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Physicists entangle a record-breaking 14 quantum bits

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14 entangled calcium-ion qubitsQuantum information science is a bit like classroom management—the larger the group, the harder it is to keep everything together.

But to build a practical quantum computer physicists will need many particles working in synchrony as quantum bits, or quibits. Each qubit can be a 0 and a 1 simultaneously, vaulting the number-crunching power of a hypothetical quantum computer well past that of ordinary computers. With each qubit in a superposition, a quantum computer can manipulate an exponentially large quantity of numbers at once—2^n numbers for a system of n qubits. So each step toward generating large sets of qubits pushes practical quantum computing closer to reality.

Thomas Monz of the Institute for Experimental Physics at the University of Innsbruck in Austria and his colleagues mark just such a step forward in the April 1 issue of Physical Review Letters. Monz and his co-authors report creating entangled states with a record 14 qubits. Other researchers had previously demonstrated entangled states with 10 qubits.

Entanglement is a key quantum phenomenon by which particles share correlated properties, even though they are spatially separated. (A common analogy for entangled particles is a pair of dice that always land on matching numbers; if one die comes up 5, for instance, the other will, too.) But entangled particles are a bit like rambunctious children—keeping them on-task is difficult work, and the situation only gets harder to control as more particles are added.

Monz and his colleagues encoded information onto a string of trapped calcium atoms, using two energy levels of the atom to represent 0 and 1. Using lasers to manipulate the atomic qubits, the group set the entire ensemble into superpositions of 0 and 1—in other words, each qubit is in a sense both a 0 and a 1 until it is measured, at which point it is forced to settle on being one or the other. With a set of qubits entangled in an certain way, measuring one qubit forces the rest of the set to follow suit, resulting in all 0s or all 1s.

Monz and his colleagues ran their experiment with qubit sets of different sizes, and managed to demonstrate multiparticle entanglement with up to 14 qubits. But the data leave some room for doubt—whereas sets of 2, 4, or even 8 qubits showed strong signs of entanglement, the set of 14 just barely cleared the benchmark for fidelity. (Entanglement does not work every time, so it is usually verified by certain statistical tests.) The researchers say that the data support 14-qubit entanglement with a confidence interval of 76 percent, so there is certainly room for improvement.

Photo credit: University of Innsbruck

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  1. 1. pyepaurn 2:58 pm 04/6/2011

    key error in the article. The dice analogy, suggesting both dice land on the same number. Two "items" cannot be in the same quantum state upon observation. If 2 dice are in quantum superposition, they both cannot end up in exactly the same quantum state after the wave collapses.

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  2. 2. Trent S 7:33 pm 04/6/2011

    I believe that specific paragraph relates to simplifying the idea of entanglement, rather than describing superpositioning. For the general reader, describing two particles that are paired no matter the distance between them can be a tricky task.

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  3. 3. jtdwyer 12:40 am 04/7/2011

    So, for a special reader, how would the superposition of >10 qubits be determined?

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  4. 4. jack.123 7:32 pm 04/17/2011

    As for simple example of entanglement,the two slit experiment is the the way to go.There no need to make things more complicated than they need to be.Now don’t get me wrong from a academic point of view bigger and better experiments means an expansion of knowledge,and thus more funds for more research.The problem is that usable products are not forthcoming.Now if the scientist were to put out something practical that the laymen could understand.A great many more funds would become available moving the science ahead at a much faster rate.

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  5. 5. jtdwyer 8:38 pm 04/17/2011

    As I understand, the simplest explanation is that entangled particles represent a single wave partitioned into a number of independently directed wavefronts, each representing a potential characteristic property state manifestation of the particle eventually detected upon collapse of each independent wave function. Separation distance is restricted by the duration of non-disrupted wave transmission and is transmitter/media dependent.

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  6. 6. jack.123 8:58 pm 05/18/2011

    First the particles are not connected the waves and their functions are,when you look at any part of the particle function,the wave function is lost.In other words it is the waves not the particals that are entangled.

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