Recent developments in the field of optical clocks and quantum information have fueled the necessity for both fiber phase noise cancellation and laser stabilisation. The Quantum Information group at Lund University is conducting research on quantum computation experiments based on rare earth ion doped crystals and also application of slow light effects induced by spectral hole burning in these rare earth ion doped crystals. Slow light effects can significantly improve laser stabilisation by increasing effective cavity length without inducing mechanical noise. In this thesis a fiber phase noise cancellation setup was designed and built. The fiber stabilisation will be used in a setup where a laser is stabilised using a slow light crystal cavity as frequency reference and a setup was designed for this application. By partially reflecting back light through a fiber and by performing a phase measurement and through feedback control, the fiber phase noise can be effectively cancelled. A method for first creating a slow light crystal cavity and then locking the laser to one of the resonance peaks of the cavity was planned and designed incorporating two fiber locking setups for phase noise cancellation. Suitable components for building the system was investigated and put together. Fiber locking experiment was then carried out.
Recent developments in the field of optical clocks and quantum information have fueled the necessity for both fiber phase noise cancellation and laser stabilisation. The Quantum Information group at Lund University is conducting research on quantum computation experiments based on rare earth ion doped crystals and also application of slow light effects induced by spectral hole burning in these rare earth ion doped crystals. Slow light effects can significantly improve laser stabilisation by increasing effective cavity length without inducing mechanical noise. In this thesis a fiber phase noise cancellation setup was designed and built. The fiber stabilisation will be used in a setup where a laser is stabilised using a slow light crystal cavity as frequency reference and a setup was designed for this application. By partially reflecting back light through a fiber and by performing a phase measurement and through feedback control, the fiber phase noise can be effectively cancelled. A method for first creating a slow light crystal cavity and then locking the laser to one of the resonance peaks of the cavity was planned and designed incorporating two fiber locking setups for phase noise cancellation. Suitable components for building the system was investigated and put together. Fiber locking experiment was then carried out.
An electromagnetic wave propagating through an optical fiber is sensitive to external disturbances, such as mechanical vibrations and temperature and pressure fluctuations, which add phase noise onto the wave. For high-precision frequency-based applications this phase noise has to be removed. By partially reflecting light from the remote end of the fiber back through the fiber, and comparing it to light that has not passed through the fiber, their beat note can be phase locked to a stable reference, using an acousto-optic modulator for phase noise compensation. This effectively cancels the phase noise at the remote end of the fiber. In this project, two such fiber noise cancellation systems have been successfully assembled. The fiber noise cancellation systems will be used for laser locking to a slow light crystal cavity. Using spectral hole burning, a very low group velocity can be obtained in rare-earth-ion-doped crystals. By depositing mirrors on such a crystal, it could act as a reference cavity with a very long effective length, but without the increased difficulty of isolating the cavity from mechanical vibrations associated with a physically long cavity. By locking a laser to a slow light crystal cavity, an improved stability could potentially be achieved. A possible setup for slow light crystal cavity laser locking experiments, where the fiber noise cancellation systems have been integrated, is also presented in this thesis.
In this thesis the application of slow light effects to the area of laser stabilisation is investigated. The frequency stability of lasers locked to external cavities is limited by the stability of the modes in the reference cavity. By using dispersive slow light effects the effective length of a cavity can be increased by several orders of magnitude, without a proportional increase in the sensitivity to fluctuations in cavity length, thereby yielding a net increase in stability. The effects are achieved through spectral hole burning in cryogenically cooled rare-earth ion doped crystals. Two sets of experiments are carried out, the first focusing on hole burning and the second focusing laser locking. Linewidths and frequency drift of the cavity spectrum are studied. Feedback control using an error signal generated from the slow light cavity is demonstrated, but successfully locking the laser is still yet to be achieved.
