Quantum processors are implemented with a variety of physical systems, and quantum processors with tens of qubits have been already reported. The leading physical systems for quantum computing include superconducting circuits, trapped ions, silicon quantum dots, and so on. However, scalable implementation of fault-tolerant quantum computers is still a major challenge for any physical system due to the inherent fragility of quantum states.
In order to protect fragile quantum states from disturbance, most of these physical systems need to be fully isolated from the external environment by keeping the systems at cryogenic temperature in dilution refrigerators or in a vacuum environment inside metal chambers.
Photonic quantum computing is one of the leading approaches to universal quantum computation. Photonic systems have several unique and advantageous features. First, quantum states of photons are maintained without vacuum or cooling systems due to their extremely weak interaction with the external environment. In other words, photonic quantum computers can work in an atmospheric environment at room temperature. Second, photons are an optimal information carrier for quantum communication since they propagate at the speed of light and offer large bandwidth for a high data transmission capacity. Therefore, photonic quantum computers are completely compatible with quantum communication.
The large bandwidth of photons also provides high-speed (high clock frequency) operation in photonic quantum computers. These advantageous features, together with mature technologies to prepare and manipulate photonic quantum states with linear optical elements and nonlinear crystals, have made photonic systems one of the leading approaches to building quantum computers.
Photonic quantum computers not only work at room temperature, they’re also integrable into existing fiber optic-based telecommunications infrastructure—one day perhaps enabling powerful quantum networks and even possibly a quantum Internet.
However, photonic quantum computers have faced problems of their own. For example, although Chinese scientists last year reported a photonic quantum computer demonstrating quantum advantage—solving a problem they say would take the world’s current top supercomputer 600 million years to accomplish. However, the bulkiness of the setup and the amount of photons it lost during operations suggest this design was not scalable. Moreover, its circuitry was not reconfigurable, and therefore could only execute a single algorithm.
Now Toronto-based Xanadu has developed a photonic quantum chip it says is programmable, can execute multiple algorithms, and is potentially highly scalable.
New record for generating multiple entangled optical qubits on a quantum computing chip
Photons have the advantage of preserving entanglement over long distances and time periods but the biggest technological challenge is the generation of multiple, stable, and controllable entangled qubit states.
In a paper, published in the journal Science, the research team outlines how it created entangled photon states with unprecedented complexity and over many parallel channels simultaneously on an integrated chip. The researchers were led by Professor David Moss, of Swinburne University of Technology, and Professor Roberto Morandotti from the Institut National de la Recherche Scientifique (INRS-EMT) in Montreal, Canada.
Breakthrough opens possibility of incorporating quantum devices directly into laptops and cell phones. “By achieving this on a chip that was fabricated with processes compatible with the computer chip industry we have opened the door to the possibility of bringing powerful optical quantum computers for everyday use closer than ever before,” Professor Morandotti says.
The research team has demonstrated that on-chip quantum frequency combs can be used to simultaneously generate multiphoton entangled quantum bit states. “This represents an unprecedented level of sophistication in generating entangled photons on a chip,” Professor Moss says. “Not only can we generate entangled photon pairs over hundreds of channels simultaneously, but for the first time we’ve succeeded in generating four-photon entangled states on a chip.” Until now, integrated systems developed by other research teams had only succeeded in generating individual two-photon entangled states on a chip.
Conventional Optical frequency combs are light sources that use mode-locked laser to create a large number of very evenly spaced frequencies. These tools can be used to make very precise measurements of different colors and are used in several applications (e.g., in atomic clocks and coherent communications).
These systems, however, are based on mode-locked lasers that generally have high complexity and large volumes. Microresonator-based Kerr frequency comb (microcomb) generation is thus an emerging technique. In this approach, high-quality-factor microresonators are used to convert a single frequency pump to a broadband comb. Microcombs offer the potential of chip-level integration and low power consumption. According to the researchers, the chip developed by his team was designed to meet numerous criteria for use in practical systems: the chip is scalable, compact, uses standard telecommunication frequencies and compatible with existing technologies.
Programmable linear optical circuit realized on a chip
A group of physicists in the UK has made a programmable photonic circuit that can be used to carry out any kind of linear optics operation and demonstrated its versatility via a series of experiments using single photons. Anthony Laing of the University of Bristol had realized that by building a device capable of reproducing any unitary operator ( matrix of complex numbers), it would be possible with that single device to carry out any linear optics experiment on the same number of input and output ports. Now Scientists and researchers at the University of Bristol and Nippon Telegraph and Telephone (NTT) in Japan have developed that device, a multifunctional photonic circuit.
The chip incorporates six wave-guides for universal linear optic transformations and 15 integrated interferometers (devices that superimpose one photon beam over another to look for anomalies in intensity or phase), each of which is individually programmable. As a result, a range of different quantum processor operations can be performed at one time.
