July 20, 2011

Using high-magnetic fields researchers managed to suppress decoherence

Scientists have taken the next major step toward quantum computing, which will use quantum mechanics to revolutionize the way information is processed. Using high-magnetic fields, Susumu Takahashi, assistant professor at the USC Dornsife College of Letters, Arts and Sciences, and his colleagues managed to suppress decoherence, one of the key stumbling blocks in quantum computing.

Nature - Decoherence in crystals of quantum molecular magnets



Think of decoherence as a form of noise or interference, knocking a quantum particle out of superposition - robbing it of that special property that makes it so useful. If a quantum computer relies on a quantum particle’s ability to be both here and there, then decoherence is the frustrating phenomenon that causes a quantum particle to be either here or there.

University of British Columbia researchers calculated all sources of decoherence in their experiment as a function of temperature, magnetic field and by nuclear isotopic concentrations, and suggested the optimum condition to operate qubits, reducing decoherence by approximately 1,000 times.

In Takahashi’s experiments, qubits were predicted to last about 500 microseconds at the optimum condition - ages, relatively speaking.

Quantum decoherence is a central concept in physics. Applications such as quantum information processing depend on understanding it; there are even fundamental theories proposed that go beyond quantum mechanics in which the breakdown of quantum theory would appear as an ‘intrinsic’ decoherence, mimicking the more familiar environmental decoherence processes. Such applications cannot be optimized, and such theories cannot be tested, until we have a firm handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate and testable predictions for environmental decoherence in molecular-based quantum magnets. Experiments on molecular magnets have successfully demonstrated quantum-coherent phenomena but the decoherence processes that ultimately limit such behaviour were not well constrained. For molecular magnets, theory predicts three principal contributions to environmental decoherence: from phonons, from nuclear spins and from intermolecular dipolar interactions. We use high magnetic fields on single crystals of Fe8 molecular magnets (in which the Fe ions are surrounded by organic ligands) to suppress dipolar and nuclear-spin decoherence. In these high-field experiments, we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally, and there are no other visible decoherence sources. In these high fields, we obtain a maximum decoherence quality-factor of 1.49 × 10^6; our investigation suggests that the environmental decoherence time can be extended up to about 500 microseconds, with a decoherence quality factor of ~6 × 10^7, by optimizing the temperature, magnetic field and nuclear isotopic concentrations.

Using crystalline molecular magnets allowed researchers to build qubits out of an immense quantity of quantum particles rather than a single quantum object - the way most proto-quantum computers are built at the moment.

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