gives the opportunity to optically initialize the ground spin state of single Ce ions with 97.5% fidelity. The coherent spin properties of single Ce ions can be investigated, after the successful optical initialization. By scanning the microwave frequency, we obtain the electron spin resonance of single Ce spin transitions. We show the spin state has a coherence time of 290 ns, which is dominated by the surrounding aluminum nuclear spin bath and impurities induced electron spin bath. The coherence time can be extended to 2 ms through dynamical decoupling method, which approaches closely to the upper limit 4.5 ms due to spin-lattice relaxation. This result indicates single Ce electron spin can be a good solid-state qubit. With the combination of optical and spin manipulations of single Ce ions, a quantum interface between a single spin and a single photon is observed by monitoring the temporal behavior of circularly polarized emission photons from the ion. We obtain the Larmor precession of the excited spin state, indicating the direct mapping of the excited spin states of single Ce ions onto the polarization states of the fluorescence photons. Furthermore, with both optical and spin access, we all-optically demonstrate coherent population trapping of a single Ce spin qubit. To scale up the system for further applications, we present the nano-scale engineering of rare earth species in YAG crystal, with the acquirement of the production yield of Ce and Pr implantation (53 and 91% respectively). The optical and spin properties of single implanted Ce ions show fairly good performance, which paves the way towards coherent coupling of single rare earth qubits in a small volume. Besides the study of single Ce ions, the spectroscopy properties of single praseodymium ions in YAG crystal are investigated. We obtain the photoluminescence excitation spectrum and the hyperfine splitting of a single praseodymium ion. All-optical control of a single praseodymium ion is achieved. In this dissertation, we solve the key problem of optically detecting, spectrally studying and coherently manipulating single rare earth ions in solids. The developed methods including optical detection, initialization, readout, coherent control and nanoscale production of single rare earth elements in crystals offer novel and powerful tools for understanding and exploring the fundamental spectroscopy properties of single rare earth elements in solids. It paves a way towards constructing scalable quantum network based on single rare earth solid-state qubits.
Single photons are highly desirable for encoding, sending and processing quantum information. Suitable engineering of single quantum emitters and their environment can lead to a reliable single photon source and to mediated photon-photon interactions, essential assets for quantum communication and computation purposes. Dye molecules, our chosen single quantum emitter, have attracted much attention in recent years due to their high photostability and narrow emission lines. In this thesis I present a detailed analysis of the optical properties of single dibenzoterrylene molecules in an anthracene matrix. Single photon emission is demonstrated at room and cryogenic temperatures. In the latter case, an analysis of the second-order correlation function of the emission of a single molecule is studied at various temperatures. The data agree with a full solution of the optical Bloch equations over a wide range of temperature, and provide some insight into the dephasing process of the molecules. I then present a technique for integrating single dye molecules with nano-photonic devices. These are hybrid plasmonic waveguides, designed to guide light and to enhance the interaction with a single emitter by a tight confinement of the waveguide mode. Single molecules are deposited over the waveguides and a first evidence of coupling is presented. I conclude with proposed improvements to the optical setup and to the deposition methods. Finally, I present another type of integrated device, where the light matter interaction is enhanced by inserting single quantum emitters into coupled optical resonators. The theoretical description for the case of resonant two-photon absorption from single atoms in coupled photonic crystal cavities is developed. I conclude by proposing a possible realisation of this system, and describing the first steps towards the fabrication of such a device.
Optical spectroscopy of single impurity molecules in low-temperature solids provides an exquisitely sensitive probe of the structure and dynamics of the specific local environment around the single molecule, unobscured by ensemble averaging. Effects such as spectral diffusion, perturbations by external fields, changes in molecular photophysics, optical switching, dynamics due to amorphous system physics, magnetic resonance of a single molecular spin, and perturbations due to nearby charge transport have all been observed. Because properly chosen single molecules act as two- and three-level quantum systems, a variety of quantum optical effects may be observed, such as photon antibunching, optical Stark effect, and single-photon emission on command. When they carry spin in their ground and excited states, single molecules and other nano-objects open many more experiments in quantum optics and nanophysics, because they are sensitive to magnetic interactions and can serve as readouts for spin systems with long coherence times. As a new class of nanoscale experiments, the ability to optically pump and probe single 2–3-nm-sized molecules in complex systems provides a new window into the local dynamics and heterogeneity of solids.
