## Quantum manipulation: Filling the gap between quantum and classical world

Quantum superposition is a fundamental and also intriguing property of the quantum world. Because of superposition, a quantum system can be in two different states simultaneously, like a cat that can be both “dead” and “alive” at the same time. However, this anti-intuitive phenomenon cannot be observed directly, because whenever a classical measuring tool touches a quantum system, it immediately collapse into a classical state. On the other hand, quantum superposition is also the core of quantum computer’s enormous computational power. A quantum computer can easily break the widely used RSA (Rivest, Shamir and Adleman) security system with Shor’s algorithm. But for now, quantum computation still suffers from the decoherence induced by environment. Obviously, the key to manipulate a quantum system is to make it stay coherent as long as possible, to achieve this, one need to isolate the system from its environment. “For ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems”, Serge Haroche and David Wineland won the 2012 Nobel Prize in Physics.

This review begins by introducing the interesting property of quantum superposition, explaining its physical meaning, potential applications and main obstacles ahead. Then the author goes on to introduce the work of the two 2012 Nobel Prize Laureates – Serge Haroche and David Wineland. Instead of manipulating a neutral atom or a photon, Wineland and his team focused on controlling a charged atom, the ion, in an electromagnetic well. In order to break the limit of Doppler cooling, a new cooling technique – Side-Band cooling was used to reach extreme low temperature. The well cooled ions made an ideal platform for building optical clock and quantum computer. Since 2001, Wineland and his team had realized several optical clocks with very high precision. They had also realized basic quantum logic gate in ion trap and demonstrated the scalability of ion system, proving their system is promising for practical quantum computation. This article covers the above topics and gives detailed review.

In the fourth section, the author introduces the work of Haroche and his collaborators. Haroche et al managed to build a high-Q microwave cavity with superconducting materials and cooled it down to superconducting phase. According to Meissner effect, photons in the cavity cannot penetrate the superconducting mirror and will be trapped inside, thus isolate the photons from its environment. Since the cavity has extremely high-Q, the Rydberg atoms inside the cavity are strongly correlated to the photon field, which makes a perfect platform for testing the fundamental principles of quantum mechanics. With the aid of quantum non-demolition measurement, quantum processes can be observed without destroying the state. Using this platform, Haroche et al had directly observed decoherence, quantum jump and several other quantum information processes.

Finally, the review introduces recent developments and further applications of quantum manipulation, and then ends with a discussion of the relationship between quantum and classical world. With advanced quantum manipulation techniques, people are able to investigate fundamental quantum mechanics. In return, a better understanding of quantum mechanics makes it possible to develop new technologies that will change our classical world.

**Publication:**

A new epoch of quantum manipulation. Yongjian Han, Zhen Wang, and Guang-Can Guo *Natl Sci Rev *(March 2014) 1 (1): 91-100 DOI:10.1093/nsr/nwt024

[http://nsr.oxfordjournals.org/content/1/1/91.abstract]

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