Integrated photonics plays a crucial role in scaling up quantum technology.
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Quantum mechanics technology is expected to transform how the world operates in the near future. The past several years have already seen quantum technologies reach important milestones, with governments heavily investing in further development. Several private quantum start-up companies have also sprung out to address different innovations in quantum computing, sensing, and communication.
Quantum technology has been explored on several platforms. For example, using cold neutral atoms, trapped ions, single-photon emitters generated in condensed matter systems, photons, and superconductors.
Most of these platforms rely on integrated photonics to manipulate, transport, and detect qubits. A qubit, also known as a quantum bit, is the quantum-mechanical analog of a classical bit. Quantum mechanical properties such as superposition and entanglement, which are fundamental building blocks of a quantum device, are leveraged through photonics.
Photonic Integrated Circuit
An integrated circuit is a chip comprising electronic elements that form a functional circuit. These chips are embedded inside a smartphone, computer, and other electronic devices. Similarly, a photonic integrated circuit (PIC) is a chip that comprises photonic modules that work with photons. Photons are particles of light.
Quantum Computing with Photons
Photonic chips are widely used for quantum computing. These chips are made of materials such as silicon, silicon nitrides, or other semiconductor materials. Quantum computing chips comprise three different functional segments.
The first segment is the input module. Classical laser light is distributed to an array of optical tweezers. Tweezers are microscopic devices that are made of small ring resonators, which, when driven by bright classical light pulses, generate a special quantum state of light called squeezed state.
A photon-based qubit can be initiated with a squeezed state. These states involve a quantum superposition of a different number of photons. Once generated, they are coupled into an array of optical fibers or waveguides which carry them to the next stage. Other quantum technology platforms also use integrated photonic chips to interface qubits generated by other means.
In the second phase of PIC, the squeezed states enter a network of beam splitters and phase shifters called an interferometer. The interferometer is programmed using software, and user instructions are loaded electronically. The control systems translate these instructions by applying a set of electrical voltages to different components on the chip.
The squeezed states interact with one another and are entangled. Entanglement is a prerequisite for quantum computing. The interferometer can be thought of as a sequence of quantum gates operating on the input. In general, the output of the interferometer is a highly entangled quantum state encoding the processed quantum information.
After the quantum states are manipulated and encoded with data, they are ready for read-out. In the third segment of the PIC, each output of the chip is directed to a special single-photon counter. These detectors measure how many photons are present in each output yielding an array of integers that are reported back to the user. The result of the computation or algorithm is encoded in the statistics of this photon number data.
Scaling Quantum Technology
While many advances have been achieved, to extract the full potential of quantum technology, the number of controllable qubits in a quantum system must be scaled up.
Hundreds of times higher qubits than what is possible today will be needed to make impactful quantum devices. However, translating results from the lab environment to everyday applications has been a major challenge.
Transformative quantum technology, when fully functional, will require more than 1,000 optical components. All these components will also have to be individually optimized.
According to scientists, more research and development is required to overcome some of the current limitations. One experimental approach that can benefit future photonic quantum technology is to develop new classical photonic integration methods in parallel.
Since classical integrated photonic devices can be used for quantum applications, chip-level integration will be a crucial development for scaling up and translating laboratory efforts into commercial technologies.
Integrated photonic platforms will require multiple materials, component designs, and integration strategies.
A major challenge to be solved will be signal losses, which are not easily compensated for in quantum systems. Major investments and commitments by government authorities will help improve integrated photonics by identifying specific technological challenges and implementing strategic developmental plans, benchmarking, and upgrading the infrastructure towards a sustainable ecosystem.
References and Further Reading
Emanuele Pelucchi, Giorgos Fagas, Igor Aharonovich, Dirk Englund, Eden Figueroa, Qihuang Gong, Hübel Hannes, Jin Liu, Chao-Yang Lu, Nobuyuki Matsuda, Jian-Wei Pan, Florian Schreck, Fabio Sciarrino, Christine Silberhorn, Jianwei Wang, Klaus D. Jöns. The potential and global outlook of integrated photonics for quantum technologies. Nature Reviews Physics, 2021; https://doi.org/10.1038/s42254-021-00398-z
Kleese van Dam, K. From Long-distance Entanglement to Building a Nationwide Quantum Internet: Report of the DOE Quantum Internet Blueprint Workshop (Brookhaven National Laboratory, 2020).
Awschalom, D. et al. Development of quantum interconnects (quics) for next-generation information technologies. PRX Quantum 2, 017002 (2021). https://doi.org/10.1103/PRXQuantum.2.017002
European Commission. New Strategic Research Agenda on Quantum technologies (European Quantum Flagship, 2020); https://digital-strategy.ec.europa.eu/en/news/new-strategic-research-agenda-quantum-technologies.