Rare-earth doped crystals have proven to be of considerable interest for quantum information processing, with demonstrations of quantum memories and the potential for coherent microwave–optical conversion as part of a quantum network. By interfacing these rare-earth based technologies with superconducting devices, they can complement existing technologies — which form the basis of many state-of-the-art quantum computers — forming a hybrid quantum information processing architecture exhibiting the advantages of fast superconducting processors alongside the extended coherence time and frequency conversion available from rare-earths. This thesis demonstrates the suitability of a widely used rare-earth doped crystal, yttrium orthosilicate (Y2SiO5 or YSO) doped with 145Nd or with 171Yb, as a substrate for fabrication of superconducting resonators. Designs for these resonators are presented with simulations of their electromagnetic modes, and their fabrication detailed. These devices are cooled to milliKelvin temperatures in a dilution refrigerator in order to investigate properties of the device and its coupling to electron spin transitions within the substrate where a coupling rate of the order gens/2π =1.2–7.1 MHz is observed. Dielectric loss due to two-level systems is measured with a loss-tangent tan δ = 4*10−6, and a coupling between resonator and spins is observed to have a high cooperativity C = 8–250. The superconducting resonator is then used to perform pulsed spectroscopy of electron spin resonance (ESR) transitions in the high-cooperativity regime, measuring two-pulse coherence times of T2 = (409 ± 14) μs for 200 ppm 145Nd:Y2SiO5 and T2 = (1170 ± 90) μs for 50 ppm 171Yb:Y2SiO5. Chirped pulses are used to invert spin magnetisation and drive arbitrary rotations, enabling the measurement of relaxation dynamics and Rabi oscillations. Dynamical decoupling sequences are explored as a method for extending coherence time. Experiments exploiting coherence-enhancing 'near-ZEFOZ' transitions in 171Yb are proposed, and continuous-wave measurements are used to demonstrate the working principle of such a scheme.
Rare-earth doped crystals have proven to be of considerable interest for quantum information processing, with demonstrations of quantum memories and the potential for coherent microwave–optical conversion as part of a quantum network. By interfacing these rare-earth based technologies with superconducting devices, they can complement existing technologies — which form the basis of many state-of-the-art quantum computers — forming a hybrid quantum information processing architecture exhibiting the advantages of fast superconducting processors alongside the extended coherence time and frequency conversion available from rare-earths. This thesis demonstrates the suitability of a widely used rare-earth doped crystal, yttrium orthosilicate (Y2SiO5 or YSO) doped with 145Nd or with 171Yb, as a substrate for fabrication of superconducting resonators. Designs for these resonators are presented with simulations of their electromagnetic modes, and their fabrication detailed. These devices are cooled to milliKelvin temperatures in a dilution refrigerator in order to investigate properties of the device and its coupling to electron spin transitions within the substrate where a coupling rate of the order gens/2π =1.2–7.1 MHz is observed. Dielectric loss due to two-level systems is measured with a loss-tangent tan δ = 4*10−6, and a coupling between resonator and spins is observed to have a high cooperativity C = 8–250. The superconducting resonator is then used to perform pulsed spectroscopy of electron spin resonance (ESR) transitions in the high-cooperativity regime, measuring two-pulse coherence times of T2 = (409 ± 14) μs for 200 ppm 145Nd:Y2SiO5 and T2 = (1170 ± 90) μs for 50 ppm 171Yb:Y2SiO5. Chirped pulses are used to invert spin magnetisation and drive arbitrary rotations, enabling the measurement of relaxation dynamics and Rabi oscillations. Dynamical decoupling sequences are explored as a method for extending coherence time. Experiments exploiting coherence-enhancing 'near-ZEFOZ' transitions in 171Yb are proposed, and continuous-wave measurements are used to demonstrate the working principle of such a scheme.
Quantum light-matter interfaces that reversibly map the quantum state of photons onto the quantum states of atoms, are essential components in the quantum engineering toolbox with applications in quantum communication, computing, and quantum-enabled sensing. In this talk I present our progress towards developing on-chip quantum light-matter interfaces based on nanophotonic resonators fabricated in rare-earth-doped crystals known to exhibit the longest optical and spin coherence times in the solid state. We recently demonstrated coherent control of neodymium (Nd3+) ions coupled to yttrium orthosilicate Y2SiO5 (YSO) photonic crystal nano-beam resonator. The coupling of the Nd3+ 883 nm 4I9/2-4F3/2 transition to the nano-resonator results in a 40 fold enhancement of the transition rate (Purcell effect), and increased optical absorption (~80%) - adequate for realizing efficient optical quantum memories via cavity impedance matching. Optical coherence times T2 up to 100 μs with low spectral diffusion were measured for ions embedded in photonic crystals, which are comparable to those observed in unprocessed bulk samples. This indicates that the remarkable coherence properties of REIs are preserved during nanofabrication process. Multi-temporal mode photon storage using stimulated photon echo and atomic frequency comb (AFC) protocols were implemented in these nano-resonators. Our current technology can be readily transferred to Erbium (Er) doped YSO devices, therefore opening the possibility of efficient on-chip optical quantum memory at 1.5 μm telecom wavelength. Integration with superconducting qubits can lead to devices for reversible quantum conversion of optical photons to microwave photons.
