Abstract : Rare-earth ions are useful for the implementation of quantum memory. In particular, the energy levels and magnetic/spin properties of select ions may enable storage of quantum states. Also, the optical activity may enable optical read-out techniques and optical transmission of qubit states. In addition, the energy levels of ions can be used for designing superconducting qubits where the ionic configuration tunes the Hamiltonian for quantum computing. In the more near term, ions embedded in superconductor heterostructure can be used to implement cryogenic memory for superconducting digital computing. Our concept involves embedding rare-earth ions in superconductor devices to provide a low dissipation environment and access to the systems Hamiltonian. In this report we show results and success in integrating rare-earth neodymium by ion implantation in thin films of niobium and niobium-based heterostructure devices. We model the ion implantation process and confirm the concentration of the neodymium in niobium by energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope (SEM). We study the effect of optical illumination and magnetic stimulus on the transport of the devices at room temperature, as it is important to understand the films and devices so they can properly designed and optimized for utility as quantum memory. We find that the magnetic field has a strong effect on the response. We also find that the selectivity in the optical effect when using a broadband halogen lamp in comparison to a light emitting diode (LED) with selectivity in the excitation wavelength, reinforces the role of the magnetic and optically active Nd rare-earth ions on the device response.
Abstract : Rare-earth ions are useful for the implementation of quantum memory. In particular, the energy levels and magnetic/spin properties of select ions may enable storage of quantum states. Also, the optical activity may enable optical read-out techniques and optical transmission of qubit states. In addition, the energy levels of ions can be used for designing superconducting qubits where the ionic configuration tunes the Hamiltonian for quantum computing. In the more near term, ions embedded in superconductor heterostructure can be used to implement cryogenic memory for superconducting digital computing. Our concept involves embedding rare-earth ions in superconductor devices to provide a low dissipation environment and access to the systems Hamiltonian. In this report we show results and success in integrating rare-earth neodymium by ion implantation in thin films of niobium and niobium-based heterostructure devices. We model the ion implantation process and confirm the concentration of the neodymium in niobium by energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope (SEM). We study the effect of optical illumination and magnetic stimulus on the transport of the devices at room temperature, as it is important to understand the films and devices so they can properly designed and optimized for utility as quantum memory. We find that the magnetic field has a strong effect on the response. We also find that the selectivity in the optical effect when using a broadband halogen lamp in comparison to a light emitting diode (LED) with selectivity in the excitation wavelength, reinforces the role of the magnetic and optically active Nd rare-earth ions on the device response.
Optically active rare-earth Neodymium (Nd) ions are integrated in Niobium (Nb) thin films forming a new quantum memory device (Nd:Nb) targeting long-lived coherence times and multi-functionality enabled by both spin and photon storage properties. Nb is implanted with Nd spanning 10-60 keV energy and 1013-1014 cm-2 dose producing a 1- 3% Nd:Nb concentration as confirmed by energy-dispersive X-ray spectroscopy. Scanning confocal photoluminescence (PL) at 785 nm excitation are made and sharp emission peaks from the 4F3/2 -< 4I11/2 Nd3+ transition at 1064-1070 nm are examined. In contrast, un-implanted Nb is void of any peaks. Line-shapes at room temperature are fit with Lorentzian profiles with line-widths of 4-5 nm and 1.3 THz bandwidth and the impacts of hyperfine splitting via the metallic crystal potential are apparent and the co-contribution of implant induced defects. With increasing Nd from 1% to 3%, there is a 0.3 nm red shift and increased broadening to a 4.8 nm linewidth. Nd:Nb is photoconductive and responds strongly to applied fields. Furthermore, optically detected magnetic resonance (ODMR) measurements are presented spanning near-infrared telecom band. The modulation of the emission intensity with magnetic field and microwave power by integration of these magnetic Kramer type Nd ions is quantified along with spin echoes under pulsed microwave π-π/2 excitation. A hybrid system architecture is proposed using spin and photon quantum information storage with the nuclear and electron states of the Nd3+ and neighboring Nb atoms that can couple qubit states to hyperfine 7/2 spin states of Nd:Nb and onto NIR optical levels excitable with entangled single photons, thus enabling implementation of computing and networking/internet protocols in a single platform.
