Measurement-based quantum computing on a fibre-optical platform

Jonas Neergaard-Nielsen

DTU Physics Faculty+ meeting 2021-05-27

We generated a "substrate" for optical quantum computing

...and implemented quantum gates and a small circuit.

But we don't have a quantum computer... yet.

Quantum computing

  • is a different model of computing than classical computers (Turing machines)
  • relies on quantum superposition (\(\alpha|0\rangle+\beta|1\rangle\)) and interference
  • may solve problems that are impossible on classical supercomputers
  • is a rapidly growing industry — major players like Google, IBM, Amazon, Microsoft, Alibaba, ..., plus tons of startups
  • is technologically very diverse:
    superconductors, semiconductors, trapped ions, photonics, and more

Google Sycamore with 54 qubits

- demonstrated quantum supremacy in 2019

- but scaling up is hard!

Our approach to scalability:

Expand temporally — reuse few physical resources over and over

Standard GATE MODEL of quantum computation

Measurement-based quantum computing (MBQC)

Gate based model

Coherent, high-fidelity qubit interactions

MBQC

Creation of massively entangled state
- the computational substrate

Photonics is ideally suited for this!

Continuous variable quantum information

Standard optical qubits:

a single photon in one of two modes

Continuous-variable states occupy the full Hilbert space, not just the two lowest states

Continuous variable cluster state

Generating qubit clusters:

Generating CV clusters:

Easier approach:

- requires just squeezed states and linear optics

Our tools

squeezers

optical fibres

homodyne detectors

Cluster state generation procedure

Cluster state verification

M. V. Larsen, X. Guo, C. Breum, J. S. Neergaard-Nielsen, U. L. Andersen

Science 366, 369 (2019)

Summer 2019:

disassemble, B309 → B307, reassemble

Implementing gates on the cluster

Implementing gates on the cluster

M. V. Larsen, X. Guo, C. Breum, J. S. Neergaard-Nielsen, U. L. Andersen

accepted in Nature Physics

Main credits: Mikkel Vilsbøll Larsen - now on his way to Canada