Periodic Reporting for period 3 - QPE (Quantum Photonic Engineering)
Reporting period: 2018-05-01 to 2019-10-31
For quantum information technologies (QITs) to have as big an impact on society as anticipated, a practical and scalable approach is needed. Recent developments in chip-scale integrated quantum photonic circuits have radically changed the way in which quantum optic experiments are performed and provides a means to deliver complex and compact quantum photonic technologies for applications in quantum communications, sensing, and computation. This research programme focuses on an engineering approach to QITs and draws upon the rapidly maturing field of silicon photonics. Silicon photonics is a promising material system for the delivery of a fully integrated and large-scale quantum photonic technology platform, where all key components could be monolithically integrated into single quantum devices.
Outlined below are a selection of achievements across the projects’ five major objectives:
1) Scaling quantum complexity: Increasing the number of photons and the number of modes within a quantum system are two ways in which the complexity of quantum photonic circuits can be scaled. To investigate large-mode systems we developed a complex quantum photonic circuit capable of generating, manipulating and analysing multi-dimensional entangled states. The circuit implemented comprises 16 integrated photon sources, and was successfully used in demonstrating 15x15 dimensional entanglement of a two photon state [2, 3]. To investigate large photon number systems, we created a silicon quantum photonic circuit capable of the on-chip generation and algorithmic processing of quantum states of light with up to eight photons .
2) Large-scale programmable quantum circuits: Programmable quantum circuits allow a single chip to perform many different functions. We realised a quantum photonic circuit able to implement any two-qubit unitary quantum operation  and was programmed to implement 98 different two-qubit quantum logic gates (including CNOT, CZ, CH, SWAP, iSWAP and SWAP). Additionally, we realised a silicon chip that programmatically generates four-photon graphs states – a key resource state in measurement-based quantum computing .
3) Integrated quantum technology platform: The project has made major strides towards a fully integrated technology platform, with the successful integration of multiple photon-sources within single integrated circuits . Ring-resonator-based sources were integrated within reconfigurable circuits to demonstrate qubit entanglement and indistinguishable photon generation . Cryogenic-compatible optical modulators have also been developed to enable future integration with cryogenic superconducting single photon detectors .
4) Ultra-compact and practical quantum communications devices: Integrated photonics provides a stable, compact, miniaturised and robust platform to implement quantum communications systems. Through this project we demonstrated the world’s first chip-to-chip quantum communications system, realising integrated quantum communications devices in both the InP  and Silicon  material platforms.
5) Quantum simulation and computation: Entanglement is a fundamental property of quantum mechanics and is a primary resource in quantum information systems. We implemented a device which can generate, manipulate, and analyse two-qubit entangled states and manipulate their entanglement through a switchable controlled-Z gate operates . Programmable quantum circuits were developed to investigate quantum computing applications in quantum simulation  and quantum Hamiltonian learning , where we successfully realised a quantum simulator that could learn the physics of another quantum system. Moving beyond a single isolated system has been achieved through the demonstration of chip-to-chip quantum teleportation  – an important requirement for future distributed quantum computing systems.
 Silverstone et al, Silicon Quantum Photonics, IEEE J. Sel. Top. Quantum Electron., 22, 1–13, 2016
 Wang et al, Multidimensional quantum entanglement with large-scale integrated optics, Science, eaar7053, 2018
 Faruque et al, On-chip quantum interference with heralded photons from two independent micro-ring resonator sources in silicon photonics, Opt. Express, 26, 20379–20395, 2018
 Paesani et al, Generation and sampling of quantum states of light in a silicon chip, Nat Phys, 273, 1, 2019
 Adcock et al, Programmable four-photon graph states on a silicon chip, Nat Comms, 10, 1, 1–6, 2019
 Qiang et al, Large-scale silicon quantum photonics implementing arbitrary two-qubit processing, Nature photonics, 12, 534–539, 2018
 Silverstone et al, Qubit entanglement between ring-resonator photon-pair sources on a silicon chip, Nat Comms, 6, 7948, 2015
 Eltes et al. An integrated cryogenic optical modulator, Submitted 2019 arxiv.org/abs/1904.10902
 Sibson et al, Chip-based quantum key distribution, Nat Comms, 8, 13984, 2017
 Sibson et al, Integrated silicon photonics for high-speed quantum key distribution, Optica, 4, 172–177, 2017
 Santagati et al, Silicon photonic processor of two-qubit entangling quantum logic, J. Opt., 19, 114006, 2017
 Santagati et al, Witnessing eigenstates for quantum simulation of Hamiltonian spectra, Science Advances, 4, 1, eaap9646, 2018
 Wang et al, Experimental quantum Hamiltonian learning, Nat Phys, 6, 031007, 2017
 Llewellyn et al, Chip-to-chip quantum teleportation and multi-photon entanglement in silicon, Submitted Nov 2019 arXiv:1911.07839