Without using any trapping lasers, rare-earth (RE) ions can be doped into some nanofabricated materials in form of nanocrystals in a solid state matrix, thus rendering a natural trap for potential application as a quantum computing device. In particular, passive nuclear spin states of RE ions can be utilized as (i) optical quantum memory (with huge density as yielded from nanocrystals), (ii) qubits by encoding in ground hyperfine states and operated as (iii) two-qubit gate when transferring them to active magnetized electronic states. A feasible implementation of qubits is realised by using those doped RE ions whose open-shell (for inner highly shielded 4f shell electrons) energy levels and statevectors are well described by the crystal field (CF) interaction and hyperfine coupling in which the Hamiltonian is composed of various freeion interactions, the CF potential, electronic and nuclear Zeeman effects as well as the hyperfine interaction. For those qubit encoded in the two ground hyperfine states of RE ions, they can have fairly long lifetimes of days and coherence times (from 1.3s for Er3+ to 6 hours for Eu3+) due to the unique spectroscopic properties of RE ions. Because of the recent progress in optical single-ion detection for addressing individual qubits with a specific frequency channel, RE ions could be packed in a solid state 3D matrix at nanometer spacing, hence rendering very high qubit densities for quantum memories. Meanwhile, this configure also leads to strong dipole-dipole interactions (or high connectivity) between qubits which are not nearest neighbors. In this study, Kramers ions like 167Er and 171Yb doped in yttrium orthosilicate Y2SiO5 (YSO) nanocrystals will be investigated as a promising system for quantum computing because of their weak spin bath and possibility for being directly milled in single nanocrystals as nano-resonators. By using the famous Wien2k package to carry out the first-principles calculations, we can obtain the CF parameters and hyperfine constants as well as various electronic properties like energy gap and band structure for those Kramers RE ions doped in YSO. We shall present some essential findings from first-principles calculations to obtain various properties of the ground and excited CF and hyperfine states for those RE ions with an aim to optimize RE ions doping in YSO nanocrystals for quantum memories and qubits, contributing to scalable quantum computing architectures.
Microwave and optical transitions of rare-earth ions enable microwave-to-optical frequency transduction. This transduction is a coherent process; therefore, the narrow linewidths and long coherence times of rare-earth ions are crucial for achieving efficient frequency conversion. Particularly long coherence times can be achieved with small-spin materials, as unpaired electronic and nuclear spins introduce magnetic noise, serving as a decoherent source of the transitions. However, complete spin-free rare-earth materials cannot exist without charge compensation. Consequently, residual spins in existing rare-earth materials act as a bottleneck, limiting both their spin and optical coherence times. Rather than focusing on nearly spin-free materials, this thesis investigates the microwave and optical transitions of rare-earth ions in materials that are fully concentrated with electronic spins. Recent studies on rare-earth magnetic materials have demonstrated narrow optical linewidths resulting from magnetic ordering and a particularly strong coupling between microwave cavity modes and the collective excitations of rare-earth ions. Although these recent studies are noteworthy, the energy-level transitions, especially optical transitions, of rare-earth ions in magnetic materials remain largely unexamined. In this thesis, various spectroscopic studies are employed to characterise their transition properties and to develop a model of energy levels. The model presented is expected to be applicable to other rare-earth magnetic materials, thereby opening avenues for further investigation in this class of systems. Two types of rare-earth magnetic materials are investigated: neodymium gallate with a stoichiometric concentration of neodymium ions, and gadolinium vanadate doped with a low (ppm) concentration of erbium ions. Microwave and optical transmission spectroscopy are employed under externally applied magnetic fields to characterise the energy-state transitions of the rare-earth ions in both materials. The magnetic phases and corresponding spin configurations influence the observed spectra, making conventional approaches to spectral simulation inadequate. To address this challenge, mean field and magnon-coupled crystal-field models are developed in this thesis to predict magnon and optical transition spectra. These simulations exhibit good agreement with experimental results, allowing for the characterisation of transition properties. Following the characterisation of the material, a new transduction approach is introduced, developed, and implemented using doped rare-earth ions in a magnetic material. In this scheme, coherent conversion between microwave and optical modes is achieved by exciting an optical transition of the doped ions.
