Integrated quantum photonics, uses photonic integrated circuits to control photonic quantum states for applications in quantum technologies. [1] [2] As such, integrated quantum photonics provides a promising approach to the miniaturisation and scaling up of optical quantum circuits. [3] The major application of integrated quantum photonics is Quantum technology:, for example quantum computing, [4] quantum communication, quantum simulation, [5] [6] [7] [8] quantum walks [9] [10] and quantum metrology. [11]
Linear optics was not seen as a potential technology platform for quantum computation until the seminal work of Knill, Laflamme, and Milburn, [12] which demonstrated the feasibility of linear optical quantum computers using detection and feed-forward to produce deterministic two-qubit gates. Following this there were several experimental proof-of-principle demonstrations of two-qubit gates performed in bulk optics. [13] [14] [15] It soon became clear that integrated optics could provide a powerful enabling technology for this emerging field. [16] Early experiments in integrated optics demonstrated the feasibility of the field via demonstrations of high-visibility non-classical and classical interference. Typically, linear optical components such as directional couplers (which act as beamsplitters between waveguide modes), and phase shifters to form nested Mach–Zehnder interferometers [17] [18] [19] are used to encode a qubit in the spatial degree of freedom. That is, a single photon is in superposition between two waveguides, where the zero and one states of the qubit correspond to the photon's presence in one or the other waveguide. These basic components are combined to produce more complex structures, such as entangling gates and reconfigurable quantum circuits. [20] [21] Reconfigurability is achieved by tuning the phase shifters, which are manipulated by using thermo- or electro-optical elements. [22] [23] [24] [25]
Another area of research in which integrated optics will prove pivotal is Quantum communication and has been marked by extensive experimental development demonstrating, for example, quantum key distribution (QKD), [26] [27] quantum relays based on entanglement swapping, and quantum repeaters.
Since the birth of integrated quantum optics experiments have ranged from technological demonstrations, for example integrated single photon sources [28] [29] [30] and integrated single photon detectors, [31] to fundamental tests of nature, [32] [33] new methods for quantum key distribution, [34] and the generation of new quantum states of light. [35] It has also been demonstrated that a single reconfigurable integrated device is sufficient to implement the full field of linear optics, by using a reconfigurable universal interferometer. [20] [36] [37]
As the field has progressed new quantum algorithms have been developed which provide short and long term routes towards the demonstration of the superiority of quantum computers over their classical counterparts. Cluster state quantum computation is now generally accepted as the approach that will be used to develop a fully fledged quantum computer. [38] Whilst development of quantum computer will require the synthesis of many aspects of integrated optics, boson sampling [39] seeks to demonstrate the power of quantum information processing via readily available technologies and is therefore a very promising near term algorithm for doing so. In fact, shortly after its introduction, there were several small scale experimental demonstrations of the effectiveness of the boson sampling algorithm [40] [41] [42] [43]
Quantum photonics is the science of generating, manipulating and detecting light in regimes where it's possible to coherently control individual quanta of the light field (photons). [44] Historically, quantum photonics has been fundamental to exploring quantum phenomena, for example with the EPR paradox and Bell test experiments,. [45] [46] Quantum photonics is also expected to play a central role in advancing future technologies, such as Quantum computing, Quantum key distribution and Quantum metrology. [47] Photons are particularly attractive carriers of quantum information due to their low decoherence properties, light-speed transmission and ease of manipulation. Quantum photonics experiments traditionally involved 'bulk optics' technology—individual optical components (lenses, beamsplitters, etc.) mounted on a large optical table, with a combined mass of hundreds of kilograms.
The application of Integrated quantum photonic circuits to quantum photonics, [1] is seen as an important step in developing useful quantum technology. Single die photonic circuits offer the following advantages over bulk optics:
Being based on well-developed fabrication techniques, the elements employed in Integrated Quantum Photonics are more readily miniaturisable, and products based on this approach can be manufactured using existing production processes and methods.