Yttrium orthosilicate ( \nY \n2 \nSi \nO \n5 \n, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces and shows promise for coherent microwave-optical conversion owing to its favorable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on \nY \n2 \nSi \nO \n5 \n and hence we investigate here the use of \nY \n2 \nSi \nO \n5 \n as an alternative to sapphire or silicon substrates for superconducting-hybrid-device fabrication. A \nNb \nN \n resonator with frequency 6.008 GHz and low-power quality factor \nQ \n≈ \n400 \n000 \n is fabricated on a \nY \n2 \nSi \nO \n5 \n substrate doped with isotopically enriched \n145 \nNd \n. Measurements of dielectric loss yield a loss tangent \ntan \n \nδ \n= \n4 \n× \n10 \n− \n6 \n, comparable to that of sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic \n145 \nNd \n spin and the coupling strength between the resonator and the electron spins is in the high-cooperativity regime ( \nC \n= \n30 \n). These results demonstrate that \nY \n2 \nSi \nO \n5 \n is an excellent substrate for low-loss, high- \nQ \n microwave resonators, especially in applications for coupling to optically accessible rare-earth spins.
Rare-earth-ion doped crystals are an interesting system for quantum computing investigations due to their long optical and hyperfine coherence times. In particular, the ground-state coherence times can be longer than one second, which makes them perfect for long-term storage of quantum states. They also have the advantage of being a solid-state system. At present, many of the fundamental requirements for quantum computing have been demonstrated using rare-earth-ions; however, quantum state readout has yet to be achieved. The strong-coupling regime is of great importance in cavity quantum electrodynamics, as in this regime interactions between single atoms and single photons can be realized. Strong coupling thereby provides a method of enabling qubit-qubit interactions. It is desirable to achieve this regime with rare-earth-ion doped resonators, as it would enable on-demand readout of quantum states, a necessity for quantum computing, and provide the scalability required for implementation of quantum networks. This thesis describes the work that has gone into investigating the suitability of rare-earth-ion doped whispering-gallery mode resonators for quantum computing applications. A theoretical analysis of the strong-coupling regime is performed, followed by experiments investigating the properties of ions in resonators, and of new rare-earth-ion materials. A final set of experiments studies a new quantum memory protocol for use with rare-earth-ions. A theoretical investigation is undertaken to determine whether the strong-coupling regime will be attainable with rare-earth-ion doped whispering-gallery mode resonators. Emphasis is given to determining which rare-earth-ion materials will have the best properties when used as resonators. Specific applications of the strong-coupling regime are examined for this system; in particular, whether it will be possible to demonstrate the reversible transfer of quantum states, and detection of single dopants. Experiments are performed, measuring the properties of millimetre-sized Pr3+:Y2SiO5 resonators. Direct measurements of cavity quantum electrodynamics parameters are made using photon echoes, giving good agreement with theoretical predictions. Interactions between the atom and cavity are observed and a theoretical model is presented to describe these effects. Thus, a more realistic picture is obtained of the experimental requirements for achieving the strong-coupling regime. The properties of different rare-earth-ion doped materials are measured and analysed to determine their suitability for quantum computing applications. The spin Hamiltonian parameters of Pr3+:YAG are measured using Raman heterodyne spectroscopy, and then used to find the optical transitions with zero first-order Zeeman shift, as this should allow the coherence time to be extended to a point where this system is useful for resonator applications. Hyperfine transitions with zero first-order Zeeman shift are also calculated for Er3+:Y2SiO5, with the aim of enabling long-term storage in a telecommunication-wavelength memory. The thesis concludes with an experimental investigation into a new quantum memory technique that does not require spectral hole burning. The hybrid photon echo rephasing protocol uses strong optical pulses to perform rephasing, thus utilizing the advantageous properties of the two-pulse photon echo. Furthermore, the use of external broadening fields removes the usual problem of noise from amplified spontaneous emission. The properties of this memory are examined using a Pr3+:Y2SiO5 crystal.
