18 qubits entangle using six photons

Arxiv – 18-qubit entanglement with photon’s three degrees of freedom

They used
* 30 single-photon interferometers in total.
* The outputs are detected by 48 single-photon detectors

Scheme and experimental setup for creating and verifying 18-qubit GHZ state consisting of six photons and three degrees of freedom. a. The generation of six-photon polarization-entangled GHZ state. An ultrafast laser with a central wavelength of 788 nm, a pulse duration of 120 fs and a repetition rate of 76 MHz is focused on a lithium triborate (LBO) and up-converted to 394 nm. The ultraviolet laser is focused on three custom-designed sandwich-like nonlinear crystals, each consists of two 2-mm thick β-barium borates (BBOs) and one half-wave plate (HWP), to produce three pairs of entangled photons. In each output, two pieces of YVO4 crystals with different thickness and orientation are used for spatial and temporal compensation for the birefringence effects. The three pairs of entangled photons are combined on two polarizing beam splitters (PBSs) to generate a six-photon polarization-entangled GHZ state. b. For each single photon, it is sent through a double PBS and two spiral phase plates (SPPs) to be prepared in a single-photon three-qubit state. c. The measurement of the spatial qubit with closed (dash line) or open (without the dash line) interferometric configuration. d. Polarization measurement. e. High-efficiency and dual-channel OAM readout by coherently convert the OAM to polarization by a swap gate (inset). f. Photo of the actual setup used in b and c. By vertical translation, it is convenient to switch between open and close g. Real-time monitoring the visibilities in the spatial (f) and OAM (h) measurements. h. Photo of the actual setup used in e. DP: Dove prism.

Genuine multipartite entanglement has previously been reported up to 14 trapped ions, 10 photons and 10 superconducting qubits.

Chinese researchers have experimentally demonstrated an 18-qubit Greenberger-Horne-Zeilinger (GHZ) entanglement by simultaneous exploiting three different DoFs of six photons, including their paths, polarization, and orbital angular momentum (OAM). They developed high-stability interferometers for reversible quantum logic operations between the photon’s different DoFs with precision and efficiencies close to unity,
enabling simultaneous readout of 2 18 (262,144) outcome combinations of the 18-qubit state