This thesis describes a number of coherent processes, such as quantum information processing, superradiance and electromagnetically induced transparency, which have been experimentally implemented using rare-earth-ion-doped crystals. The rare-earths are a class of elements that have in common, an atomic structure that allows for very long lifetimes as well as coherence times, both on optical transitions and on spin transitions. These ions can be naturally trapped inside host crystals, and with the phonon vibrations removed by cooling down to cryogenic temperatures (< 4 K), they can be used for a number of different quantum information processes. Most experiments were carried out using PrYSO, but also other crystal types were investigated, such as Nd:YVO4, and La2(WO4)3. The characterization of the ions were done with a variety of methods, including among others, photon echo techniques and electromagnetically induced transparency. Quantum computing is a rapidly growing field and there are still many potential candidates for its implementation. In our work we have utilized the spin states of the Pr rare-earth ion as a qubit, and demonstrated arbitrary single qubit gates, which are important pieces towards a quantum computing realization in these systems. Most of the experimental work done for the thesis was carried out using an ensemble approach, that has the advantage of giving a strong readout signal, but for future scaling to multiple qubits, single instances are more promising, which is discussed. Quantum memories are crucial components in applications such as quantum networks and long distance quantum communication. In this thesis, work has been done to investigate how quantum memories can be implemented in rare-earth crystals. In particular, there are two protocols, CRIB and AFC, which were suggested with rare-earth crystals directly in mind, and both of these require high optical depth, for maximum recall efficiency. In this thesis, implications of being in the high optical depth regime, with phenomena such as superradiance, a collective effect that could cause the stored light to be immediately reemitted, and slow light effects that come from performing the storage inside spectrally structured materials, were investigated. In order to carry out the phase-sensitive experiments, a laser system with a very narrow linewidth of ~1 kHz at 606 nm was constructed, by locking the laser to a semi-persistent spectral hole. In addition, many experiments required advanced pulse shapes, such as complex sechyp pulses or pulses obtained from optimal control theory. In order to be able to accurately create such shapes, an elaborate system using an arbitrary waveform generator and two well calibrated AOMs, controlled from a computer, was also built.
Abstract : The research objective of this Directors Research Initiative was to work on developing solid-state quantum memory using cryogenically cooled rare-earth-ion-doped crystal, Nd3 :GaN. The samples were grown using the US Army Research Laboratorys molecular beam epitaxy (MBE) capabilities. The approach is based on the coherent transient interaction between light and the ions in a semiconductor. We investigated the energy level structure of the neodymium (Nd) ions embedded into the semiconductor crystal lattice, gallium nitride (GaN), and developed a quantum description of the 2- and 3-pulse photon echo effect. Epitaxial GaN:Nd was grown by plasma-assisted molecular beam epitaxy in a modular Gen II reactor using liquid gallium, solid Nd, and a nitrogen plasma. The photoluminescence (PL) spectrum from the lowest 4F3/2 multiplet to all 4I9/2 multiplets was measured by a free-space variable amplifier silicon diode. A demonstration of the coherence properties requires a high-quality sample with a narrow homogeneous broadening. For this reason, the PL linewidth was characterized for various growth conditions. The experiments were carried out to observe coherent effects, and various experimental setups are described. In parallel with experiments on the coherence properties of Nd3 :GaN, we also investigated the effect of a varying electric field on the Nd PL spectrum to provide a tunable memory. To vary the applied field, we designed and grew a series of Nd-doped GaN p-i-n structures, strain-balanced superlattice structures, and periodic unbalanced structures.