An addressable quantum dot qubit with fault-tolerant control-fidelity.

Exciting progress towards spin-based quantum computing has recently been made with qubits realized using nitrogen-vacancy centres in diamond and phosphorus atoms in silicon. For example, long coherence times were made possible by the presence of spin-free isotopes of carbon and silicon. However, despite promising single-atom nanotechnologies, there remain substantial challenges in coupling such qubits and addressing them individually. Conversely, lithographically defined quantum dots have an exchange coupling that can be precisely engineered, but strong coupling to noise has severely limited their dephasing times and control fidelities. Here, we combine the best aspects of both spin qubit schemes and demonstrate a gate-addressable quantum dot qubit in isotopically engineered silicon with a control fidelity of 99.6%, obtained via Clifford-based randomized benchmarking and consistent with that required for fault-tolerant quantum computing. This qubit has dephasing time T2* = 120 µs and coherence time T2 = 28 ms, both orders of magnitude larger than in other types of semiconductor qubit. By gate-voltage-tuning the electron g*-factor we can Stark shift the electron spin resonance frequency by more than 3,000 times the 2.4 kHz electron spin resonance linewidth, providing a direct route to large-scale arrays of addressable high-fidelity qubits that are compatible with existing manufacturing technologies.

[Working and living conditions of Danish medical assistant]. / Danske underordnede laegers arbejds- og livsvilkår.

In spring 1988 almost 6,000 members of the Danish Association of Young Doctors answered a questionnaire containing 132 questions about their working and living conditions. The Danish study shows that many young doctors are worried by uncertain conditions of employment, lack of opportunities for further training, and demanding emergency duty. Traditional sex role patterns in the families afflict the women doctors who have difficulty in holding their own in the keen competition. Stress in different forms is regarded as a problem by about 25 per cent. Almost half of the respondents had wholly or partially regretted their choice of a career and one fifth had considered changing their profession. The physicians own occupational injuries are substantially (24 per cent) underreported. Swedish and Finnish studies indicate that the problems in the medical profession are similar in the Nordic countries.