Integrated photonics

Developing Integrated Photonic Technologies for Scalable Quantum Networks

Photons have emerged as a promising candidate for real-world quantum-enhanced technologies due to their long coherence times,rapid spatial propagation, and simple single photon manipulations. They have provided the platform for some of the first experiments in quantum teleportation, quantum cryptography and were the first to show experimental evidence of a violation of Bell's inequality.

Benefits of Integration

Invariably, these early demonstrations were performed using large-scale (bulk) optics bolted to optical tables with photons propagating in free-space. These experiments take up substantial room, are inherently non-scalable, and will not be sufficiently stable for reliable operation on a large scale. It became clear that scaling to systems larger than a few photons in a handful of modes would quickly become impractical to realise with this approach.

We are one of several groups moving towards an integrated photonics architecture. In this system, linear elements (beam splitters / phase shifters) are fabricated into a chip along with waveguides, providing an inherently stable, monolithic architecture allowing us to explore new physics using more photons distributed across more modes.

An integrated photonic experiment: A Silica-on-Silicon circuit consisting of many nested interferometers and phase shifters is coupled using arrays of optical fibres supplying single photons and lead to off-chip detectors

Quantum photonic networks are composed of three distinct stages:

  • Sources - A source of indistinguishable single photons
  • Circuits - Manipulation of non-classical states of light
  • Detectors - A method of counting how many photons are present in any mode

In order to fully realise the benefits of the integrated architecture we are pursuing development of all three of these stages on-chip. It is hoped that the lower loss and greater complexity afforded by these approaches will allow us to demonstrate, for the first time, a quantum enhanced system with capacity beyond that of classical components.

Integrated Sources

Many photonic quantum-enhanced technologies require low-loss sources of high purity single photons. Building such sources has proven difficult, impeding progress towards more complex experiments involving larger numbers of single photons.

Non-linear optical sources, relying on spontaneous four-wave mixing (SFWM) or spontaneous parametric down-conversion (SPDC), are the most common route to single photon production. Although these processes create photons probabilistically, one can take advantage of the pairwise production to herald the presence of a single photon by detecting its partner. Due to their relative ease of use, bulk crystal SPDC sources have historically been the workhorse for proof-of-principle quantum optics experiments. However, there is significant loss when coupling photons emitted from these crystals into the photonic chips upon which an increasing fraction of experiments are performed. Furthermore, entanglement between emitted photon pairs can lead to reduction of heralded photon purity.

These issues of photon loss and purity can be addressed by waveguide-based SPDC or SFWM sources. By constraining emission to a single guided spatial mode, single photons can be produced on (or efficiently coupled to) the chip where they are needed. This mode control also facilitates the engineering of pair emission that is not entangled, allowing straightforward heralding of high purity photons.

We have successfully demonstrated these benefits of source integration by constructing the first single photon source on a silica photonic chip (arXiv). Our SFWM source operates in an undoped silica chip fabricated via femtosecond laser writing (Optics Letter). Loss is greatly reduced with such silica-based sources due to their exceedingly low optical loss and excellent mode-matching to optical fibers, a ubiquitous component in quantum photonics. Furthermore, SFWM supports heralding of high purity states and a single waveguide can support emission over a wide spectral range from visible to telecom wavelengths. We have experimentally demonstrated heralding of single photons with >40% efficiency (80% when accounting for heralding detector inefficiency). The purity of the heralded state is measured to be 0.86 with no filtering of single-photon emission. To our knowledge, no on-chip source has demonstrated better simultaneous performance in these key areas. We anticipate such sources finding immediate application in integrated quantum optics experiments that frequently rely on similar integrated silica architectures.

Single photon source on a silica chip: Ultrashort pulses (average power~100 mW) are coupled into a birefringent waveguide residing on a silica chip. These pulses act as pump for the SFWM process, which allows low-loss heralding of high purity states.

Integrated Circuits

Our group currently employs two main technologies for integrated circuits. Both are 'direct-written', meaning there is no need to use any expensive or time-consuming lithography process. A laser simply traces out the desired circuit design, enabling rapid prototyping of different configurations.

Silica-on-Silicon (SoS)

This UV-written waveguide architecture is based on a layered structure of doped-silica supported on a silicon substrate. Beam splitters are realised by crossing two waveguides over each other at a specified angle and phase-shifting is accomplished using a small heating element deposited above the waveguides. We demonstrated the basic reconfigurable quantum operation of these devices (Optics Express) showing that they are suitable to start building more complex circuits.

A Reconfigurable quantum circuit: Thermo-optic phase shifters were used to reconfigure the integrated photonic circuit and manipulate the entanglement of multi-photon states on-chip

Three-photon Quantum Interference

In order to reach more interesting experimental regimes where classical machines may no longer be able to efficiently simulate what will happen, we must increase the number of interfering photons inside our circuit. However, this is very difficult to achieve in practice because of loss. The probability that all of our input photons make it through our circuit without being loss becomes exponentially small as the number of photons increases. We are working hard to minimise loss in all stages of our experiment. Making use of low-loss silica waveguides, high heralding efficiency down-conversion sources and optimised fiber-array coupling we recently demonstrated the first genuine quantum interference of more than two single photon input states on an integrated chip (Nature Communications).

Three photon quantum interference on a silica-on-silicon chip: Using an eight-mode integrated network, consisting of 3 nested interferometers and reconfigurable phase shifters we have shown genuine three photon quantum interference between three separate input photon states

Such multi-photon quantum interference, coupled with the complex nature of the linear optical transformation implemented by the waveguide circuit, will allow exploration of more advanced quantum information protocols such as teleportation, cluster-state generation and simple error-correcting codes.

