Could photonic qubits work?

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  • #23403
    Quan
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    @quan
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    Hi, I’m just an enthusiast with a very limited background in physics and no background in computers. Pardon me if my physics is shaky.

    I read a few articles about graphene plasmonics, and wanted a PhD’s take.

    Since a photon is neutrally charged and bosonic, it interacts much less with its environment than an electron. Will this fact be at all helpful for building a room temperature quantum computer? Are any companies or research groups taking this idea seriously? What are the biggest engineering hurtles, in your opinion? Thanks!

    #23404
    davelowe
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    Hi @quan – yes you are quite correct about photons not being interactive – they are very ‘inert’.

    There is an architecture based on photons known as the ‘linear optical quantum computer’. These fall into two major sub-types called ‘KLM protocol’ and ‘boson sampling’. Wikipedia has a good article that summarizes what these are.

    In either case, the photon is used as the physical qubit, and there are three major ways that information can be encoded into them.

    1 .time-bin encoding where the 0 & 1 basis states are ‘early and ‘late’ arrival of a photon; the 2-state quantum mechanical system is ‘time of arrival’
    2. number of photos encoding where the 0 & 1 basis states are ‘no photon’ and ‘single photon’; the 2-state quantum mechanical system is the ‘Fock state’
    3. polarization where the 0 & 1 basis states are ‘horizontal’ and ‘vertical’; the 2-state quantum mechanical system is ‘polarisation’

    These types of hardware can be simple to implement since some just rely on simple optical experimental hardware such as mirrors and beamsplitters. The major downside is the velocity of the photons which means that whatever computation you are wanting to perform needs to be performed in one go. Other hardware implementations, such as trapped ion for example, use the internal state of an atom as a qubit, and these can be trapped in one physical location pretty much indefinitely.  Unfortunately the quantum information degrades over time through a process known as decoherence.

    One interesting crossover design is the CQED or cavity quantum electrodynamics computers where the qubit is an atom trapped in a small mirrored cavity where it can undergo what is known as a Rabi oscillation. This means that the atom energy spontaneously decays and emits a photon which has the qubit information, but this is bounced off a mirror and reabsorbed by the atom and then this process repeats. You can see then that this type of design acts as an interface between trapped atoms and photons.

    So far as I’m aware, no large company is actively building an optical quantum computer but I would like to be proven wrong!

    • This reply was modified 1 year, 3 months ago by davelowe.
    #23575
    Quan
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    It sounds like a difficult challenge that could yield amazing dividends. It’d be difficult getting several thousand or more photons to interact at molecularly small points. But it’d be nice not to have a truck sized cooling system for my quantum home PC.
    <p style=”text-align: left;”>Thanks for the detailed response! Time for a wiki-walk.</p>

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