Control over photons can be achieved with integrated devices that can be realised in diverse material substrates such as silica, silicon, gallium arsenide, lithium niobate and indium phosphide and silicon nitride.
Three methods for using silica:
The main challenges of the silica platform are the low refractive index contrast, the lack of active tunability post-fabrication (as opposed to all the other substrates) and the difficulty of mass production with reproducibility and high yield due to the serial nature of the inscription process.
A big advantage of using silicon is that the circuits can be tuned actively using integrated thermal microheaters or p-i-n modulators, after the devices have been fabricated. The other big benefit of silicon is its compatibility with CMOS technology, which allows leveraging the mature fabrication infrastructure of the semiconductor electronics industry. The structures differ from modern electronic ones, however, as they are readily scalable. Silicon has a really high refractive index of ~3.5 at the 1550 nm wavelength commonly used in optical telecommunications. It therefore offers one of the highest component densities in integrated photonics. The large contrast in refractive index with glass (1.44) allows waveguides formed of silicon surrounded by glass to have very tight bends, which allows for a high component density and reduced system size. Large silicon-on-insulator (SOI) wafers up to 300 mm in diameter can be obtained commercially, making the technology both available and reproducible. Many of the largest systems (up to several hundred components) have been demonstrated on the silicon photonics platform, with up to eight simultaneous photons, generation of graph states (cluster states), and up to 15 dimensional qubits). [49] [50] Photon sources in silicon waveguide circuits leverage silicon's third-order nonlinearity to produce pairs of photons in spontaneous four-wave mixing. Silicon is opaque for wavelengths of light below ~1200 nm, limiting applicability to infrared photons. Phase modulators based on thermo-optic and electro-optic phases are characteristically slow (KHz) and lossy (several dB) respectively, limiting applications and the ability to perform feed-forward measurements for quantum computation.
Lithium niobate offers a large second-order optical nonlinearity, enabling generation of photon pairs via spontaneous parametric down-conversion. This can also be leveraged to manipulate phase and perform mode conversion at high speeds, and offers a promising route to feed-forward for quantum computation, multiplexed (deterministic) single photons sources). Historically, waveguides are defined using titanium indiffusion, resulting in large waveguides (large bend radius). [51]
Photonic waveguides made from group III-V materials on insulator, such as (Al)GaAs and InP, provide some of the largest second and third order nonlinearities, large refractive index contrast providing large modal confinement, and wide optical bandgaps resulting in negligible two-photon absorption at telecommunications wavelengths. III-V materials are capable of low-loss passive and high-speed active components, such as active gain for on-chip lasers, high-speed electro-optic modulators (Pockels and Kerr effects), and on-chip detectors. Compared to other materials such as silica, silicon, and silicon nitride, the large optical nonlinearity, simultaneously with low waveguide loss and tight modal confinement, has resulted in ultrabright entangled-photon pair generation from microring resonators. [52]
Conventional fabrication technologies are based on photolithographic processes, which enable strong miniaturisation and mass production. In quantum optics applications a relevant role has also been played by the direct inscription of the circuits by femtosecond lasers [53] or UV lasers; [17] these are low-volume fabrication technologies, which are particularly convenient for research purposes where novel designs have to be tested with rapid fabrication turnaround.
However, laser-written waveguides are not suitable for mass production and miniaturisation due to the serial nature of the inscription technique, and due to the very low refractive index contrast allowed by these materials, as opposed to silicon photonic circuits. Femtosecond laser-written quantum circuits have proven particularly suited for the manipulation of the polarisation degree of freedom [54] [55] [56] [57] and for building circuits with innovative three-dimensional designs. [58] [59] [60] [61] Quantum information is encoded on-chip in either the path, polarisation, time bin, or frequency state of the photon and manipulated using active integrated components in a compact and stable manner.
Though the same fundamental components are used in quantum as classical photonic integrated circuits, there are also some practical differences. Since amplification of single photon quantum states is not possible (no-cloning theorem), loss is the top priority in components in quantum photonics.
