Researchers at the Quantum Engineering and Technology Labs (QETLabs) at the University of Bristol has shown how to protect qubits from errors using photons in a silicon chip.
Quantum states of particles are very fragile and can pick up errors very easily, making building a photonic quantum computer difficult.
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Quantum error correcting codes are a method to protect, or to nurture, qubits, by embedding them in a more robust entangled state of many particles, and this has been demonstrated for the first time in a silicon photonic chip by the team at QETLabs.
The team showed how large states of entangled photons can contain individual logical qubits and protect them from the harmful effects of the classical world. The Bristol-led team included researchers from DTU in Copenhagen who fabricated the chip.
“The chip is really versatile. It can be programmed to deliver different kinds of entangled states called graphs. Each graph protects logical quantum bits of information from different environmental effects,” said researcher Dr Caterina Vigliar.
The chip has three modules. The first module has eight coherently pumped spiral waveguides (1.5 cm long) generating two entangled pairs of photons, which are spatially separated with integrated filters. Each photon encodes a pair of qubits for an eight-qubit system.
Next, programmable entangling gate modules are used to fuse the qubit states, and then a triangular structure of Mach–Zehnder interferometers performs arbitrary projective measurements. The photons are fibre coupled off-chip using low-loss (<1 dB) grating couplers, and routed to high-efficiency superconducting nanowire single-photon detectors.
The team demonstrated a boost in the quality of the qubits from 65 percent to 95.8 percent.
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