This paper reviews recent advances in integrated waveguide circuits, lithographically fabricated for quantum optics. With the increase in complexity of realizable quantum architectures, the need for stability and high quality nonclassical interference within large optical circuits has become a matter of concern in modern quantum optics. Using integrated waveguide structures, we demonstrate a high performance platform from which to further develop quantum technologies and experimental quantum physics using single photons. We review the performance of directional couplers in Hong-Ou-Mandel experiments, together with inherently stable interferometers with controlled phase shifts for quantum state preparation, manipulation, and measurement as well as demonstrating the first on-chip quantum metrology experiments. These fundamental components of optical quantum circuits are used together to construct integrated linear optical realizations of two-photon quantum controlled logic gates. The high quality quantum mechanical performance observed at the single photon level signifies their central role in future optical quantum technologies.
This paper reviews recent advances in integrated waveguide circuits, lithographically fabricated for quantum optics. With the increase in complexity of realizable quantum architectures, the need for stability and high quality nonclassical interference within large optical circuits has become a matter of concern in modern quantum optics. Using integrated waveguide structures, we demonstrate a high performance platform from which to further develop quantum technologies and experimental quantum physics using single photons. We review the performance of directional couplers in Hong-Ou-Mandel experiments, together with inherently stable interferometers with controlled phase shifts for quantum state preparation, manipulation, and measurement as well as demonstrating the first on-chip quantum metrology experiments. These fundamental components of optical quantum circuits are used together to construct integrated linear optical realizations of two-photon quantum controlled logic gates. The high quality quantum mechanical performance observed at the single photon level signifies their central role in future optical quantum technologies.
Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2,3]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [4]), measurement (eg. [5]) and other quantum technologies [6]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturization and scalability [7]. Here we report high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [8]. We have demonstrated controlled manipulation of up to four photons on-chip, including highfidelity single qubit operations, using a lithographically patterned resistive phase shifter [9]. We have used this architecture to implement a small-scale compiled version of Shor's quantum factoring algorithm [10], demonstrated heralded generation of tunable four photon entangled states from a six photon input [11], a reconfigurable two-qubit circuit [12], and combined waveguide photonic circuits with superconducting single photon detectors [13]. We describe complex quantum interference behavior in multi-mode interference devices with up to eight inputs and outputs [14], and quantum walks of correlated particles in arrays of coupled waveguides [15]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [16,17] and diamond [18,19] and nonlinear [20,21] photon sources.
Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [3]), measurement (eg. [4]) and other quantum technologies [5]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability [6]. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [7]. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter [8]. We have used this architecture to implement a small-scale compiled version of Shorʼs quantum factoring algorithm [9] and demonstrated heralded generation of tuneable four photon entangled states from a six photon input [10]. We have combined waveguide photonic circuits with superconducting single photon detectors [11]. We describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs [12], and quantum walks of correlated particles in arrays of coupled waveguides [13]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [14,15] and single photon sources [16,17].
Although practical realisation of a fully functioning quantum computer is still a long way off, recent progress both experimentally and theoretically is paving the way for possible implementations. One of the leading approaches is quantum optics, where photons are used as carriers of quantum information, and are manipulated in both linear and non-linear optical circuits. More recently, advances in integrated quantum photonics are enabling the realisation of compact and stable quantum gates and circuits. This approach provides routes to enhancing the complexity of quantum optic experiments, and ultimately towards the development of advanced quantum technologies. This study presents an overview of recent developments in the field of integrated waveguide quantum circuits. Key building blocks required for the realisation of quantum circuits are presented, and a rudimentary version of Shor's quantum factoring algorithm is demonstrated. Planar waveguide quantum circuits provide a high-performance platform from which quantum technologies and experimental quantum physics using single photons can be developed, and a new generation of quantum information and computing devices can be monolithically integrated onto a single optical chip.
