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Integrated sources of photon quantum states

Friday June 9 2017

Review by Professor David Moss just published in Nature: Light, Science and Applications.

The ability to generate complex optical photon states involving entanglement between multiple optical modes is critical to advancing our understanding of quantum mechanics and 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, methods of generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development.

A review of recent developments in integrated, or chip-based, sources of photonic quantum states of light has just been published in the Nature Journal – Light: Science and Applications (LSA) [1] by Professor David Moss and an international team of co-authors from Australia, Canada, the UK and Italy. The review focuses on sources of quantum states of light based on nonlinear optics, including single and entangled photons, with a particular emphasis on devices that are compatible with contemporary optical fibre telecommunications, quantum memory infrastructures, and 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.
Quantum frequency comb
Figure 1. Quantum frequency comb generation and detection setup based on time-bin entanglement in a ring resonator. A pulse is passed through an unbalanced Michelson interferometer (consisting of a 50/50 beam splitter, Faraday mirrors, and a phase shifter), generating two pulses with a phase difference in two respective time slots (time bins 1 and 2). The pulses are fed into the micro-ring resonator exciting one resonance. The spontaneous four-wave mixing generates signal-idler photon pairs on several ring resonances symmetric to the excited resonance either within time bins 1 or 2 (generation in both time bins is avoided by using low power). The superposition of the state generated in time bins 1 and 2 results in an entangled state output at several resonances simultaneously, generating a frequency comb of time-bin entangled photon pairs. For entanglement verification, or quantum state tomography, each photon of the spectrally filtered photon pair is individually passed through an interferometer, with the temporal imbalance equal to the time slot separation, and then detected using a single-photon detector (the phases A and B of the 2nd and 3rd interferometers can be individually controlled). Figure from [2].

Quantum mechanics underpins many of the scientific and technological advancements that have already had a significant impact on our society, ranging from ultrafast computing to high-sensitivity metrology and secure communications. Furthermore, it holds the promise of profound future innovations that will redefine many areas, such as quantum computing, offering unprecedented computational power, as well as emerging areas such as non-classical imaging and spectroscopy, where quantum mechanics offers a means to greatly increase sensitivity. In particular, the field of quantum telecommunications is already providing ultimate communications security that is directly guaranteed by the laws of physics rather than by complex mathematical algorithms.

Most of these technologies exploit the peculiar properties of quantum mechanics, such as the principles of superposition and entanglement, that allow a quantum system to be in two different states simultaneously. A quantum system composed of more than one component (e.g., particles or photons) is said to be entangled if it can only be described as a whole.

Light, and its ultimate constituent – the photon, or the quantum of light – have become a widespread basis for quantum experiments for several reasons:


  1. the possibility of easily transmitting quantum states encoded in a photon by means of free space optical links or through fibre optic networks;

  2. the advances in nonlinear optics that have enabled the generation of single and entangled photons; and

  3. the lack of extreme sensitivity to environmental noise (thermal, electromagnetic, etc.) that plagues solid-state approaches.


Nonlinear parametric processes have been instrumental in generating fundamental quantum states of light. The ability to achieve these functions on photonic integrated chips or circuits is absolutely key to moving quantum technologies out of the laboratory and into the real world. The main components of quantum photonic systems, such as mirrors, beam splitters, and phase shifters, are all now realisable in integrated form. Ultimately, all functions needed for quantum demonstrations – the generation, manipulation and detection of single/entangled photons – will be integrated on a single chip, but in the meantime even just the ability to integrate one function, such as the source of non-classical light, offers many advantages over bulk optical setups.

1. L.Caspani, R.Morandotti, Bajoni, M.Liscidini, M.Galli, C.Xiong, B.Eggleton, D.J. Moss, “On-chip sources of quantum correlated and entangled photons”, Nature: Light Science and Applications 6, e17100 (2017); Accepted Article Preview, doi:10.1038/lsa.2017.100. http://aap.nature-lsa.cn:8080/cms/root/light_1/index2.vm#June 6 2017

2. C.Reimer, M.Kues, P.Roztocki, B.Wetzel, F.Grazioso, B.Little, S.Chu, T.Johnston, Y.Bromberg, L.Caspani, D.J. Moss, and R.Morandotti, “Generation of multiphoton entangled quantum states by means of integrated frequency combs”, Science 351 Issue (6278) 1176 (2016).