Supercompact Photonic Quantum Logic Gate on a Silicon Chip

To build universal quantum computers, an essential step is to realize the so-called controlled-NOT (CNOT) gate. Quantum photonic integrated circuits are well recognized as an attractive technology offering great promise for achieving large-scale quantum information processing, due to the potential for high fidelity, high efficiency, and compact footprints. Here, we demonstrate a supercompact integrated quantum CNOT gate on silicon by using the concept of symmetry breaking of a six-channel waveguide superlattice. The present path-encoded quantum CNOT gate is implemented with a footprint of $4.8\times 4.45\mu {\mathrm{m}}^2$ ($\sim 3\lambda \times 3\lambda$) as well as a high-process fidelity of $\sim 0.925$ and a low excess loss of $<0.2\mathrm{dB}$. The footprint is shrunk significantly by $\sim 10000$ times compared to those previous results based on dielectric waveguides. This offers the possibility of realizing practical large-scale quantum information processes and paving the way to the applications across fundamental science and quantum technologies.

Figure 1

The proposed quantum CNOT gates. (a) Principle of a quantum CNOT gate. (b) Diagram of the CNOT gate for path-encoded photons, including three beam splitters (BS) with a power splitting ratio of $1/3:2/3$. (c) Configuration of the present CNOT gate. (d) Cross section of the superlattice waveguides in the input-output section and the corresponding localized supermodes. (e)–(j) Simulated light propagation in the designed superlattice waveguides (WGs) when light is, respectively, launched from waveguide Nos. 1–6.

Figure 2

Silicon QPIC with CNOT gates and the experimental setup. (a) Schematic of the silicon quantum CNOT gate chip. (b) Microscope picture for the fabricated silicon QPIC. (c) SEM image of the fabricated CNOT gate $4.8\times 4.45\mu {\mathrm{m}}^2$. Microscope pictures of the (d) MZI and (e) the phase shifter. (f) Experimental setup.

Figure 3

Quantum states tomography. (a)–(d) The density matrices for the four Bell states; the fidelities are $0.88\pm 0.02$, $0.91\pm 0.02$, $0.93\pm 0.03$ and $0.92\pm 0.02$, respectively.

Figure 4

Real part of the results for the $16\times 16$ matrix ${\chi }_{\exp }$ for the CNOT gate. (a) The ideal result. (b) The experimental result reconstructed from the test raw data. The process fidelity is $0.925\pm 0.008$.