Physical qubit of optical quantum computer

Question

I was researching how optical quantum computers work and discovered a KLM protocol that allows for building quantum gates using linear optics elements like mirrors, beam splitters, and phase shifters. However, I was wondering how would we get the same functionality as with superconducting QCs. With superconducting QCs, we have Josephson Junctions - JJs. Those JJs are being interacted with differently so they form a specific gate when that gate operation is needed.

Besides optical JJ, what would be the optical equivalent to Josephson Junction that we could apply quantum gates to? What are the QCs like the ones from Xanadu running on when it comes to physical qubit implementation?

Answer 1

At Xanadu, we're using integrated quantum photonics to build our photonic quantum computing chips. In this case, we have integrated chips containing waveguides --- these are coupled to lasers to generate input resource states, undergo manipulation on the chip, and then are measured via a variety of detectors available in quantum optics. These can include photon number resolvers, homodyne detectors, or even heterodyne detectors.

One thing to note is that there are several formulations for optical quantum computing:

Non-universal formulations:

• Boson sampling
• Gaussian boson sampling

Universal formulations:

• Linear optical quantum computing with the KLM protocol

• Continuous-variable quantum computation

At Xanadu, our focus is on the last approach, the continuous-variable formulation. In this approach, we are outside the regime of linear photonics, and actually include non-linear quantum photonic operations in our basic gate set.

Finally, we also differ from the KLM protocol in that we do not depend on single photon resource states, which can typically only be produced with a certain probability. Our resource states are instead squeezed states of light, which can be produced deterministically.

For more details on continuous-variables, the gates we use, and the algorithms available, check out our documentation!