NEWS & VIEWS

Shaji et al. explore electron transport requirement above, the authors call this mode far cry from the results obtained so far in for the opposite sign of bias voltage, where of transport lifetime-enhanced transport, or GaAs, but if theory and measurements of charge transport proceeds in the tunnelling LET, because it is made possible by the long spin coherence in Si/SiGe bulk systems are sequence (2,0)→(1,1)→(1,0)→(2,0), as triplet-state lifetime. Although they do not any indication, there is great potential for illustrated in Fig. 1c. In contrast with directly measure the triplet–singlet relaxation electron spin qubits in a nuclear-spin-free measurements in GaAs quantum dots, time, the analysis of the data using a rate- quantum world4. Shaji et al. find that their Si/SiGe double dot, equation model sets a lower bound on the when operated in this regime, unexpectedly relaxation time of 16 µs. References 1. Deutsch, D. Proc. Roy. Soc. Lond. A 400, 97–117 (1985). transfers electrons with almost no drop in Rapid progress in GaAs quantum dots 2. Loss, D. & DiVincenzo, D.P. Phys. Rev. A 57, 120–126 (1998). the measured conductance. The authors give has been made possible by decades of 3. Kouwenhoven, L. P. & Marcus, C. M. Phys. World two requirements for observing this effect: continual improvement of the GaAs/AlGaAs 11, 35–39 (1998). the probability of loading an electron into the materials system. The results presented in 4. Hanson, R., Kouwenhoven, L. P., Petta, J. R., Tarucha, S. & Vandersypen, L. M. K. Rev. Mod. Phys. 79, 1217–1265 (2007). T(2,0) state must be much higher than the this issue demonstrate the future potential 5. Petta, J. R. et al. Science 309, 2180–2184 (2005). S(2,0) state, and the relaxation from T(2,0) to of carbon- and silicon-based quantum 6. Levy, J. Phys. Rev. Lett. 89, 147902 (2002). S(2,0) must be very slow. If either condition devices. Spin-relaxation times exceeding 7. Mason, N., Biercuk, M. J. & Marcus, C. M. Science is not met (as in previous studies), then the 100 milliseconds have been reported for 303, 655–658 (2004). 8. Jørgensen, H. I. et al. Nature Phys. 4, 536–539 (2008). 4 system will fall back into a S(2,0) state, and GaAs quantum dots . The 16-µs relaxation 9. Shaji, N. et al. Nature Phys. 4, 540–544 (2008). current will be blocked. Owing to the second time reported for the Si/SiGe system is a 10. Ono, K. & Tarucha, S. Phys. Rev. Lett. 92, 256803 (2004). WILLIS LAMB Shift to quantum electrodynamics

of hydrogen were not quite what they were predicted by Dirac to be”. In 1947, he proved it. Exploiting the advances in microwave technology made during the war, Lamb devised, and conducted with Robert Retherford, microwave experiments of sufficient sensitivity to reveal at last

the shift in energy between the S2 ½ and 2P½ levels. Such was the importance of his result that he was invited to present it on the opening day of the Shelter Island conference, on 2 June 1947. Shelter Island was a defining moment in the history of physics in the USA: for the first time in years, an elite group of theorists gathered, free from the restrictions of wartime, ready to kick-start a programme of research that would tackle some of the most fundamental aspects of physics. Oppenheimer, one of the delegates, AIP EMILIO SEGRE VISUAL ARCHIVES VISUAL SEGRE EMILIO AIP reportedly considered the conference to be the most successful he ever Willis Lamb was, until his death on 15 May of quantum mechanics, the greatest attended. Something of the intensity 2008 aged 94, the longest surviving Nobel milestone being arguably the Dirac of discussion, particularly among the laureate in physics. He received his prize equation, published by Paul Dirac in 1928. young turks, is captured in photographs in 1955 “for his discoveries concerning Although the broad quantum picture was of the time: pictured here, left to right, the fine structure of the hydrogen taking shape, many details were lacking. are Lamb, Abraham Pais, John Wheeler, spectrum” — specifically, the Lamb shift. One of these concerned the prediction Richard Feynman, Herman Feshbach and Born in Los Angeles in 1913, Lamb arising from Dirac’s theory that the 2S½ Julian Schwinger. Suitably inspired, it was studied at the University of California, and 2P½ energy levels in the spectrum of Hans Bethe, riding home on the train from Berkeley, under Robert Oppenheimer, hydrogen should be degenerate. In the the conference, who took the first step in before joining Columbia University in years before the outbreak of war, numerous recognizing the significance of the Lamb 1938. His wartime experience in the attempts to test this experimentally had shift in quantum electrodynamics — a Columbia Radiation Laboratory, working failed. However, Lamb “just knew” — as he theory that, in the hands of Feynman, on microwave radar, was to inspire his later recalled (J. Mehra & H. Rechenberg Schwinger and others, would become lasting contribution to physics. in The Quantum Theory of Planck, Einstein, the most robust and well tested in all Throughout the 1920s, great strides Bohr and Sommerfeld Vol. 6, p1037, of physics. had been taken in developing the theory Springer, 1982) — “that the energy levels Alison Wright

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