The first quantum technology that harnesses quantum mechanical effects for its core operation has arrived in the form of commercially available quantum key distribution systems. This technology achieves enhanced security by encoding information in photons such that an eavesdropper in the system can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, which promise exponentially greater computational power for particular tasks. Photonics is destined to have a central role in such technologies owing to the high-speed transmission and outstanding low-noise properties of photons.

Quantum computing technology promises to be significantly faster than traditional computing, which reads and writes data encoded as bits that are either a zero or one. Instead of bits, quantum computing uses qubits that can be in two states at the same time and will interact, or correlate, with each other. These qubits, which can be an electron or photon, allow many processes to be performed simultaneously.

There are several advantages to encoding a quantum bit onto a photon: the preparation and manipulation of single photons is easy, and the photonic qubit is a natural candidate for quantum imaging and communications. Furthermore, the qubit can be encoded in various degrees of freedom such as on the position, phase, time‐bin, energy, angular momentum, polarization of the photon—or a combination of these. This flexibility leads to enormous opportunities for scientific exploration and in practical applications.

**Challenges**

Unfortunately optical QC has a serious drawback: the difficulty in implementing two-qubit gates. Realizing the nonlinearity required for entangling two qubits is challenging, so alternatives such as the teleportation of nondeterministic quantum gates have been investigated. While this approach is still impractical due to the large amount of required resources, another solution may be found in measurement-based QC. In 2014, Pfister’s group succeeded in generating more than 3,000 quantum modes in a bulk optical system. However, using this many quantum modes requires a large footprint to contain the thousands of mirrors, lenses and other components that would be needed to run an algorithm and perform other operations.

Despite their success, photonic quantum technologies currently face a significant roadblock to widespread application: the circuits are severely limited in complexity. One measure of circuit complexity is the product of its breadth (number of photons), and its depth (number of interactions between the photons). According to “The Australian Centre of Excellence for Quantum Computation & Communication Technology “, Increasing circuit complexity will require solutions to the following inherent difficulties:

1) It is difficult to store photons, since they interact weakly with other particles and move at the speed of light. This limits circuit breadth, since many protocols require holding information in one part of the circuit while waiting for information to be processed in parallel.

2) It is difficult to efficiently produce and detect single photons. The current best photon sources are spontaneous, i.e. the photons are produced at random times with probability, p<1. This quickly limits circuit breadth, since the probability of producing 1 photon per mode decreases exponentially (for N input modes it is p^N << 1). A major challenge is the design of telecom photon emitters with high efficiency, good yield, spectral purity, stability and the possibility of mass manufacture.

3) Current photonic entangling gates are inherently random—with success rates varying between 9% and 25% which means they cannot be scaled.

### Photonic Integrated Circuits (PIC) or Integrated Photonic circuits (IPC)

The ongoing miniaturization of photonic structures due to the availability of sophisticated nanofabrication has provided huge opportunities for physical research of novel phenomena in nanophotonic systems and quantum technological applications. Just as Integrated circuit (IC) is a microelectronic device that houses multiple electric circuits on a chip, a photonic integrated circuit (PIC) or Integrated Photonic circuits (IPC) are devices that integrate multiple photonic functions on a chip. A typical IPC may consist of single photon sources, nonlinear photon processing circuits and photon detectors all integrated onto a solid-state chip. Photonic integrated circuits (PICs) have attracted considerable attention owing to their small footprint, scalability, reduced power consumption and enhanced processing stability.

The technology of integrated quantum photonics has enabled the generation, processing and detection of quantum states of light at a steadily increasing scale and level of complexity, progressing from few-component circuitry occupying centimetre-scale footprints and operating on two photons, to programmable devices approaching 1,000 components occupying millimetre-scale footprints with integrated generation of multiphoton states. Recently Photonic integrated circuits are also being used for Quantum Information Sciences like Quantum sensors, computing, Quantum key cryptography and simulation. Moreover, with the advancements in quantum computing, the adoption of photonic ICs is increasing as they allow multitasking that quantum computing readily requires.

