Integrated optics is an engineering solution proposed for exquisite control of photonic quantum information. Here we use silicon photonics and the linear combination of quantum operators scheme to realise a fully programmable two-qubit quantum processor. The device is fabricated with readily available CMOS based processing and comprises four nonlinear photon-sources, four filters, eightytwo beam splitters and fifty-eight individually addressable phase shifters. To demonstrate performance, we programmed the device to implement ninety-eight various two-qubit unitary operations (with average quantum process fidelity of 93.2±4.5%), a two-qubit quantum approximate optimization algorithm and efficient simulation of Szegedy directed quantum walks. This fosters further use of the linear combination architecture with silicon photonics for future photonic quantum processors.The range and quality of control that a device has over quantum physics determines the extent of quantum information processing (QIP) tasks that it can perform. One device capable of performing any given QIP task is an ultimate goal 1 and silicon quantum photonics 2 has attractive traits to achieve this: photonic qubits are robust to environmental noise 5 , single qubit operations can be performed with high precision 16 , a high density of reconfigurable components have been used to manipulate coherent light 5,6 and established fabrication processes are CMOS compatible. However, quantum control needs to include entangling operations to be relevant to QIPthis is recognised as one of the most challenging tasks for photonics because of the extra resources required for each entangling step 5,6 . Here, we demonstrate a programmable silicon photonics chip that generates two photonic qubits, on which it then performs arbitrary twoqubit untiary operations, including arbitrary entangling operations. This is achieved by using silicon photonics to reach the complexity required to implement an iteration of the linear combination of unitaries architecture 8,9 that we have adapted to realise universal two-qubit processing. The device's performance shows that the design and fabrication techniques used in its implementation work well with the linear combination architecture and can be used to realise larger and more powerful photonic quantum processors.Miniaturisation of quantum-photonic experiments into chip-scale waveguide circuits started 10 from the need to realise many-mode devices with inherent sub-wavelength stability for generalised quantum-interference experi-ments, such as multi-photon quantum walks 11 and boson sampling 12-14 . Universal six-mode linear optics implemented with a silica waveguide chip (coupled to free-space photon sources and fibre-coupled detectors) demonstrated the principle that single photonic devices can be configured to perform any given linear optics task 15 . Silicon waveguides promise even greater capability for large-scale photonic processing, because of their third order nonlinearity that enables photon pair generation within integ...

Imperfections in integrated photonics manufacturing have a detrimental effect on the maximal achievable visibility in interferometric architectures. These limits have profound implications for further technological developments in photonics and in particular for quantum photonic technologies. Active optimization approaches, together with reconfigurable photonics, have been proposed as a solution to overcome this. In this Letter, we demonstrate an ultrahigh (>60  dB) extinction ratio in a silicon photonic device consisting of cascaded Mach-Zehnder interferometers, in which additional interferometers function as variable beamsplitters. The imperfections of fabricated beamsplitters are compensated using an automated progressive optimization algorithm with no requirement for pre-calibration. This work shows the possibility of integrating and accurately controlling linear-optical components for large-scale quantum information processing and other applications.

No abstract

No abstract