Physicists from School of Physical Sciences in University of Science and Technology of China, and their colleagues from the Chinese University of Hong Kong has recently reported an observation of an anomalous decoherence effect(ADE) of single electron spin in diamond. ADE is a decisive evidence of the controllability of the environment and therefore the research takes an important step towards the exploring the quantum bath as a resource for the quantum information processing.
Due to the coupling to the environment, quantum system loses coherence irreversibly. Such a process is known as decoherence and is of paramount importance in quantum information processing. According to the classical noise theory, the environment is modeled by a classical fluctuation which naturally leads to a universal conclusion that stronger noise causes faster decoherence and it has been successfully used to describe a wide range of decoherence process. However, such a classical noise theory is based on an assumption that the environment is large enough compared to the central system so that the backaction from the central system to the environment can be neglected. As the development of the modern quantum science, the typical size of the environment relevant to the central system becomes smaller and smaller and a quantum-mechanics based model has to be used to dealing with decoherence. A theoretical study has predicted that decoherence of a single electron spin surrounded by nuclear spins is anomalous and can’t be described by classical noise theory.
The physicists study the decoherence behaviors of a three-level single electronic spin in ultra-pure diamond. In such a system, the central electron spin is coupled to hundreds of 13C nuclear spins and the coupling strength of coherence between |1> and |-1> (double transition coherence) is twice stronger than that of |1> and |0> (single transition coherence). At room temperature, the random orientation of the nuclear spins brings a large thermal fluctuation which causes a very strong decoherence and such decoherence is totally classical. In order to see the non-classical behavior of the nuclear spin bath, the physicists use dynamical decoupling(DD) pulses to remove these classical noises. The observation is striking: as the orders of DD increase, the coherence time of the double transition coherence increases faster than that of the single coherence and finally becomes longer. In order to understand the observation, numerical calculation is performed, without using any assumed fitting parameters, both the experimental and the theoretical data agree very well.
The observed ADE demonstrates the quantumness of the 13C nuclear spin bath. By flipping the central electron spin by DD, the nuclear spins are coherently controlled via the system-bath interactions. The research shows that, instead of source of noises, a mesoscopic quantum bath can be potentially used for quantum information tasks. The work was recently published in Nature Communications[1].
Reference:
[1], Observation of an anomalous decoherence effect in a quantum bath at room temperature
Pu Huang , Xi Kong , Nan Zhao, Fazhan Shi , Pengfei Wang , Xing Rong , Ren-Bao Liu and Jiangfeng Du
Nature Communications (2011) DOI: 10.1038/ncomms1579
http://www.nature.com/ncomms/journal/v2/n12/full/ncomms1579.html