The researchers succeeded in developing the optical device integrated on a photonic chip using planar lightwave circuit (PLC) technology. The arrangement of the optical waveguide network in the device can be modified in seconds by configuring electric voltages applied to thin-film heaters arranged across the chip. The researchers further successfully demonstrated the versatility of the device by performing various photonic QIP experiments with single photons. The demonstrations range from implementations of key components for quantum computation (entanglement generations and quantum gate operations) to the performance of state-of-the-art quantum tasks.
Laboratory for Quantum, Nonlinear and Mechanical Photonics, University of Rochester are also exploring and developing chip-scale approaches that are capable of generating, processing, storing, and detecting versatile photonic quantum states on a single chip, aiming for broad applications in computing, communication, and sensing, by taking advantage of the intriguing quantum mechanical principles.
Silicon Chip Engineered for Quantum Information Processing reported in 2018
An international team led by the University of Bristol has demonstrated the ability to control two qubits of information within a single silicon chip. This programmable two-qubit quantum processor could be used as a tool to perform quantum information experiments and could facilitate the use of silicon photonics for future photonic quantum processors.
To encode the qubits, the researchers used large-scale silicon photonic circuits to guide photons along waveguides. The quantum processor was fabricated with CMOS-compatible processing and comprises more than 200 photonic components. The researchers programmed the device to implement 98 different two-qubit unitary operations, a two-qubit quantum approximate optimization algorithm, and efficient simulation of Szegedy-directed quantum walks. “What we’ve demonstrated is a programmable machine that can do lots of different tasks,” said researcher Xiaogang Qiang.
The chip consists of many interferometers, which split the photons into different spatial modes. Each mode passes through a specific waveguide, so having a photon in one waveguide represents a 1, while in another it represents a 0. Knowing which path one photon is following tells you which path its entangled partner is on. The photons are encoded using thermo-optical phase shifters, which are controlled by electrical voltages. “Different settings of the phase shifters control the photon’s transmission behaviors in the interferometers, enabling different qubit-state encoding and different quantum operations,” Xiaogang says.
“It’s a very primitive processor, because it only works on two qubits, which means there is still a long way before we can do useful computations with this technology,” said Qiang. “But what is exciting is that the different properties of silicon photonics that can be used for making a quantum computer have been combined together in one device. This is just too complicated to physically implement with light using previous approaches.”
The team believes that its small device built from silicon could be scaled up in a cost-effective way, and emphasizes the importance of building quantum computers from technology that will allow precision on a very large scale. It sees integrated photonics as an alternative to bulky optical elements that could be too large and unstable to be used for the large, complex circuits that will be needed to build quantum computers.
“We need to be looking at how to make quantum computers out of technology that is scalable,” said researcher Jonathan Matthews. “We think silicon is a promising material to do this, partly because of all the investment that has already gone into developing silicon for the microelectronics and photonics industries.”
The small quantum processor has become a tool for further research, said the team, who has used the device to implement several different quantum information experiments using almost 100,000 different reprogrammed settings. “Since there’s been so much research and investment in silicon chips, this innovation might be found in the laptops and smartphones of the future,” said University of Queensland professor Timothy Ralph. “This is just the beginning; we’re just starting to see what kind of exponential change this might lead to.”
New Dutch silicon nitride photonics company, QuiX, aims at quantum computing
Dutch scientists from the University of Twente and the research institute AMOLF (Amsterdam) have teamed up to create the first quantum photonic processor based on silicon nitride waveguides. Supported by pre-seed investor RAPH2INVEST, Ad Lagendijk, Willem Vos, Klaus Boller, Pepijn Pinkse, and Jelmer Renema have launched QuiX with the aim to create the a road to quantum computing that builds on their fundamental research.
For quantum computers, the main advantage of photonics over other quantum computing technologies is that processors operate at room temperature, whereas most other quantum computing platforms function just above 0 K, thereby requiring costly liquid-helium cryogenics.
QuiX B.V. aims to introduce the first single-purpose photonic quantum computer on the market for use in machine learning and quantum simulation applications. The technology is based on research that has, for example, resulted in a silicon nitride waveguide based reconfigurable 8×8 integrated linear optical network for quantum information processing. In two years, QuiX will make the first components of this computer available, in the form of a photonic processor with specifications aimed at far beyond the current state-of-the-art. Such a device could be of strong interest to the academic and commercial quantum computing communities.
Jelmer Renema, the chief technical officer of QuiX, notes that the company’s photonic integrated-circuit technology is based on the TripleX technology of integrated-optics giant LioniX International (Enschede, Netherlands). “Their ultralow-loss waveguide technology enables us to produce sufficiently large matrices to facilitate complex calculations and thereby outperform classical computers,” says Renema.
