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Posted: Jun 08, 2012

The curse of measurement - Uncontrolled back-action effects influence transistors

(Nanowerk News) Cutting-edge computer processors consist of 1.4 billion transistors. Such tiny structures, however, have a major drawback: The read-out process can influence their states in an uncontrolled way. A new model is able to detect and to avoid these "back-action effects" particularly at the quantum level.
The ongoing miniaturization trend in computer technologies, which is driven by cost efficiency and performance demands, will soon reach its limit. The development of a so-called quantum computer offers a possible alternative, as it opens up the prospect of achieving much higher efficiencies by using quantum algorithms. For instance, the quantum mechanical states of single electrons could replace classical transistors. However, quantum states are very fragile and are even more susceptible to back-action than traditional computers. So how realistic is the idea of the quantum computer?
back-action at the quantum level
Making quantum effects visible
LMU-physicist Dr. Stefan Ludwig and his colleagues have experimentally detected and theoretically modeled back-action at the quantum level. The scientists made back-action on single electrons directly visible by taking advantage of quantum effects. Moreover, they discovered a way of utilizing fundamental quantum effects to minimize its undesirable effects. This breakthrough was achieved in collaboration with two Canadian groups (see paper in Nature Physics: "Quantum interference and phonon-mediated back-action in lateral quantum dot circuits").
Interplay between detector and transistor
The researchers succeeded in obtaining a detailed understanding of the microscopic interaction processes which lead to back-action: The single-electron transistor they used consists of an electron confined within two adjacent wells. The detector measures whether the electron is presently in one or the other well. Here is where the back-action effect comes into play: In a simple picture, detector and transistor mutually affect each other. Consequently, the detection process itself modifies the state to be detected. This in turn changes the outcome of the measurement. In the specific setup used here, the interplay between detector and single-electron transistor is governed by a complex combination of charge fluctuations and sound waves.
"One of our main findings is that different back-action components can cancel each other out by destructive interference," says Ludwig, a member of the Nanosystems Initiative Munich (NIM). "Indeed, the back-action process itself causes these quantum interferences. This could be harnessed as a new method for minimizing measurement back-action and can be seen as a milestone in our efforts to use quantum physics for computation in the future."
Source: Nanosystems Initiative Munich
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