Boson Sampling

As stated, a long-term ambition of many quantum physicists these days is to construct and understand complex quantum systems. Despite steady progress in manipulating and measuring simple quantum systems, a gulf remains between the theoretical promise of quantum computation and what has been shown in the laboratory. In 2010, Scott Aaronson and Alex Arkhipov reported a study of the computational power inherent in the quantum interference of indistinguishable, non-interacting bosons - providing a new class of problem able to solved faster with a quantum machine than a classical one without the huge technical challenges involved in builing a full quantum computer(BosonSampling).

The specific problem they posed, termed boson sampling, is appealing due to its relatively straight-forward implementation with linear optical elements. From a computational perspective, the key advance by Aaronson and Arkhipov was to show strong evidence that boson sampling is in fact interesting – that is, it cannot be solved efficiently by classical computation.

The essence of the boson sampling problem can be readily understood by examining the quantum machine which solves it. Our machine begins by injecting indistinguishable single photons into a linear optical network of single-mode waveguides. Due to quantum interference as the photons traverse the network, the photons emerge in a complicated entangled state. The positions of the photons are then measured by single-photon detectors. The machine bears some resemblance to the classical Galton board, illustrated below, in which balls randomly fall through an array of pegs. However, while these distinguishable classical balls take familiar, distinct paths down the board, rolling either left or right off of each peg they encounter, the photons in some sense collectively take all possible paths through their network. Surprisingly, the interference of these paths makes it hard for a classical computer to predict where the photons tend to emerge! In the quantum case, the probability for a particular measurement outcome after N photons are injected into a network described by a unitary matrix U, is related to the permanent of a specific N x N submatrix of U. The permanent, a function of a matrix similar to the determinant, is noteworthy since its computation is hard. Aaronson and Arkhipov showed that an efficient classical algorithm that samples from a distribution of matrix permanents, precisely the task accomplished by the quantum machine, is very unlikely to exist.

Boson Sampling: Left: A classical Galton board produces an easy-to-simulate gaussian probability distribution. Right: A quantum Boson Sampling machine produces an output probability distribution which is exponentially hard to sample from using only classical resources

This year, our group was one of two teams to report initial boson sampling experiments in Science. In fact, a remarkable interest in boson sampling became apparent last December when four groups - with experiments undertaken in Oxford, Brisbane, Vienna, and Rome - presented results on arXiv.org within the same week. In our measurements, the principles of boson sampling were verified with three and four photons in a silica-on-silicon photonic chip. A striking feature of the boson sampling problem is that it appears to be amenable to the tools of complexity theorists as well as those of quantum experimenters. Meeting these disparate requirements is the crucial challenge to achieving a direct test of quantum computational power. Future work on boson sampling will benefit from an on-going effort to understand how unavoidable experimental imperfections relate to computational complexity. Meanwhile quantum optics labs, including our own, will continue to work towards experiments with increasing numbers of single photons.

Femtosecond-written Silica

We have formed a collaboration with Engineering Science, within the University, to provide us with the 'in-house' ability to write our own waveguides. In this technology, a femtosecond laser beam is focussed down to a tight spot inside a fused silica substrate causing a local increase in refractive index, creating a guiding region.

A key advantage to this technology is the ability to write 3D waveguide circuits. Unique to our setup is the ability to finely control the waveguide size and shape at any depth within the silica and do this adaptively during the writing process(Optics Letter). This will allow a greater variety of structures to be created which we will use for demonstrations of both quantum simulations and quantum logic operations.

Integrated Detectors

The most ubiquitous single photon detectors are Silicon Avalanche Photodiodes (Si-APDs) which are sensitive to visible wavelengths up to ~900nm and have typical quantum efficiencies of 40% - 60%. This has restricted much of the work undertaken to date to wavelengths around 700nm where APDs are most efficient. It is well known, however, that the minimum propagation loss for silica occurs at 1550nm. As such, there is a wealth of developed integrated optics technology available to use at this wavelength. However, the only commercially available detectors at this wavelength are InGaAs APDs which suffer from low efficiencies and high dark count rates, all but destroying this advantage. Further, APDs are simply ‘click’ detectors which only discriminate between zero and more than zero photons arriving.

In a joint collaboration with Prof. Peter Smith at the Optoelectronics Research Centre (Southampton University, UK) and Sae Woo Nam at the National Institute of Science and Technology (Boulder, Colorado) we are developing an integrated high efficiency telecom-band single photon detector which is intrinsically number-resolving. These detectors are based on the Transition Edge Sensor (TES) which have previously shown quantum efficiencies of >98% at 1550nm.

We have successfully demonstrated the integration of these detectors with the Silica-on-Silicon platform (PRA) showing the first proof-of-principle test of this concept.

Integrated TES: a) Schematic of the integrated detector showing the TES on top of a waveguide and how the optical mode below is absorbed into the detector. b) Picture of the actual device fabricated showing the TES well aligned to the centre of the UV-written waveguide

We are currently working on improving this efficiency and have recently realised an integrated TES detector with around 88% total detection efficiency by combining geometric optimisation of the detector along with spatial multiplexing along the waveguide. The concept of our new, high-efficiency, on-chip detectors is shown below:

On-chip, high-efficiency, TES detector scheme: (a) Three TES detectors with extended absorbers are operated in series on a single waveguide, resulting in a combined 79 % single-pass detection efficiency. (b) photo of fabricated device (c) Simulated mode profile of light propagating in the waveguide

TESs in Oxford

We are currently in the process of bringing this state-of-the-art detector technology to Oxford to be able take advantage of the technology available at 1550nm without sacrificing any loss in detector efficiency. We have a brand-new lab space dedicated to running the detectors which houses all the necessary cryogenic systems required to run them. We hope to be able to use this new capability to explore ever more complex quantum states of light for simulation and information processing.