Scientists at Estonia’s Institute of Physics at the University of Tartu are using rare earth ions as qubits in their development of a new variation of optical quantum computers. Qubits of microcrystals, synthesized on the basis of mixed optical fluoride crystal matrices doped with rare earth element ions such as erbium and praseodymium, enabled the researchers to perform ultrafast optical quantum computations. The successful demonstration is a cursory step to achieving what the researchers described as an “ultrafast working cycle” — an achievement that would allow scientists to overcome the very short coherence time of qubits, or the extent of their operational existence in a pure quantum state. The faster the full computation cycle, the less a given surrounding environment causes interference that negatively influences the performance of the qubits. The use of rare earth ions, the researchers said, additionally supports the increased reliability of performance in quantum computations, in addition to faster operation speeds. The research team used a spectral-hole burning method that the scientists said was previously developed at the university. That process, which by definition involves the frequency-specific bleaching of the absorption spectrum of a material, generating increased transmission in the form of a spectral hole at the selected frequency, was effective in selecting a set of qubits in a microcrystal acting as a computer device. The method enabled the researchers to locate ions in a microcrystal that are well suited to function as computer qubits. According to Vladimir Hizhnyakov, a member of the Estonian Academy of Sciences and a researcher on the work from the University of Tartu, the electronic states of multiple properties of ions influence their amenability to use in quantum systems. “They must have at least two states in which the ion interaction is very weak. These states are suitable for basic quantum-logic operations on single quantum bits,” he said. “In addition, a state or states are needed in which the ion interaction is strong — these states enable quantum-logic operations with two or more qubits. All these states must have a long (milli- or microsecond) lifetime and optical transitions must be allowed between these states.” Previously, according to Hizhnyakov, scientists have primarily investigated the spin states of atomic nuclei for the role of qubits, as opposed to electronic states of rare earth ions. Finding the desired state(s) of rare earth ions was in fact not considered possible, he said. Because the frequency of atomic nuclei are a million times lower than the frequency of the quantum bits used in the new work, quantum computers created on the basis of atomic nuclei qubits would perform slower than those relying on electronic-states-based quantum bits. Researchers in the laser spectroscopy lab at the University of Tartu have begun work on a pilot prototype of a quantum computer based on the new method. The completed research is part of the joint project “Spectroscopy of entangled states of clusters of rare-earth impurity ions for quantum computing.” The Laboratory of Laser Spectroscopy and the Laboratory of Solid State Theory at the University of Tartu Institute of Physics are conducting the project. The team of Hizhnyakov, Vadim Boltrushko, Helle Kaasik, and Yurii Orlovski published the research in Optics Communications (www.doi.org/10.1016/j.optcom.2020.126693).