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Protecting quantum information from qubit loss

Date: 2008-08-07
  Proc. Natl. Acad. Sci. USA 2008 105:11050-11054; published ahead of print August 5, 2008,


Quantum computers that make use of the principles of quantum mechanics are expected to solve problems such  as factoring large integers,database search and quantum physics simulation much faster than any classical  computer. However, building such devices in practice has proved extremely difficult. A significant challenge is that  the unavoidable coupling of the quantum computers to the environment quicklydestroys the fragile quantum  information. It is thus important to find ways to reduce the decoherence and carry out coherent quantum  operationsin the presence of noise. A major source of the noise is that the physical qubits can be lost during  quantum computing. The fundamental unit for quantum computingthe qubit, is supposed to be an isolated two- level system consisting of a pair of different quantum states. However, for most proposed quantum hardware, the  qubits could leak out of the desired qubit space and lost into a larger Hilbert space. Take, single photons, for  example; they canbe lost during processing or owing to inefficient photon sources anddetectors. This causes loss
of quantum information, posing a significantobstacle for practical quantum computation.

Fortunately, it is possible to tackle the problem by designing appropriate codes, which are usually called as “loss-tolerant quantum code”. A group of physicists led by Dr. Jian-Wei Pan from the Universitof Science and  Technology of China and the University of Heidelberg has experimentally demonstrated such codes for the first  time. It is shown that by grouping together four photons to encode the logical quantuminformation of a single  photon, even if any one of the four photons is lost, the essential quantum information is still preserved and can  brecovered from the remaining three photons. This result proves that the qubit loss error could in principle be  overcome by quantum codes, thus representing a necessary step toward scalable quantum informatioprocessing.

reference: Lu et al. Proc. Natl. Acad. Sci. 105, 11050-11054 (2008).