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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Saturday, March 14, 2009

Long-Distance Teleportation between Two Atoms

Figure 1: Teleportation Team (rear from left: Christopher Monroe, Dzmitry Matsukevich; front from left: Peter Maunz, Steven Olmschenk, David Hayes) (Photo credit: Jonathan Mizrahi)

Quantum teleportation is the faithful transfer of quantum states between systems. A team from the Joint Quantum Institute (JQI) at the University of Maryland (UMD) and the University of Michigan has succeeded in teleporting a quantum state directly from one atom to another over a distance of one meter [Figure 1]. In the Jan. 23 issue of the journal Science [1], the scientists report that, by using their protocol, atom-to-atom teleported information can be recovered with perfect accuracy about 90% of the time.

>>Link to `Trapped Ion Quantum Information Group' led by Christopher Monroe, University of Maryland
>>Link to past 2Physics article on the work of this group

Teleportation works because of a remarkable quantum phenomenon, called “entanglement.” Once two objects are put in an entangled state, their properties are inextricably entwined. Although those properties are inherently unknowable until a measurement is made, measuring either one of the objects instantly determines the characteristics of the other, no matter how far apart they are.

The JQI team set out to entangle the quantum states of two individual ytterbium ions so that information embodied in the condition of one could be teleported to the other. Each ion was isolated in a separate high-vacuum trap. [Figure 2] The researchers identified two readily discernible ground (lowest energy) states of the ions that would serve as the alternative “bit” values of an atomic quantum bit, or qubit.

Figure 2: Experimental setup. Single photons from each of two ions in separate traps interact at a beamsplitter. If both detectors record a photon simultaneously, the ions are entangled. At that point, Ion A is measured, revealing exactly what operation must be performed on Ion B in order to teleport Ion A’s information. (Image Credit: Curt Suplee, JQI)

At the start of the experimental process, each ion (designated A and B) is initialized in a given ground state. Then ion A is irradiated with a specially tailored microwave burst from one of its cage electrodes, placing the ion in some desired superposition of the two qubit states – in effect writing into memory the information to be teleported.

Immediately thereafter, both ions are excited by a picosecond laser pulse. The pulse duration is so short that each ion emits only a single photon as it sheds the energy gained from the laser pulse and falls back to one or the other of the two qubit ground states. Depending on which one it falls into, each ion emits a photon whose color is perfectly correlated with the two atomic qubit states. It is this entanglement between each atomic qubit and its photon that will eventually allow the atoms themselves to become entangled.

The emitted photons are captured by lenses, routed to separate strands of fiber-optic cable, and carried to a 50-50 beamsplitter where it is equally probable for either photon to pass straight through the splitter or to be reflected. On either side of the beamsplitter are detectors that can record the arrival of a single photon. Because of the quantum interference of the two photons [2], a simultaneous detection at both output ports of the beamsplitter occurs only if the photons are in a particular quantum state. Since state of the photons was initially correlated with the state of the atomic qubits this measurement leaves atomic qubits in an entangled state [3]. The simultaneous detection of photons at the detectors does not occur often, so the laser stimulus and photon emission process has to be repeated many thousands of times per second. But when a photon appears in each detector, it is an unambiguous signature of entanglement between the ions.

Figure 3: Quantum state of the atom is teleported by 1 meter. (Image credit: N.R. Fuller, National Science Foundation)

When an entangled condition is identified, the scientists immediately take a measurement of ion A. The act of measurement forces it out of superposition and into a definite condition: one of the two qubit states. But because ion A’s state is irreversibly tied to ion B’s, the measurement of A also forces B into a complementary state. Depending on which state ion A is found in, the researchers now know precisely what kind of microwave pulse to apply to ion B in order to recover the exact information that had originally been stored in ion A. Doing so results in the accurate teleportation of the information.

This method combines the unique advantages of both photons and atoms. Photons are ideal for transferring information fast over long distances, whereas atoms offer a valuable medium for long-lived quantum memory. The combination represents an attractive architecture for a ‘quantum repeater,’ that would allow quantum information to be communicated over much larger distances than can be done with just photons.

The work reported in Science was supported by the Intelligence Advanced Research Project Activity program under U.S. Army Research Office contract, the National Science Foundation (NSF) Physics at the Information Frontier Program, and the NSF Physics Frontier Center at JQI. This report is written by Curt Suplee.

[1] "Quantum Teleportation between Distant Matter Qubits," S. Olmschenk, D. N. Matsukevich, P. Maunz, D. Hayes, L.-M. Duan, and C. Monroe, Science 323, 486 (2009). Abstract.
[2] “Measurement of subpicosecond time intervals between two photons by interference,” C. K. Hong, Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 59, 2044 (1987). Abstract.
[3] “Robust Long-Distance Entanglement and a Loophole-Free Bell Test with Ions and Photons,”

C. Simon, W. T. M. Irvine, Phys. Rev. Lett. 91, 110405 (2003). Abstract.

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