A qubit candidate shines brighter

Dec 29, 2014

Credit: Evelyn Hu/Harvard

By Phys.org

In the race to design the world’s first universal quantum computer, a special kind of diamond defect called a nitrogen vacancy (NV) center is playing a big role. NV centers consist of a nitrogen atom and a vacant site that together replace two adjacent carbon atoms in diamond crystal. The defects can record or store quantum information and transmit it in the form of light, but the weak signal is hard to identify, extract and transmit unless it is intensified.

Now a team of researchers at Harvard, the University of California, Santa Barbara and the University of Chicago has taken a major step forward in effectively enhancing the fluorescent  of diamond nitrogen vacancy centers – a key step to using the atom-sized defects in future quantum computers. The technique, described in the journal Applied Physics Letters, from AIP Publishing, hinges on the very precise positioning of NV centers within a structure called a photonic cavity that can boost the light signal from the defect.

A Potential Qubit Power Couple

NV centers contain an unpaired electron that can store information in a property known as spin. Researchers can “read” the spin state of the electron by observing the intensity of particular frequencies of the light that the NV center emits when illuminated by a laser.

At room temperatures, this pattern of light emission couples to multiple “sideband” frequencies, making it difficult to interpret. To amplify the most important element of the signal researchers can use a structure called a photonic cavity, which consists of a pattern of nanoscale holes that serve to enhance the NV center’s light emission at its main frequency.

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One comment on “A qubit candidate shines brighter”

  • 1
    Lorenzo says:

    The concept of quantum computing has always baffled me -even more so since I got to know the classical Quantum Mechanics, which makes storage of information at least really hard to concieve. Here’s the problem:

    […] an unpaired electron that can store information in a property known as spin.

    Classically, the concept of storing information (and then retrieving it) is based on the fact that you have a system that can be found in any of some states, you assign a predetermined meaning to each of those states, then you put the system in one of those states and someone comes later to the system, findd it in that state and gets the message. For any information to be exchanged, the state can’t be known in advance -otherwise, what’s the point of “reading” anyway?

    Now, QM poses a problem here: if a system has more than one states to choose from, it will find itself in all of those states, except when a measure is performed: then it degenerates into one of those states with probabilities that depends on how the wave function of the system is organized. And here’s the trick:
    a) If the states have the same energy, then the system will oscillate among them -with a period dependent of the energy iteslf.
    b) If it’s not degenerate, then it will return to the ground state soon or later -it will not stay where you put it. The more the states are separated in energy, the faster the return will be. The closest, the more a) the system will look like -and the more difficult to read it becomes.

    A way around that I can think of is for you to exactly know your energies of an a) system, thus you can know when to read it to find the right configuration, and just do that… but this apporach doesn’t look to me like a true quantum commputer as often described, only a very strong miniaturization of a classical deterministic machine -with all the risks that comes form even minor faults in predicting the system’s energy an thus period…

    Whatever; fascinating.

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