When doped into solid state transparent crystals, rare-earth ions can have optically excited states with milliseconds coherence time and up to six hours coherence time for hyperfine levels after being cooled down to liquid helium temperature. This long coherence property makes them unique as solid state material and attractive for quantum information applications.The rare-earth ions have narrow homogeneous linewidths on the order of ~ kHz. When doped into a crystal, the lattice structure is slightly distorted due to the size mismatch between the dopant and the host ions, resulting in slight differences in the local crystal fields at the dopant sites. Therefore, the resonance frequencies of the rare-earth ions are shifted relatively, resulting in an inhomogeneously broadened absorption profile, typically on the order of a few GHz. This provides the possibility of addressing each group of ions individually and engineering the absorption profile for different purposes by optical pumping using a narrow linewidth laser, and makes rare-earth-ion-doped crystals suitable for ensemble based quantum memories.Quantum memories are vital components for developing longdistance quantum communication. Part of this thesis project focused on making quantum memories with high efficiency and long storage time, especially using materials with low optical depth. To achieve this goal, a low finesse, unsymmetrical cavity was employed and experiments were carried out at (or close to) the impedance matched condition. Atomic frequency comb (AFC) protocol proposed for such an inhomogeneously broadened system was used to store coherent light for a certain time. 56% overall efficiency was achieved, and the main limiting factor for reaching even higher efficiency was the inefficient coupling between incoming light and the cavity mode. Other possible loss mechanisms were analyzed as well.Long-time, on-demand, quantum memory experiments were conducted with the help of two efficient radio-frequency rephasing pulses. Storage time was extended from microseconds to milliseconds after compensating (most of) the residual earth magnetic field in the lab. The opportunities and challenges of combing the cavity assistant quantum memory and the long time, on demand quantum memory were analyzed and possible solutions were proposed. Another part of the thesis project involves slow light-based applications in rare-earth-ion-doped crystals. The strong slow light effect at the presence a spectral hole is due to the fact that the light interacts with the absorbing ions outside the spectral hole off-resonantly, and most of the light energy is stored inside the ions as a polarization. For example, with a group velocity of 300 km/s, 99.9% of the pulse energy is stored in the ions when the pulse propagates inside the crystal. The slower the light propagates, the more energy is stored inside the medium. Based on this, a frequency shifter that is capable of shifting the light frequency of ± 4 MHz was demonstrated, and frequency shift up to GHz was proposed. Group velocity control by a factor of 20 and pulse compression in time by a factor of 2 were also demonstrated. The frequency shift and group velocity of the light are solely controlled by the external electric field, which makes them ideal for weak light situation. The unique property of such a frequency shifter and group velocity controller is that they could be made to accept light from an arbitrary incoming angle and polarization, for example, scattered light.
Slow light and Whispering Gallery Mode Resonators are two interesting and useful fields with applications in optical sensing and laser stabilisation among others. Slow light can lead to long interaction times which are useful in light-matter interactions such as sensitive non-linear optics. Whispering Gallery Mode Resonators have small mode volumes and strong beam confinement leading to high Quality Factors and intense trapped light. Whispering Gallery Mode Resonators exhibit very narrow resonance linewidths which are beneficial in frequency sensitive measurements. This thesis uses a Praseodymium REI doped Yttrium orthosilicate crystal, formed into a disc shaped Whispering Gallery Mode Resonator. The aim is to design a setup which can be used to combine the two fields, slow light and Whispering Gallery Mode Resonators, in an attempt to achieve ultra-high Quality Factors and ultra-narrow resonance linewidths. The thesis details the experimental mode coupling procedure learnt from a group at Max Planck Institute for the Science of Light in Erlangen, Germany. The thesis provides a foundation for Whispering Gallery Mode Resonator experimentation in the Quantum Information group in Lund, where the thesis work was undertaken. The resulting knowledge gained is applied to design a setup holder for use in a cryostat, which provides the necessary temperatures to create slow light structures. Two beam alignment setups are constructed and used to couple into the Whispering Gallery Mode Resonator at room temperature. The first of the two setups (free beam) was difficult to operate and control over the beam alignment and focal spot shape was poor. The resulting mode coupling had a low efficiency. The second setup (fibre coupled) provided a more user friendly setup with good control over the beam alignment and focal spot shape. Results using this setup were taken for a variety of wavelengths between 593.1 nm and 610.4 nm to investigate the effect of the ions wavelength dependent absorbance, on the losses. Due to the low efficiency of the coupling in both regimes it was not possible to isolate the cause. A mismatch of focal spot size and shape is suggested as a potential area that introduces loss and therefore requires more extensive investigation. With improved beam alignment, in addition to the focal spot geometry, it should be possible to perform the absorbance measurements again in order to determine the contribution. Suggestions for further measurements including the surface roughness and mode simulations are made in an attempt to optimise the mode coupling regime to be able to accurately estimate the Quality Factor of the Whispering Gallery Mode Resonator. This is critical in order to observe changes in the Quality Factor value at cryogenic temperatures, which can be attributed to slow light effects alone.