“Photonic quantum technologies have reached a number of important milestones over the last 20 years. However, scalability remains a major challenge when it comes to translating results from the lab to everyday applications. Applications often require more than 1,000 optical components, all of which have to be individually optimized. Photonic quantum technologies can, though, benefit from the parallel developments in classical photonic integration,” explains Jöns. According to the scientists, more research is required.

“The integrated photonic platforms, which require a variety of multiple materials, component designs and integration strategies, bring multiple challenges, in particular signal losses, which are not easily compensated for in the quantum world,” continues Jöns. In their paper, the authors state that the complex innovation cycle for integrated photonic quantum technologies (IPQT) requires investments, the resolution of specific technological challenges, the development of the necessary infrastructure and further structuring towards a mature ecosystem. They conclude that there is an increasing demand for scientists and engineers with substantial knowledge of quantum mechanics and its technological applications.

Integrated quantum photonics uses classical integrated photonic technologies and devices for quantum applications, whereby chip-level integration is critical for scaling up and translating laboratory demonstrators to real-life technologies. Jöns explains that “efforts in the field of integrated quantum photonics are broad-ranging and include the development of quantum photonic circuits, which can be monolithically, hybrid or heterogeneously integrated. In our paper, we discuss what applications may become possible in the future by overcoming the current roadblocks.” The scientists also provide an overview of the research landscape and discuss the innovation and market potential. The aim is to stimulate further research and research funding by outlining not only the scientific issues, but also the challenges related to the development of the necessary manufacturing infrastructure and supply chains for bringing the technologies to market.

### Army researchers predict quantum computer based on photonic circuits will no longer need extremely cold temperatures to function could become a reality after about a decade.

Unfortunately, one of the major drawbacks of quantum systems is the fragility of the strange states of the qubits. Most prospective hardware for quantum technology must be kept at extremely cold temperatures — close to zero kelvins — to prevent the special states being destroyed by interacting with the computer’s environment. “Any interaction that a qubit has with anything else in its environment will start to distort its quantum state,” Jacobs said. “For example, if the environment is a gas of particles, then keeping it very cold keeps the gas molecules moving slowly, so they don’t crash into the quantum circuits as much.”For years, solid-state quantum technology that operates at room temperature seemed remote.

Researchers have directed various efforts to resolve this issue, but a definite solution is yet to be found. At the moment, photonic circuits that incorporate nonlinear optical crystals have presently emerged as the sole feasible route to quantum computing with solid-state systems at room temperatures.

While the application of transparent crystals with optical nonlinearities had emerged as the most likely route to this milestone, the plausibility of such a system always remained in question. Now, Army scientists have officially confirmed the validity of this approach. Dr. Kurt Jacobs, of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, working alongside Dr. Mikkel Heuck and Prof. Dirk Englund, of the Massachusetts Institute of Technology, became the first to demonstrate the feasibility of a quantum logic gate comprised of photonic circuits and optical crystals.

“If future devices that use quantum technologies will require cooling to very cold temperatures, then this will make them expensive, bulky, and power hungry,” Heuck said. “Our research is aimed at developing future photonic circuits that will be able to manipulate the entanglement required for quantum devices at room temperature.” Unlike quantum systems that use ions or atoms to store information, quantum systems that use photons can bypass the cold temperature limitation. However, the photons must still interact with other photons to perform logic operations. This is where the nonlinear optical crystals come into play.

Researchers can engineer cavities in the crystals that temporarily trap photons inside. Through this method, the quantum system can establish two different possible states that a qubit can hold: a cavity with a photon (on) and a cavity without a photon (off). These qubits can then form quantum logic gates, which create the framework for the strange states. In other words, researchers can use the indeterminate state of whether or not a photon is in a crystal cavity to represent a qubit. The logic gates act on two qubits together, and can create “quantum entanglement” between them. This entanglement is automatically generated in a quantum computer, and is required for quantum approaches to applications in sensing.

