Alexander S. Solntsev, Frank Setzpfandt, Alex S. Clark, Chunlin Wu, Matthew J. Collins, Chunhe Xiong, A. Schreiber, Fabian Katzschmann, Falk X. Eilenberger, Roland Schiek, W. Sohler, Arnan Mitchell, Christine Silberhorn, Benjamin J. Eggleton, Thomas Pertsch, Andrey A. Sukhorukov, Dragomir N. Neshev, Yuri S. Kivshar
Quantum entanglement underpins the realization of quantum simulators and computers, which can be used for unbreakable cryptography and powerful computational algorithms. Entangled photons are an ideal medium for creating and manipulating quantum states due to the low noise and ease of transmission. A qubit encoded into a photon can be easily sent between different photonic elements along an optical fiber, in analogy with the transmission of classical bits along electrical wires. Furthermore, logic operations can be performed on entangled photons by exploiting the nonlinearity inherent to quantum measurements [1]. The practical implementation of complex applications requires the minimization of coupling losses, as well as stable quantum interference. Low losses and interferometric stability allow strong entanglement between output photons. One of the biggest milestones in achieving these requirements is the integration of photon sources together with optical circuits on the same photonic chip.
Quantum entanglement underpins the realization of quantum simulators and computers, which can be used for unbreakable cryptography and powerful computational algorithms. Entangled photons are an ideal medium for creating and manipulating quantum states due to the low noise and ease of transmission. A qubit encoded into a photon can be easily sent between different photonic elements along an optical fiber, in analogy with the transmission of classical bits along electrical wires. Furthermore, logic operations can be performed on entangled photons by exploiting the nonlinearity inherent to quantum measurements [1]. The practical implementation of complex applications requires the minimization of coupling losses, as well as stable quantum interference. Low losses and interferometric stability allow strong entanglement between output photons. One of the biggest milestones in achieving these requirements is the integration of photon sources together with optical circuits on the same photonic chip.
Integrated optical platforms enable the realization of complex quantum photonic circuits for a variety of applications including quantum simulations, computations, and communications. The development of on-chip integrated photon sources, providing photon quantum states with on-demand tunability, is currently an important \nresearch area. A flexible approach for on-chip generation of entangled photons is based on spontaneous nonlinear \nfrequency conversion, with possibilities to integrate several photon-pair sources [1] and realize subsequent post \nprocessing using thermo-optically or electro-optically controlled interference [2, 3]. However, deterministic postprocessing can only provide a limited set of output states, whereas quantum gates with probabilistic operation are \nneeded to generate arbitrary two-photon states [4].
Quantum technology is a fundamentally new way of harnessing Nature and it has potential for a truly revolutionary innovation and promise the next generation of products with exciting and astounding properties that will affect our lives profoundly. They will have a great influence in defence, aerospace, energy and telecommunications sectors. If this process is to continue in the future, new, quantum technology must replace or supplement what we have now. In particular, Quantum Information Technology can support entirely new modes of information processing based on so called quantum bits or qubits. Its eventual impact may be as great as or greater than that of its classical predecessor. There is almost daily progress in developing promising technologies for realizing quantum information processing with various advantages over its classical counterpart. Logic gates and wires are becoming smaller, and soon they will be made out of only a handful of atoms. Photons are unsurpassed as qubits in terms of decoherence times, mobility, and achievability of high-fidelity single-qubit operations. Thus far, entanglement experiments in optics have been very clean, and an optical quantum processor would obviously have an advantage in connecting to a quantum “network” (no need to convert between stationary and flying qubits). All-optical quantum computing became feasible when, in 2001, a breakthrough known as the KLM (Knill-Laflamme-Milburn) scheme showed that scalable quantum computing is possible using only single-photon sources, detectors, and linear optical circuits. This scheme relies on quantum interference with auxiliary photons at a beam splitter and single-photon detection to induce not-deterministic interactions. Nowadays, the limits of the original KLM linear optics quantum computing proposal, as well as the more recent cluster and graph-state approaches, in addition to various nonlinear optical approaches are explored, but several challenges must be overcome. The most important issues are the practical scalability of quantum circuits, the ability to perform quantum logic gates with error rates below the fault-tolerant threshold and their incorporation into large-scale quantum circuits with realistic physical resources and single-photon detectors packed on an optical chip. Optical integrated circuits offer great potential for realizing previously unfeasible large-scale quantum circuits. However, this architecture suffers not only of the overhead of multiple sources, detectors and ancillary qubits, but it needs to maintain the complete coherence between single photons in which qubits are encoded. The idea to reduce the overhead due to multiple single photon sources with very high degree of reciprocal coherence exploits the possibility to encode the whole state space in a complex optical circuit based on Multiple Rail (MR) Architecture: a particular state in input is encoded in the quantum superposition of a coherent light beam trough the different waveguides, so that it is possible to reproduce the mathematical and physical aspects of quantum entanglement in classical beams of light. Multiple Rail Architecture provides the realization of so called Classical or, better, Intra-System Entanglement. In practics a reconfigurable optical matrix of interconnected MachZehnder provides the representation of the complex Hilbert-Schmidt space where the quantum operation lives. This integrated system on chip can be both Photonics and/or Plasmonics. By exploiting this idea we are able to obtain all the universal quantum gates as SWAP, CNOT, CPHASE, TOFFOLI for two qubits and for multiqubits cases. The most important goal is the possibility to perform deterministic Bell measurements: in usual optical schemes the probability of success of Bell measurements is ½, while the novel architecture provides the possibility to discriminate all Bell states, so that the teleportation can become completely deterministic.
Photon entanglement has a range of applications from secure communication to the tests of quantum mechanics. Utilizing optical nonlinearity for the generation of entangled photons remains the most widely used approach due to its quality and simplicity. The on-chip integration of entangled light sources has enabled the increase of complexity and enhancement of stability compared to bulk optical implementations. Entanglement over different optical paths is uniquely suited for photonic chips, since waveguides are typically optimized for particular wavelength and polarization, making polarization- and frequency-entanglement less practical. In this review we focus on the latest developments in the field of on-chip nonlinear path-entangled photon sources. We provide a review of recent implementations and compare various approaches to tunability, including thermo-optical, electro-optical and all-optical tuning. We also discuss a range of important technical issues, in particular the on-chip separation of the pump and generated entangled photons. Finally, we review different quality control methods, including on-chip quantum tomography and recently discovered classical-quantum analogy that allows to characterize entangled photon sources by performing simple nonlinear measurements in the classical regime.
The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent advances in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world.
Integrated optical circuits enable stable and scalable realization of quantum logic devices, which can form a basis for the mass production of photonic chips for quantum communications and computations. An important challenge is the integration of single-photon sources, which should provide on-chip generation and preparation of quantum photon states. Spontaneous parametric down-conversion (SPDC) provides an attractive solution for experimental generation of correlated and entangled photon pairs [1,2]. However, internal losses in the waveguides may still be present, affecting nontrivially the emerging photon state [3].
Abstract High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices—including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different discrete degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of quantum states, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we focus on the high-dimensional versions of qubits—qudits—and review the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources, and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.