Atom-photon link demoed
By
Eric Smalley,
Technology Research NewsCertain states of atoms and photons can represent the 1s and 0s of computer information. Atoms are relatively stable and so are well-suited for storing information, whereas photons are fleeting but hold onto their information as they travel, which makes them well-suited for transmitting information.
Practical quantum information processing is likely to require atoms to process and store information, and photons to transmit information within and between quantum computers. The trick is finding a way to transfer information from atoms to photons and back.
Researchers from the University of Michigan have taken a significant step in that direction by entangling a cadmium ion held in a vacuum by radio waves, and a single, free-flying ultraviolet photon. An ion is a charged atom.
Entanglement, dubbed spooky-action-at-a-distance by Einstein, is a weird ability of particles like atoms and photons. When particles are entangled, their properties, like polarization, remain linked regardless of the distance between them. Polarization is the orientation of a photon's electric field. Entanglement is most often accomplished between like particles.
Entangling an ion and a photon makes it possible to instantly know the state of the ion by measuring the photon, wherever the photon is. "Even if the photon traveled [several] light years to Alpha Cantauri before detection, the Alpha Centaurian who detected the photon would know what state the ion... on Mother Earth... was in," said Boris Blinov, a research fellow at the University of Michigan.
This is potentially useful in quantum cryptography, which taps the properties of particles to provide theoretically perfect security. Two people can share a series of quantum particles and use them as random numbers to encrypt and decrypt messages. The process provides perfect security because when an eavesdropper observes the particles, he unavoidably alters them, making the security breach detectable.
Ion-photon entanglement also promises to advance quantum computing. Quantum computers have the potential to solve certain problems like cracking secret codes and searching large databases far faster than the best possible classical computer. Quantum computers work by checking every possible solution to a problem using one set of operations rather than checking possibilities one by one as today's classical computers do.
The researchers' entanglement method could also be used to teleport quantum information over long distances and within large quantum computers, said Blinov. Quantum teleportation works like a fax machine for particles, with the original destroyed in the process.
The researchers entangled an atom and a photon by trapping a single cadmium ion in a vacuum using radio-frequency electro-magnetic fields, then exciting the ion using a 50-nanosecond ultraviolet laser pulse, said Blinov. A nanosecond is one billionth of a second. "The ion quickly decayed from this excited state while emitting a single ultraviolet photon," he said. The researchers detected the ion and measured its polarization.
Before the researchers measured the photon's polarization, the photon was in a superposition of both possible polarizations -- another weird quantum trait. The photon's polarization superposition was entangled with the superposition of the ion's hyperfine levels, which are subtly different energy levels of the ion's lowest energy, or ground, state. Once the photon was measured, it assumed a single polarization and at the same instant the ion assumed a related hyperfine level.
Other experiments have probably produced atom-photon entanglement, but the entanglement hasn't been directly detected. There are also several proposals for methods of entangling atoms and photons -- these usually involve special optical cavities that cause photons to bounce back and forth many times in a small space containing an atom.
The advantage of the researchers' method is its simplicity. "All that's required is a trapped ion which can be put into an excited state and the photon detection system looking at light emitted by the trapped ion," said Blinov. "The ion excitation is accomplished with conventional lasers, and emitted photons are collected with conventional lenses," he said.
Such a simple approach also has a drawback, said Blinov. The entanglement is probabilistic, meaning it does not result in entanglement every try. This is because the single photon emitted by the ion is emitted in a random direction, said Blinov. "We had only a small probability of catching it with our detectors, covering only about 2 percent of all possible directions the photons could travel," he said.
The success rate of registering a photon in each experiment was about one out of 10,000, or about 0.0001 percent, due also to factors like the imperfect efficiency of the detectors, said Blinov. The researchers repeated the experiment millions of times to get a large number of successful trials.
The probability can be increased to as high as a few percent by optimizing the setup said Blinov. "As long as the success probability is non-zero, the source of entanglement is still very useful and has many practical applications," he added.
The researchers' next step is to use the entangled atom and photon to generate long-distance entanglement of a pair of trapped ions by doubling the experiment to have a pair of ion traps separated by some distance, said Blinov. "Ions in both traps will be excited simultaneously, and the photons transmitted by both ions will be collected and detected," he said. The right measurements of the photons arriving from the two ions causes the ions to become entangled. "We can then use the remote entangled ion pair to implement quantum communication protocols, quantum cryptography, teleportation and quantum computation," Blinov said.
The scheme could be used for quantum cryptography in five to ten years, according to Blinov. "Quantum state teleportation and scalable quantum computation is a more difficult task, but some practical results may appear in a 10 to 15 year period," he added.
Blinov's research colleagues were David Moehring, Luming Duan and Christopher Monroe. The work appeared in the March 11, 2004 issue of Nature. The research was funded by the National Security Agency (NSA), the Army Research Office (ARO), and the National Science Foundation (NSF).
Timeline: 5-10 years, 10-15 years
Funding: Government
TRN Categories: Quantum Computing and Communications; Physics
Story Type: News
Related Elements: Technical paper, "Observation of Entanglement between a Single Trapped Atom and a Single Photon," Nature, March 11, 2004
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June 2/9, 2004
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Atom-photon link demoed
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