Converting quantum bits
Alex Kuzmich and Dzmitry Matsukevich have transferred atomic state information from two clouds of rubidium atoms to a single photon.
A team of physicists at the Georgia Institute of Technology has taken a significant step toward the development of quantum communications systems by successfully transferring quantum information from two different groups of atoms onto a single photon.
The work, to be published in the October 22 issue of the journal Science, represents a "building block" that could lead to development of large-scale quantum networks. Sponsored by the Research Corporation and NASA, the work is believed to be the first to demonstrate transfer of quantum information from matter to light.
Conversion of quantum states from atomic-based systems to photonic systems is necessary for long-distance communication. While the matter-based systems can provide long-term storage of information, efficient transfer of information requires that it be converted into a photonic state for transmission across optical fiber networks.
Once converted into a photonic qubit, the information can be processed and may not need to be converted back to a matter-based qubit. "If you want to realize a quantum repeater, you must have two such quantum nodes," Kuzmich explained. "But in this quantum communications approach, you dont ever need to convert the photon back to atomic format."
For their research, the Georgia Tech physicists used light at a wavelength of 780 nanometers. For transmission in conventional optical fiber networks, however, they will have to switch to the 1550 nanometer wavelength that has become standard in the telecommunications industry. The Science paper reported on atom clouds containing approximately a billion rubidium atoms. Kuzmich says having 10 billion atoms compressed into the same space would boost efficiency. "We should be able to increase our efficiency by a factor of ten at least," he said. Practical applications are still at least 7-10 years away, Kuzmich estimates.
Detailed Explanation of Experiment Diagram: A magneto-optical trap is used to provide an optically thick atomic cloud of a billion rubidium atoms for the experiment. The classical coherent laser pulses used in the generation and verification procedures define the two distinct pencil-shape components of the atomic ensemble that form the memory qubit, L and R.
An infrared write pulse (780 nm wavelength) is split into two beams by a polarizing beam splitter (PBS1) focused into two regions of the atomic cloud about 1 mm apart and passed through it. The light induces spontaneous Raman scattering of a signal photon with slightly shorter wavelength. The classical light is dumped away by the PBS2, while the quantum signal photon is transmitted by the dichroic mirror DM, passed through an arbitrary polarization state transformer R and a polarizer PBS5, and is directed onto a single-photon detector D1. Detection of the signal photon by D1 prepares the atomic ensemble in any desired state and thereby concludes the preparation of the quantum memory qubit.
Following memory state preparation, the read-out stage is performed. After a user-programmable delay a classical coherent read pulse of 795 nm wavelength illuminates the two atomic ensembles. This results in a single (i.e., quantum) idler photon being emitted in the forward direction. This accomplishes a transfer of the memory state onto the idler. The idler is reflected off the dichroic mirror DM. After passing through the state transformer R and PBS6, the two polarization components are directed onto single-photon detectors (D2, D3) thus accomplishing measurement of the idler photon, and hence the memory qubit, in a controllable arbitrary basis.
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