The generation of non-classical states of light [1] is an important scientific challenge with many potential applications. For example, squeezed states of light [2] have generated much interest in the past decade, because measurements can be performed with noise levels that are lower than those available with classical light. A particularly novel non-classical source of light is a triggered single-photon source: a source that has the property to emit with a high degree of certainty one (and only one) photon at a user-specified time. Such a deterministic source of single photons can be important for quantum information processing [3], quantum cryptography [4] and certain quantum computation problems [5]. A single quantum emitter such as a molecule or an atom is a good potential source because it can emit only one photon at a time. The emitted light is antibunched [6, 7] and consists of single photons separated by random time intervals, which depend on the excited state lifetime and on the pumping rate. Thus triggered emission of single photons can be obtained with controlled excitation of a single emitter. The first demonstration of a single photons source [8] using a single molecule in a solid is based on a rapid adiabatic passage method [9] to excite the molecule. In the effective spin picture, the Bloch vector follows the driving magnetic field adiabatically if the laser-molecule frequency detuning is swept slowly through the resonance. Since the passage inverts the field, the system goes from the ground to the excited state. From this fluorescent state the molecule can emit only a single photon. The molecular resonance is swept by Stark effect at fixed laser frequency. This is done easily by applying a radio frequency electric field to the molecule. The results showed photon states with statistics that differ radically from those of coherent states. But this method requires cryogenic temperatures (2K) since narrow zero phonon lines are needed for the adiabatic following. At room temperature a source of triggered single photons based on the concept of pulsed optical excitation of a single highly fluorescent molecule has been demonstrated [10]. A short pulse of green laser light pumps the molecule from the ground state to a vibronically excited level of the first electronic excited state. After fast (ps) relaxation to the lowest electronic excited state, the molecule subsequently emits a single photon. A frequency-doubled mode-locked Nd-YAG laser is used to pump highly stable single terrylene molecules contained in a p-terphenyl molecular crystal where they are protected from exposure to diffusing quenchers (such as oxygen). The result shows that the photons are emitted after the laser pulses at times separated by the laser repetition time within the lifetime of terrylene in p-terphenyl. Since the pump pulse width is very short compared to the fluorescence lifetime, the probability of two (or more) photon emission per pulse is always zero. At the maximum laser power, 90% of the laser pulses lead to a single photon emission. The parameters of this source (repetition rate and single photon generation probability) were limited only by the laser system used; nevertheless, this performance already surpasses that of previous work. The high performance combined with the vanishing two photon emission probability and the simplicity of this source suggests that it can be considered for a variety of quantum optical experiments and for secure quantum cryptography [11]. Semiconductor quantum dots (QD) are other potential systems for single photon generation [12]. QDs are often referred to as “artificial atoms” because many of their optical properties, such as ultranarrow transitions, arise from discrete, atomic-like energy levels [13, 14]. To reinforce this analogy a study of the photon antibunching in this system was needed. QDs differ from atoms and molecules since the creation of multiple excitons is possible in QDs. Consequently, if two electron-hole pairs can radiatively recombine, the simultaneous emission of two photons from a QD is then possible. Thus different regimes of photon statistics were expected because the probability of multiple exciton creation depends on excitation intensity. The fluorescence intensity correlation function of a single CdSe quantum dot has been investigated [15] using a coincidence setup (start-stop experiment). A strong photon antibunching has been observed over a large range of intensities (0.1-100 kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process [16], which suppresses multi-photon emission in these colloidal quantum dots. This result shows that room temperature single photon sources can be achieved with colloidal nanoparticles.