Canadian startup Xanadu says their quantum computer is cloud-accessible, Python programmable, and ready to scale
Scientists from the Ontario, Canada-based quantum computing firm Xanadu and the US National Institute of Standards and Technology have taken a big step towards that future by building a light-based chip that can be programmed through cloud access.
“For a long time, photonics was considered an underdog in the quantum computing race,” says study co-author Zachary Vernon, head of hardware at Xanadu. “With these results, alongside the growing intensity of progress from academic groups and other photonic quantum computing companies, it’s becoming clear that photonics is not an underdog, but in fact one of the leading contenders.”
The new 4 millimeter by 10 millimeter X8 chip is effectively an 8-qubit quantum computer. The scientists say the silicon nitride chip is compatible with conventional semiconductor industry fabrication techniques, and can readily scale to hundreds of qubits. Infrared laser pulses fired into the chip are coupled together with microscopic resonators to generate so-called “squeezed states” consisting of superpositions of multiple photons. The light next flows to a series of beam splitters and phase shifters that perform the desired computation. The photons then flow out the chip to superconducting detectors that count the photon numbers to extract the answer to the quantum computation.
Xanadu has made the chip available over the cloud. Remote users with no knowledge of how the hardware works can still program the device using Strawberry Fields, Xanadu’s Python library for simulating and executing programs on photonic quantum hardware, and PennyLane, the company’s Python library for quantum machine learning, quantum computing and quantum chemistry.
“Quantum hardware and algorithm development have barely scratched the surface of what’s possible,” Vernon notes. “The more people working on something, the better. In order to reap the full potential of quantum computing, as many people as possible should be working on application development. If someone develops a great app with Company A’s hardware, that app will in all likelihood be equally deployable on Company B’s hardware. So it matters less where an app is developed and tested. The important part is that the app was developed in the first place.”
The researchers executed three different quantum algorithms on their fully reprogrammable chip. One, Gaussian boson sampling, analyzes random patches of data, and has many practical applications, such as identifying which pairs of molecules are the best fits for each other. Another, molecular vibronic spectra, calculates the energy of shifts between different states of a molecule, and has use in quantum chemistry. The last, graph similarity, looks for similar traits between different sets of data, and has use in data science, Vernon says.
Xanadu notes a current limitation of its systems are the superconducting photon detectors they use, which require ultra-cold temperatures. However, the company notes that future detectors may not require superconductivity or cryogenic temperatures, and that the entire machine is otherwise contained in a standard server rack.
The scientists note the greatest challenge they face in scaling up their quantum computer is reducing the amount of lost photons zipping around inside the computer’s circuitry. They suggest their quantum machines could achieve acceptably low losses using integrated beam splitters and phase shifters built using more precise, commercially-available chip fabrication tools. Xanadu now aims to make their quantum computer more useful for practical applications through error correction strategies to make them more tolerant of noise, defects and other problems, Vernon says.
The scientists detailed their findings in the March 4 issue of the journal Nature.
Technical University Of Denmark: Optical Chip Protects Quantum Technology From Errors, reported in Sep 2021
Researchers from DTU Fotonik have co-created the largest and most complex photonic quantum information processor to date – on a microchip. It uses single particles of light as its quantum bits, and demonstrates a variety of error-correction protocols with photonic quantum bits for the first time.
“We made a new optical microchip that processes quantum information in such a way that it can protect itself from errors using entanglement. We used a novel design to implement error correction schemes, and verified that they work effectively on our photonic platform,” says Jeremy Adcock, postdoc at DTU Fotonik and co-author of the Nature Physics paper.
“Error correction is key to developing large-scale quantum computers” Jeremy Adcock, postdoc at DTU Fotonik
“Chip-scale devices are an important step forward if quantum technology is going to be scaled up to show an advantage over classical computers. These systems will require millions of high-performance components operating at the fastest possible speeds, something that is only achieved with microchips and integrated circuits, which are made possible by the ultra-advanced semiconductor manufacturing industry,” says co-author Yunhong Ding, senior researcher at DTU Fotonik.
To realize quantum technology that goes beyond today’s powerful computers requires scaling this technology further. In particular, the photon (particles of light) sources on this chip are not efficient enough to build quantum technology of useful scale.
“At DTU, we are now working on increasing the efficiency of these sources – which currently have an efficiency of just 1 per cent – to near-unity. With such a source, it should be possible to build quantum photonic devices of vastly increased scale, and reap the benefits of quantum technology’s native physical advantage over classical computers in processing, communicating, and acquiring information, says postdoc at DTU Fotonik, Jeremy Adcock. He continues: “With more efficient photon sources, we will be able to build more and different resource states, which will enable larger and more complex computations, as well as unlimited range secure quantum communications.”
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