Lasers with high spectral purity are indispensable for optical clocks and coherent manipulation of atomic and molecular qubits for applications such as quantum computing and quantum simulation. Stabilisation of the laser to a reference can provide a narrow linewidth and high spectral purity. However, widely-used diode lasers exhibit fast phase noise that prevents high fidelity qubit manipulation. Here we demonstrate a self-injection locked diode laser system utilizing a medium finesse cavity. The cavity not only provides a stable resonance frequency, but at the same time acts as a low-pass filter for phase noise beyond the cavity linewidth of around 100 kHz, resulting in low phase noise from dc to the injection lock limit. We model the expected laser performance and benchmark it using a single trapped 40Ca+-ion as a spectrum analyser. We show that the fast phase noise of the laser at relevant Fourier frequencies of 100 kHz to >2 MHz is suppressed to a noise floor of between -110 dBc/Hz and -120 dBc/Hz, an improvement of 20 to 30 dB over state-of-the-art Pound-Drever-Hall-stabilized extended-cavity diode lasers. This strong suppression avoids incoherent (spurious) spin flips during manipulation of optical qubits and improves laser-driven gates in using diode lasers with applications in quantum logic spectroscopy, quantum simulation and quantum computation.
The main aim of the present thesis is to demonstrate the efficient quantum memories for light employing coherent processes in rare-earth-ion-doped crystals. Rare-earth (RE) ions between lanthanum, with atomic number 57, and lutetium, with atomic number 71, have a partly filled 4f shell, which is spatially inside the full shells 5s, 5p, and 6s. Therefore, 4f-4f transitions are shielded from environmental noise. Less noise means longer lifetime and coherence times and, when the crystals are cooled down to about 4 K, these can be about 8 orders of magnitude longer than what is typical for electronic transitions in solids. In quantum information processing, we are looking for physical systems which are isolated from the environment and consequently have good coherence properties. The long coherence time of both optical and spin transitions, is the main attraction of rare-earth ions for quantum information processing. In the present thesis, we map a quantum state of light employing a 4f-4f transition of praseodymium doped in yttrium silicate. We mainly employed the ensemble approach and the atomic frequency comb (AFC) protocol for quantum memory demonstrations. Among several parameters that define quantum memory performance, we mostly focused on quantum memory efficiency. The overall memory efficiency is simply defined as the energy of the pulse recalled from the memory divided by the energy of the pulse sent in for storage in the memory. To maximize quantum memory efficiency, most protocols require high optical depth. This is a limitation for materials with rather good coherence properties but low optical depth. In the research described in the present thesis, we combined the use of a low finesse cavity and the ensemble approach and designed an impedance-match cavity to obtain the advantages of both approaches and thereby to implement an efficient quantum interface in a weakly absorbing solid-state medium. Based on the cavity configuration, we obtained 56% storage and retrieval efficiency, which is the highest storage and retrieval efficiency based on the AFC protocol reported in the research so far. In the present thesis, we demonstrated a narrow-band (about 2 MHz) spectral filter with a >30 dB intensity absorption/transmission ratio, which is based on the large (6 orders of magnitude) ratio between the inhomogeneous and homogeneous broadening of Pr3+ : Y2SiO5. Slow-light effects due to the strong dispersion created by the spectral filter improved the suppression of the filter by also utilizing the time domain. As an application, we detected weak diffuse light shifted by ultrasound from the carrier about 2 MHz, after a 9 cm thick biological tissue. The coupling between light and matter is directly related to the relative direction of the light polarization and transition dipole moment of atoms. I developed a simulation program that was able to investigate the polarization direction and ellipticity of polarization of optical radiation propagating through the crystal. This might open up new opportunities for the investigation of interesting physics regarding propagation effects and for the improvement of spectral engineering of RE ions in the future. In order to deal with the sharp homogeneous transitions of the RE ions, the frequency of our dye laser was stabilized against a highly stabilized Fabry-Perot cavity using the Pound-Drever-Hall locking scheme.