In the context of quantum information technologies superconducting qubits (SQs) are very attractive devices for the manipulation of quantum states, and present themselves as one of our best candidates to build a quantum processor. They couple naturally to microwave photons, for which suitable quantum memories or a long distance propagation channel don’t exist. A way around these limitations is to turn these microwave photons into optical ones by building a quantum frequency converter: a device by which the frequency of a photon can be changed while preserving its non-classical correlations. Then, optical fibres could be used to link distant SQ-based devices together, facilitating the creation of a network of quantum computers. Moreover, SQs could then be coupled to quantum memories compatible with photons at optical frequencies, which are the most well developed kind of quantum memories at the present time. This thesis explores the possibility to convert single microwave photons into optical photons using erbium doped in a yttrium orthosilicate crystal (Er3+:Y2SiO5). Er3+:Y2SiO5 is a good candidate because it has a naturally occurring optical transition near 1536 nm close to the point where silica optical fibres show their minimum loss. A microwave transition can be found in two different ways: one way is to use the 167 isotope of erbium, which is the only stable isotope that shows hyperfine splitting as it has non-zero nuclear spin. The hyperfine structure of the ground state of 167Er3+:Y2SiO5 spans over about 5 GHz. The other possibility is to use the other stable isotopes of erbium and Zeeman split their ground state using an external magnetic field. A microwave transition near 5 GHz can be achieved with moderate magnetic fields due to the high 𝑔-factors of Er3+:Y2SiO5. The physical process of interest is a three wave mixing process involving two fields at optical frequencies and one field at microwave frequencies. In order to boost the efficiency of the frequency conversion process the Er3+:Y2SiO5 crystal is placed inside a microwave and an optical resonator. The problem is first explored from the theoretical point of view, where a nonlinear coefficient Λ(2) is derived (analogous to the 𝜒(2) often used in nonlinear optics), and the interaction between cavity modes and the nonlinear medium is studied. It is predicted that with a sample cooled down to millikelvin temperatures total frequency conversion between microwave and optical fields can be achieved. A preliminary hole burning spectroscopy experiment is performed with the objective of reconstructing the hyperfine structure of the excited state of 167Er3+:Y2SiO5, but the complexity of the problem makes it too difficult to achieve this goal. Then a series of experiments are shown, aimed at determining whether or not frequency conversion at the single photon level is achievable using the even isotopes of erbium in a magnetic field. These experiments are based in the Raman heterodyne spectroscopy technique, which is used in combination with electron paramagnetic resonance and optical absorption spectroscopy. In all experiments the sample is cooled down to cryogenic temperatures near 4 K. A first experiment shows that the frequency conversion process exists in Er3+:Y2SiO5, in a setup where only a microwave resonator is used, but not an optical one. A second experiment is performed in a similar setup, this time presenting a quantitative study of the properties of the frequency conversion process, and its comparison with the theoretical model previously derived. A third experiment is performed, which incorporates an optical cavity to the system. The interaction between the erbium ions and the optical cavity introduces a whole new range of experimental complications, which are studied and discussed. Then, the frequency conversion signal is studied anew, showing an unexpected highly non-linear scaling behaviour with the input powers. A hypothesis explaining this unexpected behaviour is given, referring to stray optical absorption in the inhomogeneous line of Er3+:Y2SiO5 (and in particular 167Er3+:Y2SiO5), which can be bleached out under certain circumstances due to spectral hole burning effects. The overall maximum frequency conversion efficiency observed is of 3 × 10^−4 per Watt of pump laser power. While this value is still far from the target several ways of improvement are proposed, including cooling down the system to millikelvin temperatures, increasing the dopant concentration and modifying the geometry of the resonators.
Quantum light-matter interfaces that can reversibly map quantum information between photons and atoms are essential for building future quantum networks. Crystals doped with rare-earth ions (REIs) are an attractive solid-state platform for such light-matter interfaces due to their exceptional optical and spin coherence properties at cryogenic temperatures. Building scalable REI-based technology has proven to be challenging due to the inherently weak coupling of REIs with light. This thesis explores the integration of REIs with nanophotonic resonators to overcome this weak light-matter interaction and enable efficient, scalable quantum light-matter interfaces. Specifically, this work focuses on the development of quantum nanophotonics with ytterbium in yttrium orthovanadate. This thesis begins with an introduction to a nanophotonic platform based on photonic crystal cavities fabricated directly in rare-earth host materials and highlights the initial successes of this platform with neodymium-doped materials. This motivates an examination of the optical and spin coherence properties of 171Yb:YVO4, a REI material that was previously unexplored for quantum technology applications. This material is found to have strong optical transitions compared to other REI-doped materials, a simple energy level structure, and long optical and spin coherence lifetimes. The focus then turns to the detection and coherent manipulation of single ytterbium ions coupled to nanophotonic cavities. The Purcell-enhancement in these cavities enables efficient optical detection and spin initialization of individual ytterbium ions. We identify ions corresponding to different isotopes of ytterbium and show that the coupling of electron and nuclear spin in ytterbium-171 at zero-field gives rise to strong electron-spin-like transitions that are first-order insensitive to magnetic field fluctuations. This allows for coherent microwave control and the observation of long spin coherence lifetimes at temperatures up to 1 K. We then make use of the optical selection rules and energy structure of 171Yb:YVO4 to demonstrate high-fidelity single-shot optical readout of the spin state. These results establish nanophotonic devices in 171Yb:YVO4 as a promising platform for solid-state quantum light-matter interfaces.
Rare-earth ions (REIs) in solid hosts have important applications in the storage and conversion of quantum information at optical and microwave frequencies. Yttrium orthosilicate is a widely used host for REIs, with excellent optical properties, but its compatibility with superconducting resonators and circuits for quantum technology is not known. This study uses Y2SiO5 directly as a substrate for superconducting devices, revealing dielectric losses comparable to those of sapphire. This enables fabrication of high-quality microwave devices coupled to optically accessible rare-earth spins, pointing to applications in quantum storage and microwave-to-optical conversion.