Single photon sources are built from building blocks (waveguides, directional couplers, phase shifters). Typically, optical ring resonators, and long waveguide sections provide increased nonlinear interaction for photon pair generation, though progress is also being made to integrate solid state systems single photon sources based on quantum dots, and nitrogen-vacancy centers with waveguide photonic circuits. [62]
This is a timeline of quantum computing.
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.
Optical computing or photonic computing uses light waves produced by lasers or incoherent sources for data processing, data storage or data communication for computing. For decades, photons have shown promise to enable a higher bandwidth than the electrons used in conventional computers.
Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commonly at the 1.55 micrometre wavelength used by most fiber optic telecommunication systems. The silicon typically lies on top of a layer of silica in what is known as silicon on insulator (SOI).
A spaser or plasmonic laser is a type of laser which aims to confine light at a subwavelength scale far below Rayleigh's diffraction limit of light, by storing some of the light energy in electron oscillations called surface plasmon polaritons. The phenomenon was first described by David J. Bergman and Mark Stockman in 2003. The word spaser is an acronym for "surface plasmon amplification by stimulated emission of radiation". The first such devices were announced in 2009 by three groups: a 44-nanometer-diameter nanoparticle with a gold core surrounded by a dyed silica gain medium created by researchers from Purdue, Norfolk State and Cornell universities, a nanowire on a silver screen by a Berkeley group, and a semiconductor layer of 90 nm surrounded by silver pumped electrically by groups at the Eindhoven University of Technology and at Arizona State University. While the Purdue-Norfolk State-Cornell team demonstrated the confined plasmonic mode, the Berkeley team and the Eindhoven-Arizona State team demonstrated lasing in the so-called plasmonic gap mode. In 2018, a team from Northwestern University demonstrated a tunable nanolaser that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism.
Michal Lipson is an American physicist known for her work on silicon photonics. A member of the National Academy of Sciences since 2019, Lipson was named a 2010 MacArthur Fellow for contributions to silicon photonics especially towards enabling GHz silicon active devices. Until 2014, she was the Given Foundation Professor of Engineering at Cornell University in the school of electrical and computer engineering and a member of the Kavli Institute for Nanoscience at Cornell. She is now the Eugene Higgins Professor of Electrical Engineering at Columbia University. In 2009 she co-founded the company PicoLuz, which develops and commercializes silicon nanophotonics technologies. In 2019, she co-founded Voyant Photonics, which develops next generation lidar technology based on silicon photonics. In 2020 Lipson was elected the 2021 vice president of Optica, and serves as the Optica president in 2023.
Yoshihisa Yamamoto is the director of Physics & Informatics Laboratories, NTT Research, Inc. He is also Professor (Emeritus) at Stanford University and National Institute of Informatics (Tokyo).
An optical transistor, also known as an optical switch or a light valve, is a device that switches or amplifies optical signals. Light occurring on an optical transistor's input changes the intensity of light emitted from the transistor's output while output power is supplied by an additional optical source. Since the input signal intensity may be weaker than that of the source, an optical transistor amplifies the optical signal. The device is the optical analog of the electronic transistor that forms the basis of modern electronic devices. Optical transistors provide a means to control light using only light and has applications in optical computing and fiber-optic communication networks. Such technology has the potential to exceed the speed of electronics, while conserving more power. The fastest demonstrated all-optical switching signal is 900 attoseconds, which paves the way to develop ultrafast optical transistors.
A single-photon source is a light source that emits light as single particles or photons. Single-photon sources are distinct from coherent light sources (lasers) and thermal light sources such as incandescent light bulbs. The Heisenberg uncertainty principle dictates that a state with an exact number of photons of a single frequency cannot be created. However, Fock states can be studied for a system where the electric field amplitude is distributed over a narrow bandwidth. In this context, a single-photon source gives rise to an effectively one-photon number state.