However, scientists based the idea to make quantum logic gates using nonlinear optical crystals entirely on speculation — up until this point. While it showed immense promise, doubts remained as to whether this method could even lead to practical logic gates. The application of nonlinear optical crystals had remained in question until researchers at the Army’s lab and MIT presented a way to realize a quantum logic gate with this approach using established photonic circuit components. “The problem was that if one has a photon travelling in a channel, the photon has a ‘wave-packet’ with a certain shape,” Jacobs said. “For a quantum gate, you need the photon wave-packets to remain the same after the operation of the gate. Since nonlinearities distort wave-packets, the question was whether you could load the wave-packet into cavities, have them interact via a nonlinearity, and then emit the photons again so that they have the same wave-packets as they started with.”

Once they designed the quantum logic gate, the researchers performed numerous computer simulations of the operation of the gate to demonstrate that it could, in theory, function appropriately. Actual construction of a quantum logic gate with this method will first require significant improvements in the quality of certain photonic components, researchers said. “Based on the progress made over the last decade, we expect that it will take about ten years for the necessary improvements to be realized,” Heuck said. “However, the process of loading and emitting a wave-packet without distortion is something that we should able to realize with current experimental technology, and so that is an experiment that we will be working on next.”

### Photonic Quantum Technologies

One important challenge in the development of quantum computers is finding a way to measure and manipulate the thousands of qubits needed to process extremely large data sets. For photon-based methods, the number of qubits can be increased without using more photons by increasing the number of modes encoded in photonic degrees of freedom— such as polarization, frequency, time and location—measured for each photon. This allows each photon to exhibit more than two modes, or states, simultaneously. The researchers previously used this approach to fabricate the world’s largest photonic quantum chips, which could possess a state space equivalent to thousands of qubits.

However, incorporating the new photonic quantum chips into a quantum computer requires measuring all the modes and their photonic correlations at a single-photon level. Until now, the only way to accomplish this would be to use one single-photon detector for each mode exhibited by each photon. This would require thousands of single-photon detectors and cost around 12 million dollars for a single computer. “It is economically unfeasible and technically challenging to address thousands of modes simultaneously with single-photon detectors,” said Jin. “This problem represents a decisive bottleneck to realizing a large-scale photonic quantum computer.”

For the first time, researchers have demonstrated a way to map and measure large-scale photonic quantum correlation with single-photon sensitivity. The ability to measure thousands of instances of quantum correlation is critical for making photon-based quantum computing practical. A multi-institutional group of researchers reports the new measurement technique, which is called correlation on spatially-mapped photon-level image (COSPLI). They also developed a way to detect signals from single photons and their correlations in tens of millions of images.

Although commercially available CCD cameras are sensitive to single photons and much cheaper than single-photon detectors, the signals from individual photons are often obscured by large amounts of noise. After two years of work, the researchers developed methods for suppressing the noise so that single photons could be detected with each pixel of a CCD camera. The other challenge was to determine a single photon’s polarization, frequency, time and location, each of which requires a different measurement technique. With COSPLI, the photonic correlations from other modes are all mapped onto the spatial mode, which allows correlations of all the modes to be measured with the CCD camera.

To demonstrate COSPLI, the researchers used their approach to measure the joint spectra of correlated photons in ten million image frames. The reconstructed spectra agreed well with theoretical calculations, thus demonstrating the reliability of the measurement and mapping method as well as the single-photon detection. The researchers are now working to improve the imaging speed of the system from tens to millions of frames per second.

“COSPLI has the potential to become a versatile solution for performing quantum particle measurements in large-scale photonic quantum computers,” said the research team leader Xian-Min Jin, from Shanghai Jiao Tong University, China. “This unique approach would also be useful for quantum simulation, quantum communication, quantum sensing and single-photon biomedical imaging.” “We know it is very hard to build a practical quantum computer, and it isn’t clear yet which implementation will be the best,” said Jin. “This work adds confidence that a quantum computer based on photons may be a practical route forward.”

