Three qubits in one

Phys. Rev. Lett. 120, 260502 (2018)

The number of entangled qubits determines, to first order, the power of a quantum computer. Hence, increasing this number is a key challenge in the field of quantum technology. Photons are special qubits in that they possess several independent properties such as polarization, position or orbital angular momentum. These degrees of freedom can all be employed to encode quantum information. The number of accessible qubits can thus be increased beyond the number of particles by a so-called hyperentanglement — the simultaneous entanglement of multiple photons and multiple degrees of freedom. Yet, this has not been achieved for more than two degrees of freedom. Wang et al. now show that they can entangle three degrees of freedom of six photons, providing control over an 18-qubit ensemble.

The researchers employ the paths, the polarization and the orbital angular momentum as independent qubits. By means of a complex optical set-up including beam splitters and 30 single-photon interferometers, they coherently control and entangle the three independent properties of all six photons. 48 single-photon detectors then discern between the 218 possible outcome combinations. The measured state fidelity of 0.7 demonstrates the entanglement of all 18 qubits and the potential of this approach to extend the power of optical quantum information processing.