Quantum information science has shown that harnessing quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. Already a number of photonic quantum circuits have been realized for quantum metrology, lithography and quantum logic gates. However, these demonstrations have relied on large-scale (bulk) optical elements bolted to large optical tables, thereby making them inherently unscalable and confining them to the research laboratory. This paper reports on the implementation of quantum optic integrated circuits, which not only dramatically reduces the footprint of quantum circuits, but allows unprecedented stability and control of the optical path length; this reveals the possibility for realizing previously unfeasible large scale quantum circuits. Results from femtosecond laser directly-written quantum circuits are also discussed. A large range of chip-scale "photonic building blocks" have been realized using this so-called direct-write technique and they include multimode optical interconnects, directional couplers and interferometers. The directly written quantum circuits exhibited high 2 and 3-photon Hong-Ou-Mandel (HOM) interference visibilities of 95.8% plusmn 0.5 and 84% plusmn 3% respectively.
Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization, and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8 +/- 0.5%; a controlled-NOT gate with an average logical basis fidelity of 94.3 +/- 0.2%; and a path-entangled state of two photons with fidelity of >92%. These results show that it is possible to directly "write" sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, including information processing, communication, metrology, and lithography, as well as the fundamental science of quantum optics.
In the past decades quantum optics has been at the forefront of quantum innovative technologies. For practical applications, scalable platforms for implementation of quantum optical circuits are vital. This thesis presents two new platforms for scalable implementation of quantum optical circuits, namely, modular approach and monolithic integration. Here, we take the first steps towards the integration of three main elements of every quantum optics circuits: Single-photon emitters, single-photon detectors, and quantum logics. Until now, most quantum optical circuits used separate platforms for single-photon generation and detection. The main challenge in the integration of these technologies, which have different requirements, has slowed down the research in the field. Here, we integrate sources and detectors by first fabricating the devices on their own platform and then transferring and combining them together. Plasma enhanced chemical vapor deposition of silicon nitride followed by etching optical waveguides connect these elements. Removing the Poissonian optical excitation field from the quantum circuit is necessary for integration. Classical optical excitation can be avoided if the sources are electrically pumped. However, fabrication of high-quality electrically pumped sources, suitable for integration, has been limited. The experiments described in chapter 4 are our first step towards addressing the mentioned problem. Defect-free nanowires are grown on <100> direction and their optoelectronic performance are characterized. Nanowire quantum dots, thanks to their waveguiding, purity, coherence and their potentials for deterministic integration with other optical circuits, are promising single-photon sources for on-chip quantum optics. However, precise control of the emission energy of the quantum dots by growth has not become possible yet. Chapter 5 describes a method for on-chip tuning of emission energy of nanowire quantum dots using strain fields. We show the emission energy of independent nanowire quantum dots can be brought into degeneracy without affecting their single-photon emission properties. The quantum optical components have to be routed and connected together to form functional circuits. On a chip, this is usually carried out using optical waveguides. Moreover, manipulation of single photons has to be done in a scalable fashion. Again optical waveguides and ring resonators are very good candidates for this task. Therefore, understanding the behavior of these circuits such as their losses, polarization dependence, and temperature behavior is important. The experiment described in chapter 6 studies the behavior of plasma enhanced silicon nitride waveguides in cryogenic temperatures. We concluded in this chapter that due to weak thermo-optic sensitivity of silicon nitride at cryogenic temperatures, the available thermal budget on the system should be carefully considered. An important step in achieving a scalable platform for quantum optical circuits is deterministic and efficient integration of single-photon sources. In chapter 7, we demonstrate successful integration of III-V nanowire quantum dots with silicon nitride waveguides. The nanowires are deterministically selected and transferred from the original growth chip to the new substrate where they are integrated with low-loss silicon nitride waveguides. Our measurements show that the integrated sources preserve their high quality emission properties. In chapter 8, we describe an alternative approach: amodular method for scalable quantum optics. The proposed technique is based on coupling the single-photon from sources into optical fibers where the photons can be processed and then fed into the single-photon detectors. This approach has high flexibility and is easier to implement but as described in the chapter, at the moment, losses in the interfaces between optical fibers and single-photon sources are a major limiting factor. We conclude the thesis with some possible future directions and exciting new results on integration of single-photon detectors with sources and waveguides. Finally, primary results on on-chip single-photon filtering and removal of the optical excitation field are demonstrated.