Quantum photonics is the science of creating, detecting, controlling, and manipulating individual photons. It includes the generation, detection and coherent manipulation of photonic quantum states. It is the field of study thought by some scientists that will usher in the 'second quantum revolution'. The first quantum revolution occurred with the development of technologies such as semiconductors, transistors, lasers, and other devices exploiting quantum mechanics. The second quantum revolution will allegedly create technologies capable of exploiting quantum photonics for improved information processing in computing, cryptography, and sensing technologies. Applications include quantum computing, quantum communication, quantum simulation and quantum metrology. Quantum photonics can potentially be used to improve the security of information transfer, speed up algorithms, and increase the accuracy of measurements.
Physical realization of quantum bits (qubits), include trapped ions and atoms, superconducting circuits, quantum dots, solid-state color centers and photons. Compared to the others, information carried by photons is resistant to decoherence since photons interact weakly with transparent optical media and not at all among themselves. In quantum information processing, decoherence of the quantum state is caused by interactions with the uncontrolled environment and results in errors. Another advantage of photons is that they possess many degrees of freedom that can be used for encoding quantum information: spatial and temporal modes, frequency, polarization and angular momentum. Photons also possess continuous quantum variables including quantized field quadratures.
Conventional optical tools can be used to manipulate quantum information encoded in these degrees of freedom. Photons can be efficiently manipulated, due to their high speed transmission rates, to explore and exploit quantum phenomenon because they are not subject to thermal coupling from the external world (low-noise properties). These fundamental physical properties of photons allows for the study of quantum photonics to exist, and can potentially be leveraged to create photonic quantum technologies such as quantum computation and Quantum Information Processing (QIP). These technologies use single photons and/or the quantum states of high energy laser beams to operate.
QIP exploits of the field of quantum photonics to create quantum entanglement properties found within superposition states to improve how technological systems handle information by increasing their speed, versatility, overall performance, and measuring of quantum information.
In quantum optical computation the fundamental unit is phase-sensitive interference of two optical modes at a beam splitter. Integrated quantum photonics is an area of research with the goal of integrating components like quantum optical sources, circuits and detectors on a chip.
QPICs hold promise for scalable, reconfigurable architectures, small system footprint, enhanced light-matter interaction, high stability of optical elements and interfacing with complementary metal-oxide-semiconductor (CMOS) electronics. Many of the criteria for production of QPICs are the same as for classical photonic integrated circuits (PICs). The need to process quantum information has further requirements such as generation of high-purity quantum optical states of light and ultralow tolerance for losses, spectral mismatches, instabilities and detector imperfections which are imposed by quantum fault tolerance thresholds.
Integrated programmable waveguide circuits for classical and quantum photonic processing were developed by Ph.D. student Caterina Taballione supervised by Klaus Boller and Pepijn Pinkse at University of Twente, Netherlands. The optical components of thier photonic chip can perform quantum operations by sending single photons through the system instead of continuous light. Caterina Taballione at presented a chip with components that can either split or combine light in and from separate channels and has be likened to a rail yard. Ring-shaped resonators act as a filter in the chip. Components are controlled from the outside.
Quantum computing using photons has an advantage over qubits which work at very cold temperatures. When separately detectable single photons are used as inputs, the components support typical quantum effects like coalescence, entanglement and superposition. Temperature is used to control the components. While the single photon light source and detector operate at low temperatures, the quantum processor itself operates at room temperature. QuiX is a University of Twente spinoff company, founded by Pepijn Pinkse, Ad Lagendijk, Willem Vos, Klaus Boller and Jelmer Renema, further developing their phonic chip for complex calculations.
Important photonic devices for linear quantum information information protocols include pump sources, non-classical light sources, filters, waveguides, directional couplers, switches, quantum memories, detectors and fiber couplers. Deterministic QIP protocols also require devices that feature single-photon level nonlinearities. Solid-state quantum emitters could enable strong nonlinearity in QPICs.
Material for QPICs include silicon-based, III-V semiconductors, diamond, lithium niobite, silicon carbide, silicate glasses and plasmonic materials. Platforms may contain naturally compatible materials or composite materials.
An interface between a single atom and a single optical photon is a building block for quantum applications. Since light and matter interact weakly it is a challenge to create conditions where they interact. It is possible to create artificial photonic nanostructures that enhance light-matter coupling. Solid-state quantum emitters such as vacancy centers in diamond, molecules or quantum dots can replace single atoms which are difficult to control experimentally due to the need for trapping and cooling. The next challenge is to scale up and deterministically couple multiple quantum emitters.
Diamonds are an ideal material for the technological exploitation of quantum photonics in industry because they have a unique combination of physical properties not found in other materials. They have the largest optical bandgap of any known material, are biocompatible, and are chemically robust. These properties make them ideal for several electronic, optical, and thermal applications using quantum photonics, which includes quantum computing and QIP.
Technological advancements involving the nanafabrication of high-quality synthetic diamonds, and perfection of naturally occurring diamonds, allow for the precise control of optically active color centers within diamonds. These optically active color centers can be used to enable quantum, sensing, and labelling applications through exploiting their spin properties; particularly negatively charged nitrogen vacancy (NV) centers.
Through a process called 'doping' pure diamonds can be made into room temperature solid state semi-conductors by intentionally inserting impurities into them by altering their optically active color centers. Doped diamonds have the potential to be used for detecting ultraviolet light, ultraviolet light emitting diodes and optics, high-power microwave electronics, quantum spintronics, and quantum photonics. Researchers believe by exploiting diamond spintonics and quantum photonics they could pave the way towards creating practical quantum computing solutions.
A key challenge in quantum photonics is to obtain an efficient interface for coherent transfer of quantum states between atomic and photonic qubits. Optical nanofibers with subwavelength diameter are used in quantum photonics. Using nanofiber-bases systems it is possible to demonstrate a single-photon switch and two-qubit quantum gates to create atom-photon and photon-photon entanglement.