The superconducting nanowire single-photon detector is a type of optical and near-infrared single-photon detector based on a current-biased superconducting nanowire. It was first developed by scientists at Moscow State Pedagogical University and at the University of Rochester in 2001. The first fully operational prototype was demonstrated in 2005 by the National Institute of Standards and Technology (Boulder), and BBN Technologies as part of the DARPA Quantum Network.
Photonic molecules are a form of matter in which photons bind together to form "molecules". They were first predicted in 2007. Photonic molecules are formed when individual (massless) photons "interact with each other so strongly that they act as though they have mass". In an alternative definition, photons confined to two or more coupled optical cavities also reproduce the physics of interacting atomic energy levels, and have been termed as photonic molecules.
Linear optical quantum computing or linear optics quantum computation (LOQC), also photonic quantum computing (PQC), is a paradigm of quantum computation, allowing (under certain conditions, described below) universal quantum computation. LOQC uses photons as information carriers, mainly uses linear optical elements, or optical instruments (including reciprocal mirrors and waveplates) to process quantum information, and uses photon detectors and quantum memories to detect and store quantum information.
Roberto Morandotti is a physicist and full Professor, working in the Energy Materials Telecommunications Department of the Institut National de la Recherche Scientifique. The work of his team includes the areas of integrated and quantum photonics, nonlinear and singular optics, as well as terahertz photonics.
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.
Boson sampling is a restricted model of non-universal quantum computation introduced by Scott Aaronson and Alex Arkhipov after the original work of Lidror Troyansky and Naftali Tishby, that explored possible usage of boson scattering to evaluate expectation values of permanents of matrices. The model consists of sampling from the probability distribution of identical bosons scattered by a linear interferometer. Although the problem is well defined for any bosonic particles, its photonic version is currently considered as the most promising platform for a scalable implementation of a boson sampling device, which makes it a non-universal approach to linear optical quantum computing. Moreover, while not universal, the boson sampling scheme is strongly believed to implement computing tasks which are hard to implement with classical computers by using far fewer physical resources than a full linear-optical quantum computing setup. This advantage makes it an ideal candidate for demonstrating the power of quantum computation in the near term.
Kerr frequency combs are optical frequency combs which are generated from a continuous wave pump laser by the Kerr nonlinearity. This coherent conversion of the pump laser to a frequency comb takes place inside an optical resonator which is typically of micrometer to millimeter in size and is therefore termed a microresonator. The coherent generation of the frequency comb from a continuous wave laser with the optical nonlinearity as a gain sets Kerr frequency combs apart from today's most common optical frequency combs. These frequency combs are generated by mode-locked lasers where the dominating gain stems from a conventional laser gain medium, which is pumped incoherently. Because Kerr frequency combs only rely on the nonlinear properties of the medium inside the microresonator and do not require a broadband laser gain medium, broad Kerr frequency combs can in principle be generated around any pump frequency.
Andrew Marc Weiner OSA NAE NAI was an American electrical engineer, educator and researcher known for contributions to the fields of ultrafast optics and optical signal processing. He was the Scifres Family Distinguished Professor of Electrical and Computer Engineering at Purdue University.
Photonic topological insulators are artificial electromagnetic materials that support topologically non-trivial, unidirectional states of light. Photonic topological phases are classical electromagnetic wave analogues of electronic topological phases studied in condensed matter physics. Similar to their electronic counterparts, they, can provide robust unidirectional channels for light propagation. The field that studies these phases of light is referred to as topological photonics.
Adriana Eleni Lita is a Romanian materials scientist who is a member of the faint photonics group at National Institute of Standards and Technology. She works on fabrication and development of single-photon detectors such as transition-edge sensors and superconducting nanowire single-photon detector devices.
Mark Stockman was a Soviet-born American physicist. He was a professor of physics and astronomy at Georgia State University. Best known for his contributions to plasmonics, Stockman has co-theorized plasmonic lasers, also known as spasers, in 2003.