Defect center room-temperature quantum processors.

Q uantum information devices promise unique opportunities in information technology. Physicists are intrigued with building such devices because they probe our understanding of the nature of quantum mechanics. Quantum effects, although providing the basis of atomic, molecular, and solid state physics, usually are not observed in everyday life because the highly fragile nature of coherence and entanglement requires extensive shielding against environmental effects. Early pioneers in the field of quantum information envisioned the quantum processor to be kept at Kelvin temperatures in an ultrahigh vacuum. And, indeed, most approaches today are based on such technology. Weber et al. (1) now pave the way for a systematic search of hardware for roomtemperature quantum devices. The existence of such hardware would truly revolutionize our thinking about quantum hardware and bring quantum technology to a desktop experience. The report by Weber et al. (1) aims at identifying suitable parameters for an efficient search of defects in wide band gap semiconductors in which quantum states might prevail long enough to be useful for quantum processing even under ambient conditions. In their description, Weber and colleagues assume two prerequisites: (i) that room-temperature quantum hardware should be like the only currently known viable room-temperature solid state quantum bit, the nitrogen vacancy center in diamond (2), and (ii) that future information carriers should be spins of electrons rather than the charge of electrons as in current electronics. To describe the key parameters characterizing room-temperature hardware, it is best to start by analyzing what makes the nitrogen vacancy center in diamond such a viable candidate among the many other quantum hardware candidates such as atoms, ions, quantum dots, or superconducting quantum bits. As pointed out by Weber et al. (1), a combination of different effects define the uniqueness of the system. First, the host material itself has particular physical properties. Diamond is known, e.g., for its outstanding mechanical properties, which makes it the hardest natural material known. The same property, a stiff lattice, results in a very low abundance of effective lattice vibrations even at room temperature (3). Because those lattice vibrations and, in particular, the short wavelength optical phonons cause rapid decoherence of quantum states, one characteristic of a roomtemperature solid state quantum hardware would be low coupling to phonons. Indeed, Weber et al. (1) identify other potential solid state hosts for defects that, in addition to diamond, satisfy this criterion, namely silicon carbid (SiC), nitrides, or zinc oxide (ZnO). A convincing point with SiC and ZnO is that, similar to diamond, the stable spinless nuclear isotopes of this material guarantee long electronand nuclear spin Qubit-dephasing times. The relatively small number of critical parameters for suitable host systems and the fact that most of them are well known, e.g., bandgap, spin-orbit coupling, and isotope composition, makes it relatively easy to identify suitable room-temperature hosts. This is in sharp contrast to the properties of the embedded defect. First, there is an enormous number of defects. Although defect center spectroscopy is a well-established branch in science, only a small fraction of all defects in, e.g., diamond or SiC, have been characterized in sufficient detail. This is why predicting key properties from first principles would be a great asset for an efficient search. However, some critical parameters are difficult to predict. Once again the nitrogen-vacancy (NV) center is a good defect to exemplify. Low coupling to the environment is a necessary but not sufficient requirement. Certainly, quantum bits should be addressable and their quantum states controllable and readable. In the NV center and in what Weber and colleagues envision for other systems, state readout and Qubit addressing is done via optical excitation and detection. This requires the defect to be situated well within the band gap of the host such that no bulk excitons are created upon Qubit readout. A strongly allowed optical transition favors single defect center detection. Typical benchmark values for absorption cross-sections are on the order of 10 cm, a value characteristic for an allowed atomic electric dipole transition. A handful of defects in diamond have been identified as a single quantum system. All of them show the required absorption and emission characteristics, some with outstanding brightness (4). But only one, the NV center, has been proven to exhibit an electron paramagnetic ground state. Optical readout of spin states, however, is only feasible when the excitation–emission cycle leads to spin polarization among electron spin levels and when the emission depends on the spin state of the system. Both conditions are fulfilled in the NV center. Optical illumination results in a strong spin polarization (better than 90%) (5), and the room-temperature fluorescence intensity varies by 30% when the ground spin state is changed (6). Although the details of the NV emission and excitation cycle have been analyzed in detail and are theoretically now quite well understood, it would have been hard to predict them a priori. This is because selection rules for transitions usually are not as simple as they are in atoms or atomlike systems, e.g., quantum dots. Defects usually are described by their molecular orbitals, which in the case of diamond and, e.g., SiC, are slightly mixed due to spinorbit coupling. In addition, some transitions, especially those that govern spin polarization, are phonon assisted. To determine at least a rough guideline for an efficient search for quantum defects, Weber et al. (1) thus concentrated on NV-like defects in wide band gap materials similar to diamond. In tetrahedrally coordinated semiconductors, vacancies Conduc on band