Quantum technologies are the way of the future, but will that future ever arrive?

Maybe so. Physicists have cleared a bit more of the path to a plausible quantum future by constructing an elementary network for exchanging and storing quantum information. The network features two all-purpose nodes that can send, receive and store quantum information, linked by a fiber-optic cable that carries it from one node to another on a single photon.

The network is only a prototype, but if it can be refined and scaled up, it could form the basis of communication channels for relaying quantum information. A group from the Max Planck Institute of Quantum Optics (M.P.Q.) in Garching, Germany, described the advance in the April 12 issue of Nature. (Scientific American is part of Nature Publishing Group.)

Quantum bits, or qubits, are at the heart of quantum information technologies. An ordinary, classical bit in everyday electronics can store one of two values: a 0 or a 1. But thanks to the indeterminacy inherent to quantum mechanics, a qubit can be in a so-called superposition, hovering undecided between 0 and 1, which adds a layer of complexity to the information it carries. Quantum computers would boast capabilities beyond the reach of even the most powerful classical supercomputers, and cryptography protocols based on the exchange of qubits would be more secure than traditional encryption methods.

Physicists have used all manner of quantum objects to store qubits—electrons, atomic nuclei, photons and so on. In the new demonstration, the qubit at each node of the network is stored in the internal quantum state of a single rubidium atom trapped in a reflective optical cavity. The atom can then transmit its stored information via an optical fiber by emitting a single photon, whose polarization state carries the mark of its parent atom's quantum state; conversely, the atom can absorb a photon from the fiber and take on the quantum state imprinted on that photon's polarization.

Because each node can perform a variety of functions—sending, receiving or storing quantum information—a network based on atoms in optical cavities could be scaled up simply by connecting more all-purpose nodes. "We try to build a system where the network node is universal," says M.P.Q. physicist Stephan Ritter, one of the study's authors. "It's not only capable of sending or receiving—ideally, it would do all of the things you could imagine." The individual pieces of such a system had been demonstrated—atoms sending quantum information on single emitted photons, say—but now the technologies are sufficiently advanced that they can work as an ensemble. "This has now all come together and enabled us to realize this elementary version of a quantum network," Ritter says.

Physicists proposed using optical cavities for quantum networks 15 years ago, because they marry the best features of atomic qubits and photonic qubits—namely that atoms stay put, making them an ideal storage medium, whereas photons are speedy, making them an ideal message carrier between stationary nodes. But getting the photons and atoms to communicate with one another has been a challenge. "If you want to use single atoms and single photons, as we do, they hardly interact," Ritter adds.

That is where the optical cavity comes in. The mirrors of the cavity reflect a photon past the rubidium atom tens of thousands of times, boosting the chances of an interaction. "During this time, there's enough time to really do this information exchange in a reliable way," Ritter says. "The cavity enhances the coupling between the light field and the atom."

11 Comments

1. 1. edelbrp 08:07 PM 4/11/12

   It should be important to note that there is an important distinction between 'quantum' information and 'classical'. Classical is what we would generally call useful in terms of sending useful messages and data. 'Quantum information' is only useful to other quantum systems and has to be coupled with classical information transfer systems to generally be of any use to us.
   
   Also, there have been off the shelf quantum-based data systems on the market for years now. (You can google for them yourself.) I'm not calling this article uninteresting, just a little ambiguous as to what is actually new here and what it can be used for in practical terms.

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2. 2. sidelight 09:13 PM 4/11/12

   Begging the question, isn't the whole Universe a quantum information system ?
   It will turn out easier that they are making it.
   

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3. 3. egoburnswell 07:05 AM 4/12/12

   I would like to suggest an alternate title for this article:
   "Mr. Photon--come here--I want to see you".

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4. 4. TomGrinley 08:27 AM 4/12/12

   ...and the first message was "Heisenberg, come quickly, your cat needs you"

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5. 5. ggodwin 12:06 AM 4/13/12

   I'm not sure what Schrodinger would think of that

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6. 6. brublr 08:27 PM 4/13/12

   "Consciousness is a singular for which there is no plural." - Erwin Shrodinger

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7. 7. nirmalgopa 07:58 PM 4/14/12

   What do you mean by a photon ? If so, what is the mass of it ? In the equation of Planck energy E = hγ ,we know, h = Planck Constant. Is h a photon energy ? If so, this is totally incorrect. This h is a bunch of photon energy. I explain it in my book "Complete Unified Theory" (page- 424, 1998). Photon is a ultimate particle. All matter is made up of photons. In what way the particles, matter, thus the universe created, I explained in my book. There is no variation of results between calculated results and experimental results. You can get almost all type of answer from this book.
   Nirmalendu Das.
   Dated: 15-04-2012.

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8. 8. MutantBuzzard 04:55 PM 4/15/12

   WOW who'd a thunk somthing so complcated could b so simpel?

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9. 9. MutantBuzzard 04:59 PM 4/15/12

   so what r photons made of?

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10. 10. MazarSc 03:01 PM 4/18/12

    I think they can't generalize it to say Network, while they still R/W to only one bit of information.

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11. 11. bucketofsquid in reply to MazarSc 01:52 PM 4/25/12

    It is a network in the same way as a very primitive 2 celled organism is a network. Not a network as we currently know it but calling it a network is grammatically correct.

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