Unexpected Error

26 November 2010

The University of Sydney's School of Physics Dr Michael J Biercuk, a quantum physicist, and his international colleagues, have discovered a new source of error in atomic qubits cooled and manipulated using lasers.

In their paper Decoherence due to Elastic Rayleigh Scattering, published recently in Physical Review Letters, and profiled in the prestigious Physics, the group found that a particular form of spontaneous emission, long considered irrelevant by the physics community, can actually be the dominant source of error in atomic qubits.

"Elastic, or Rayleigh, scattering has mostly been ignored as a source of qubit decoherence," says Dr Biercuk who is based with the School of Physics. "However we have derived a new formalism to describe circumstances when it is important, and we've validated our predictions through experiment."

Rayleigh scattering is the elastic (energy-conserving) scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules.

When light scatters from an atom, it carries away information about the electrons in the atom. This can obviously produce errors in the case of inelastic scattering, where the atom changes state after a scattering event. For decades, however, scientists thought that Rayleigh scattering processes that didn't change the atomic state could be ignored, and that no significant disruption to the atomic state would occur.

By presenting these new calculations, backed by experiments, the quantum physicists point to cases where this assumption is wrong. "We found that in our system Rayleigh Scattering could produce decoherence with probability four times larger than predicted by previous theories, and could dominate other forms of spontaneous emission. This has very important implications for quantum science using trapped atoms."

The paper cites the calculation of the probability that an atomic qubit, which can be in a high-energy, spin "up" state or a low-energy, spin "down" state, will elastically scatter a photon of a particular frequency, and identifies a window of frequencies where the two states combined elastic scattering is a significant source of decoherence.

The scientists then take this knowledge to the lab, where, using Beryllium ions in a magnetic field, the group has demonstrated that the atoms act as spin "up" or "down" qubits by preparing the lone electron in a particular quantum state. They then expose the ions to a tunable laser, allowing them to study the experimental conditions where elastic scattering of the laser light from the Beryllium ions causes the encoded spin information to be most rapidly lost.

Dr Biercuk says this discovery will be fundamental in predicting error sources in future quantum information experiments with trapped atoms.

H. Uys, M.J. Biercuk, A.P. VanDevender, C. Ospelkaus, D. Meiser, R.
Ozeri, J. J. Bollinger, "Decoherence due to elastic Rayleigh scattering." Phys. Rev. Lett. 105, 200401 (2010).

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