The interaction of ultrasound and light in biological tissues results in a small amount of the scattered light being shifted relative to the carrier frequency (typically 1 part in 108). We have developed an inherently efficient and low noise quantum memory based technique to selectively absorb these ‘ultrasound tagged’ photons in a pair of atomic frequency combs, and recover them delayed in time as a photon echo. In this manner we have demonstrated record ultrasoundmodulated sideband-to-carrier discrimination (49dB). Further, we confirm that the technique is compatible with highly scattering samples, and present initial acoustic pulse tracking measurements. This strongly suggests the suitability of the technique for biological tissue imaging.
The interaction of ultrasound and light in biological tissues results in a small amount of the scattered light being shifted relative to the carrier frequency (typically 1 part in 108). We have developed an inherently efficient and low noise quantum memory based technique to selectively absorb these ‘ultrasound tagged’ photons in a pair of atomic frequency combs, and recover them delayed in time as a photon echo. In this manner we have demonstrated record ultrasoundmodulated sideband-to-carrier discrimination (49dB). Further, we confirm that the technique is compatible with highly scattering samples, and present initial acoustic pulse tracking measurements. This strongly suggests the suitability of the technique for biological tissue imaging.
We present results of a novel and highly sensitive technique for the optical detection of ultrasound using the selective storage of frequency shifted photons in an inherently highly efficient and low noise atomic frequency comb (AFC) based quantum memory. The ultrasound ‘tagged’ optical sidebands are absorbed within a pair of symmetric AFCs, generated via optical pumping in a Pr3+:Y2SiO5 sample (tooth separation Δ = 150 kHz, comb finesse fc ~ 2 and optical depth αL ~ 2), separated by twice the ultrasound modulation frequency (1.5 MHz) and centered on either side of a broad spectral pit (1.7 MHz width) allowing transmission of the carrier. The stored sidebands are recovered with 10-20% efficiency as a photon echo (as defined by the comb parameters), and we demonstrate a record 49 dB discrimination between the sidebands and the carrier pulse, high discrimination being important for imaging tissues at depth. We further demonstrate detector limited discrimination (~29 dB) using a highly scattered beam, confirming that the technique is immune to speckle decorrelation. We show that it also remains valid in the case of optically thin samples, and thus represents a significant improvement over other ultrasound detection methods based on rare-earth-ion-doped crystals. These results strongly suggest the suitability of our technique for high-resolution non-contact real-time imaging of biological tissues.
Quantum memories are inherently highly efficient and display low noise, making them particularly suitable for the optical detection of ultrasound. Here, we use an atomic-frequency-comb based quantum memory to demonstrate sensitive ultrasound detection, realising a 49 dB discrimination between the sidebands and the carrier. The method remains valid in the case of optically thin samples and thus represents a significant improvement over other ultrasound detection methods based on rare-earth-ion-doped crystals. Furthermore, we show that this non-destructive non-contact approach is also compatible with highly scattering samples and suggest its particular suitability for the real-time imaging of biological tissues.
This thesis describes experiments aimed at developing a key component in applications such as quantum networks and long-distance quantum communication systems: quantum memories. Quantum memories for light using photon-echo and coherent rephrasing techniques have been developed. The work is carried out on rare-earth-ion-doped inorganic crystals. Several properties are associated with these crystals such as long coherence times and long-lived ground state sublevels that are particularly interesting in connection applications in the area of quantum information science. It is demonstrated the preparation of optical inhomogeneous absorption profiles such that the light only interacts with a specific transition at a selected frequency. Narrow absorbing structures with widths of <100 kHz have been prepared with no absorption in the surrounding spectral interval. Most of the experiments were carried out using Pr3+:Y2SiO5. However, in our search for suitable a Λ-system in the material under consideration, we also performed spectral hole-burning spectroscopy in Nd:Y2SiVO4. As part of the search for host materials for Pr, Pr:La2(WO4)3 crystal was characterized using a variety of methods, such as, hole burning and photon-echoes. Electromagnetically induced transparency was also observed in this crystal. The most important criteria for evaluating quantum memory performance and the most important physical requirements are discussed. After investigating the standard photon echo techniques and controlled reversible inhomogeneous broadening (CRIB) for quantum state storage, we considered an efficient multimode quantum memory protocol; the atomic frequency comb (AFC) protocol. Combs with peaks of widths 100-300 kHz and an optical depth of~ 10 were created by optical pumping inside an emptied region of the inhomogeneous profile of Pr3+:Y2SiO5, allowing us to improve the efficiency of light storage. Light storage combining a photon-echo technique based on an AFC and spin-wave storage have been demonstrated, and an experimental AFC scheme for the storage of weak coherent light pulses in Pr3+:Y2SiO5 is presented. Finally, we have investigated superradiance and slow light effects. Both these effects occur in the high optical depth regime and can influence the memory performance. Under certain conditions superradiance results in the immediate re-emission of the stored light in a single burst of coherent radiation, and inside spectrally structured materials, slow light effects may result when storage is performed.
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.
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.
Long-lived sub-levels of the electronic ground-state manifold of rare-earth ions in crystals can be used as atomic population reservoirs for photon echo-based quantum memories. We measure the dynamics of the Zeeman sub-levels of erbium ions that are doped into a lithium niobate waveguide, finding population lifetimes at cryogenic temperatures down to 0.7 K as long as seconds. Then, using these levels, we prepare and characterize atomic frequency combs (AFCs), which can serve as a memory for quantum light at 1532 nm wavelength. The results allow predicting a 0.1% memory efficiency, limited mainly by unwanted background absorption that we believe to be caused by excitation-induced erbium spin flips and frequency shifting due to two-level systems or non-equilibrium phonons. Hence, while it should be possible to create an AFC-based quantum memory in E r 3 + : T i 4 + : L i N b O 3 , improved crystal growth together with optimized AFC preparation will be required to make it suitable for applications in quantum communication.