## Periodic Reporting for period 3 - QPE (Quantum Photonic Engineering)

Reporting period: 2018-05-01 to 2019-10-31

By harnessing the unique properties of quantum mechanics (superposition and entanglement) to encode, transmit and process information, quantum information science offers significant opportunities to revolutionise information and communication technologies. The far-reaching goal of this project is to build quantum technology demonstrators that can outperform conventional technologies in communications and computation.

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.

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.

Work has focused on the development of a silicon quantum photonic technology platform and components [1], aimed toward the project goal of achieving a scalable approach to quantum information technologies through silicon quantum photonic integration.

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 [4].

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 [5] 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 [5].

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 [2]. Ring-resonator-based sources were integrated within reconfigurable circuits to demonstrate qubit entanglement and indistinguishable photon generation [7]. Cryogenic-compatible optical modulators have also been developed to enable future integration with cryogenic superconducting single photon detectors [8].

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 [9] and Silicon [10] 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 [11]. Programmable quantum circuits were developed to investigate quantum computing applications in quantum simulation [12] and quantum Hamiltonian learning [13], 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 [14] – an important requirement for future distributed quantum computing systems.

[1] Silverstone et al, Silicon Quantum Photonics, IEEE J. Sel. Top. Quantum Electron., 22, 1–13, 2016

[2] Wang et al, Multidimensional quantum entanglement with large-scale integrated optics, Science, eaar7053, 2018

[3] 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

[4] Paesani et al, Generation and sampling of quantum states of light in a silicon chip, Nat Phys, 273, 1, 2019

[5] Adcock et al, Programmable four-photon graph states on a silicon chip, Nat Comms, 10, 1, 1–6, 2019

[6] Qiang et al, Large-scale silicon quantum photonics implementing arbitrary two-qubit processing, Nature photonics, 12, 534–539, 2018

[7] Silverstone et al, Qubit entanglement between ring-resonator photon-pair sources on a silicon chip, Nat Comms, 6, 7948, 2015

[8] Eltes et al. An integrated cryogenic optical modulator, Submitted 2019 arxiv.org/abs/1904.10902

[9] Sibson et al, Chip-based quantum key distribution, Nat Comms, 8, 13984, 2017

[10] Sibson et al, Integrated silicon photonics for high-speed quantum key distribution, Optica, 4, 172–177, 2017

[11] Santagati et al, Silicon photonic processor of two-qubit entangling quantum logic, J. Opt., 19, 114006, 2017

[12] Santagati et al, Witnessing eigenstates for quantum simulation of Hamiltonian spectra, Science Advances, 4, 1, eaap9646, 2018

[13] Wang et al, Experimental quantum Hamiltonian learning, Nat Phys, 6, 031007, 2017

[14] Llewellyn et al, Chip-to-chip quantum teleportation and multi-photon entanglement in silicon, Submitted Nov 2019 arXiv:1911.07839

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 [4].

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 [5] 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 [5].

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 [2]. Ring-resonator-based sources were integrated within reconfigurable circuits to demonstrate qubit entanglement and indistinguishable photon generation [7]. Cryogenic-compatible optical modulators have also been developed to enable future integration with cryogenic superconducting single photon detectors [8].

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 [9] and Silicon [10] 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 [11]. Programmable quantum circuits were developed to investigate quantum computing applications in quantum simulation [12] and quantum Hamiltonian learning [13], 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 [14] – an important requirement for future distributed quantum computing systems.

[1] Silverstone et al, Silicon Quantum Photonics, IEEE J. Sel. Top. Quantum Electron., 22, 1–13, 2016

[2] Wang et al, Multidimensional quantum entanglement with large-scale integrated optics, Science, eaar7053, 2018

[3] 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

[4] Paesani et al, Generation and sampling of quantum states of light in a silicon chip, Nat Phys, 273, 1, 2019

[5] Adcock et al, Programmable four-photon graph states on a silicon chip, Nat Comms, 10, 1, 1–6, 2019

[6] Qiang et al, Large-scale silicon quantum photonics implementing arbitrary two-qubit processing, Nature photonics, 12, 534–539, 2018

[7] Silverstone et al, Qubit entanglement between ring-resonator photon-pair sources on a silicon chip, Nat Comms, 6, 7948, 2015

[8] Eltes et al. An integrated cryogenic optical modulator, Submitted 2019 arxiv.org/abs/1904.10902

[9] Sibson et al, Chip-based quantum key distribution, Nat Comms, 8, 13984, 2017

[10] Sibson et al, Integrated silicon photonics for high-speed quantum key distribution, Optica, 4, 172–177, 2017

[11] Santagati et al, Silicon photonic processor of two-qubit entangling quantum logic, J. Opt., 19, 114006, 2017

[12] Santagati et al, Witnessing eigenstates for quantum simulation of Hamiltonian spectra, Science Advances, 4, 1, eaap9646, 2018

[13] Wang et al, Experimental quantum Hamiltonian learning, Nat Phys, 6, 031007, 2017

[14] Llewellyn et al, Chip-to-chip quantum teleportation and multi-photon entanglement in silicon, Submitted Nov 2019 arXiv:1911.07839

The silicon-based quantum technology platform developed by the PI through this project is an unconventional approach to quantum optics and has allowed the team to realise many ground-breaking experiments in a relatively short period of time. Key breakthroughs include the world’s most complex quantum photonics circuit through integration of many photon sources (comprising over 600 photonic components), and the largest photon state generated on-chip (eight-photon states). The overarching trend of this project has been to demonstrate circuits of increasing complexity and functionality, which has only been possible by developing complex quantum circuits and systems – rather than focusing on the optimisation of individual components.