Fig. 2: A reprogrammable silicon-photonic quantum chip for the arbitrary preparation, operation, and measurement of four-qubit hypergraph states.
From: Demonstration of hypergraph-state quantum information processing

a Circuit diagram. The four-qubit hypergraph device is fabricated on a silicon-nanophotonic quantum chip. The chip integrates more than 400 components for the generation, operation, and measurement of four-qubit {Q1, Q2, Q3, Q4} hypergraph states. Two-qubit states are mapped to a four-dimensional qudit state in one single-photon as: {\({\left\vert 00\right\rangle }_{{{{{{{{\rm{qubit}}}}}}}}}\to {\left\vert 0\right\rangle }_{{{{{{{{\rm{qudit}}}}}}}}}\), \({\left\vert 01\right\rangle }_{{{{{{{{\rm{qubit}}}}}}}}}\to {\left\vert 1\right\rangle }_{{{{{{{{\rm{qudit}}}}}}}}}\), \({\left\vert 10\right\rangle }_{{{{{{{{\rm{qubit}}}}}}}}}\to {\left\vert 2\right\rangle }_{{{{{{{{\rm{qudit}}}}}}}}}\), \({{\left\vert 11\right\rangle }_{{{{{{{{\rm{qubit}}}}}}}}}\to {\left\vert 3\right\rangle }_{{{{{{{{\rm{qudit}}}}}}}}}}\)}. The multi-qubit controlled gate Cm-Z is enabled by a process of “entanglement generation--space expansion--local operation--coherent compression". Reconfiguring the unitary operations Uij (i, j = 0,1) allows the implementations of Cm-Z gates. A sequence of controlled gates are compiled by setting Uij accordingly. Right inset: a photograph for packaged silicon-photonic quantum chip, which is wire bonded (gold lines) on a printed circuit board. b Characterization of the CCCZ gate. Truth tables are measured in four different input-output sets of conjugate product bases. {\(\left\vert 0\right\rangle,\left\vert 1\right\rangle\)} represents the computational basis, and \(\left\vert \pm \right\rangle\) = \((\left\vert 0\right\rangle \pm \left\vert 1\right\rangle )/\sqrt{2}\) represents the Hadamard basis. The probability distributions are coded by colors and the key is provided at the right bottom. The classical statistic fidelity (Fc) shown in the plots is used to characterize each truth table. The CCCZ gate is characterized by the Hofmann fidelity estimated from four tables in b.