New publication appeared in npj Quantum Information

15.02.2018

Our publication on “Integrated-optics heralded controlled-NOT gate for polarization-encoded qubits” appeared in npj Quantum Information.

Recent progress in integrated-optics technology has made photonics a promising platform for quantum networks and quantum computation protocols. Integrated optical circuits are characterized by small device footprints and unrivalled intrinsic interferometric stability. Here, we take advantage of femtosecond-laser-written waveguides’ ability to process polarization-encoded qubits and present an implementation of a heralded controlled-NOT gate on chip. We evaluate the gate performance in the computational basis and a superposition basis, showing that the gate can create polarization entanglement between two photons. Transmission through the integrated device is optimized using thermally expanded core fibers and adiabatically reduced modefield diameters at the waveguide facets. This demonstration underlines the feasibility of integrated quantum gates for all-optical quantum networks and quantum repeaters.

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npj Quantum Information 4, 13 (2018).

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Experimental setup. Frequency-doubled pulses from a Ti:sapphire laser create photon pairs in two nonlinear β-barium borate crystals via degenerate type-II spontaneous parametric down conversion. Each crystal produces a maximally entangled polarization state. The photons from the first crystal are used as maximally entangled ancillas; the photons from the second crystal are also initially maximally entangled, and can be converted to any two-qubit control–target state using suitable projections and rotations. Before entering the waveguide, the photons are synchronized in the delay stage: the free-space length of the delay stage is tuned independently for each photon using servo motors, as indicated by the double-headed arrows. The TEC fibers coupled to the chip have thermally expanded cores near their exit facets (indicated by the red tips), to increase mode overlap with the waveguides. After interfering on the chip, the four output photons are collimated with a single lens at the output facet of the chip (omitted in the figure), and sent to the detection stage for polarization analysis. Each photon passes through a quarter-waveplate, a half-waveplate, and a polarizing beam splitter before being coupled to multi-mode fiber (omitted in the figure) and sent to an avalanche photodiode. Feed-forward is simulated in post-processing: the control–target measurement outcome is reassigned to one of four values depending on the ancilla measurement outcome