Toshiba shrinks quantum system to semiconductor size

QKD addresses the demand for cryptography which will remain secure from attack by the supercomputers of tomorrow. In particular, a large-scale quantum computer will be able to efficiently solve the difficult mathematical problems that are the basis of the public key cryptography widely used today for secure communications and e-commerce. In contrast, the protocols used for quantum cryptography can be proven secure from first principles and will not be vulnerable to attack by a quantum computer, or indeed any computer in the future.

According to Toshiba’s long-term estimates – extrapolated from existing short to mid-term estimates by research firms – the QKD market is expected to grow to approximately $20bn worldwide by 2035. Large quantum-secured fibre networks are currently under construction in Europe and South-East Asia and there are plans to launch satellites that can extend the networks to a global scale. In October 2020, Toshiba released two products for fibre-based QKD, which are based on discrete optical components. With its project partners, Toshiba has already implemented quantum-secured metro networks and long-distance fibre optic backbone links in the UK, Europe, US and Japan.

For quantum cryptography to become as ubiquitous as the algorithmic cryptography we use today, it is important that the size, weight and power consumption are further reduced. This is especially true for extending QKD and quantum random number generators (QRNG) into new domains such as the last-mile connection to the customer or IoT. The development of chip-based solutions is essential to enabling mass market applications, which will be integral to the realisation of a quantum-ready economy.

Toshiba has now developed techniques for shrinking the optical circuits used for QKD and QRNG into tiny semiconductor chips. These are not only much smaller and lighter than their fibre-optic counterparts, they also consume less power. Many can also be fabricated in parallel on the same semiconductor wafer using standard techniques used within the semiconductor industry, allowing them to be manufactured in much larger numbers. For example, the quantum transmitter chips developed by Toshiba measure just 2x6mm, allowing several hundred chips to be produced simultaneously on a wafer.

QKD systems typically comprise a complex fibre-optic circuit, integrating discrete components, such as lasers, electro-optic modulators, beam-splitters and fibre couplers. As these components are relatively bulky and expensive, the purpose of the Toshiba team’s work at its Cambridge Research Lab was to develop a QKD system in which the fibre-optic circuit and devices are written in millimetre-scale semiconductor chips.

Toshiba has developed the first complete QKD prototype in which quantum photonic chips of different functionality are deployed. Random bits for preparing and measuring the qubits are produced in quantum random number generator (QRNG) chips and converted in real-time into high-speed modulation patterns for the chip-based QKD transmitter (QTx) and receiver (QRx) using field-programmable gate arrays (FPGAs).

Photons are detected using fast-gated single photon detectors. Sifting, photon statistics evaluation, time synchronisation and phase stabilisation are done via a 10Gb/s optical link between the FPGA cores, enabling autonomous operation over extended periods of time. As part of the demonstration, the chip QKD system was interfaced with a commercial encryptor, allowing secure data transfer with a bit rate up to 100Gb/s.

Taofiq Paraiso, lead author of the research paper describing the chip-scale QKD system, said: “We are witnessing with photonic integrated circuits [PICs] a similar revolution to that which occurred with electronic circuits. PICs are continuously serving more and more diverse applications. Of course, the requirements for quantum PICs are more stringent than for conventional applications, but this work shows that a fully deployable chip-based QKD system is now attainable, marking the end of an important challenge for quantum technologies. This opens a wide-range of perspectives for the deployment of compact, plug-and-play quantum devices that will certainly strongly impact our society.”

Andrew Shields, head of quantum technology at Toshiba Europe, said: “Photonic integration will allow us to manufacture quantum security devices in volume in a highly repeatable fashion. It will enable the production of quantum products in a smaller form factor and subsequently allow the roll out of QKD into a larger fraction of the telecom and datacom network.”

Taro Shimada, chief digital officer of Toshiba Corporation, said: “Toshiba has invested in quantum technology R&D in the UK for over two decades. This latest advancement is highly significant, as it will allow us to manufacture and deliver QKD in much larger quantities. It is an important milestone towards our vision of building a platform for quantum-safe communications based upon ubiquitous quantum security devices.”

The details of Toshiba’s research have been published in the scientific journal, Nature Photonics.

Earlier this month, BT and Toshiba announced plans to build the world’s first commercially available, quantum-secured metro network in London. The network will connect sites in London’s West End, the City and the M4 corridor, providing data services secured using quantum key distribution (QKD) and post-quantum cryptography (PQC) encryption.

This week, researchers from Los Alamos National Laboratory published a paper which describes the use of a novel theorem to demonstrate that convolutional neural networks can always be trained on quantum computers, thus overcoming the obstacle of ‘barren plateaus’.