### Quantum computing platform accelerates the transition from bulk optics to integrated photonics on a silicon chip smaller than a penny, reported in August 2021

A research team led by Xu Yi, assistant professor of electrical and computer engineering at the University of Virginia School of Engineering and Applied Science, has carved a niche in the physics and applications of photonic devices, which detect and shape light for a wide range of uses including communications and computing. His research group has created a scalable quantum computing platform, which drastically reduces the number of devices needed to achieve quantum speed, on a photonic chip the size of a penny.

Olivier Pfister, professor of quantum optics and quantum information at UVA, and Hansuek Lee, assistant professor at the Korean Advanced Institute of Science and Technology, contributed to this success. Nature Communications recently published the team’s experimental results, A Squeezed Quantum Microcomb on a Chip. Two of Yi’s group members, Zijiao Yang, a Ph.D. student in physics, and Mandana Jahanbozorgi, a Ph.D. student of electrical and computer engineering, are the paper’s co-first authors. A grant from the National Science Foundation’s Engineering Quantum Integrated Platforms for Quantum Communication program supports this research.

Quantum computing promises an entirely new way of processing information. Your desktop or laptop computer processes information in long strings of bits. A bit can hold only one of two values: zero or one. Quantum computers process information in parallel, which means they don’t have to wait for one sequence of information to be processed before they can compute more. Their unit of information is called a qubit, a hybrid that can be one and zero at the same time. A quantum mode, or qumode, spans the full spectrum of variables between one and zero — the values to the right of the decimal point. Researchers are working on different approaches to efficiently produce the enormous number of qumodes needed to achieve quantum speeds.

Yi’s photonics-based approach is attractive because a field of light is also full spectrum; each light wave in the spectrum has the potential to become a quantum unit. Yi hypothesized that by entangling fields of light, the light would achieve a quantum state. You are likely familiar with the optical fibers that deliver information through the internet. Within each optical fiber, lasers of many different colors are used in parallel, a phenomenon called multiplexing. Yi carried the multiplexing concept into the quantum realm.

“The future of the field is integrated quantum optics,” Pfister said. “Only by transferring quantum optics experiments from protected optics labs to field-compatible photonic chips will bona fide quantum technology be able to see the light of day. We are extremely fortunate to have been able to attract to UVA a world expert in quantum photonics such as Xu Yi, and I’m very excited by the perspectives these new results open to us.”

Yi’s group created a quantum source in an optical microresonator a ring-shaped, millimeter-sized structure that envelopes the photons and generates a microcobe, a device that efficiently converts photons from single to multiple wavelengths. Light circulates around the ring to build up optical power. This power buildup enhances chances for photons to interact, which produces quantum entanglement between fields of light in the microcomb. hrough multiplexing, Yi’s team verified the generation of 40 qumodes from a single microresonator on a chip, proving that multiplexing of quantum modes can work in integrated photonic platforms. This is just the number they are able to measure.

“We estimate that when we optimize the system, we can generate thousands of qumodes from a single device,” Yi said. Yi’s multiplexing technique opens a path toward quantum computing for real-world conditions, where errors are inevitable. This is true even in classical computers. But quantum states are much more fragile than classical states. The number of qubits needed to compensate for errors could exceed one million, with a proportionate increase in the number of devices. Multiplexing reduces the number of devices needed by two or three orders of magnitude.

Yi’s photonics-based system offers two additional advantages in the quantum computing quest. Quantum computing platforms that use superconducting electronic circuits require cooling to cryogenic temperatures. Because the photon has no mass, quantum computers with photonic integrated chips can run or sleep at room temperature. Additionally, Lee fabricated the microresonator on a silicon chip using standard lithography techniques. This is important because it implies the resonator or quantum source can be mass-produced.

“We are proud to push the frontiers of engineering in quantum computing and accelerate the transition from bulk optics to integrated photonics,” Yi said. “We will continue to explore ways to integrate devices and circuits in a photonics-based quantum computing platform and optimize its performance.”

### References and Resources also include:

https://www.sciencedaily.com/releases/2021/08/210820153706.htm

https://phys.org/news/2022-01-photonics-quantum-technologies.html