• Toshiba Europe has announced a breakthrough in quantum communications over standard fibre-optic cable and at normal room temperature
• Simple chip technology replaces complex cryogenics to allow quantum communications over regular telecom infrastructure 
• It has also enabled the first deployment of twin-field quantum-key distribution (TF-QKD) over greater distances than possible with point-to-point QKD
• Possibility of building quantum internet between cities and countries gets closer

Toshiba Europe has announced the successful completion of coherent, secure quantum communications over an existing telecom network using standard fibre optic cable at normal room temperature and completely without the need for any highly complex cryogenic components. 

It’s a remarkable feat. Instead of relying on low temperatures and temperamental equipment, the trial ran quantum comms over 155 miles of off-the-shelf fibre optic equipment and relied throughout on straightforward, comparatively inexpensive, simple, and easily available semiconductor-based devices. The trial took place in Germany between sites in Kehl and Frankfurt over Deutsche Telekom’s fibre network.

If it proves possible to scale-up and apply the technology operationally at full telco or data network level and then make it commercially available to telcos, service providers and datacentre operators, it would be a breakthrough of tremendous importance and greatly lower the barrier for the real-world deployment of quantum-safe communications.

Currently, quantum communications are almost entirely reliant on the extremely low temperatures that are required to maintain the extremely fragile quantum states of the qubits that are the fundamental units of quantum systems. That intense level of cooling is usually chained by using liquid helium. At temperatures even a fraction of a degree above absolute zero (0 kelvin, -273.15 degrees celsius, or, -460 degrees fahrenheit) quantum communications systems including trapped ions, photons and superconducting circuits become vulnerable to environmental interference of many types, including electro-magnetics, random thermal fluctuations, and vibration. Any of these factors, and many more can, and do, cause qubits to ‘flip’ between their states and that leads to the instant propagation of errors in computing calculations and data transmission.

Quantum coherence, the ability of a qubit to exist in a state of superposition of having a value of 0 and 1 simultaneously, or to be in a state of entanglement, can be disturbed very easily and when that happens the quantum state ‘decoheres’ practically instantaneously and the computation and communication stops. Cryogenics helps to stabilise quantum states and minimise the incidence of decoherence, but keeping devices and networks supercooled to close to absolute zero (and that means maintaining a temperature of less than 0.01 kelvin) is very difficult, very precise, very delicate, very time consuming and very expensive. 

Until now, most quantum experimentation has, perforce and in the absence of any viable alternative, involved cryogenics despite the known limitations and difficulties of managing ultra-low-temperature environments. It’s better than nothing, but incredibly difficult to achieve and maintain. 

However, and notwithstanding the problems, some extremely important improvements have been made in cryogenic technologies. For example, they have enabled the exploration of previously unobserved and undiscovered quantum qualities, such as the intriguing area of topological qubits that can encode information in the ‘topology’ (the fundamental structure) of a material or system itself and store it according to how the material is arranged, bent, braided or twisted rather than in the properties of the individual particles themselves. Such topological protection provides robust resistance to errors caused by external noise or interference and may well help the development of more reliable and scalable quantum computers. 

Meanwhile, of course, research continues into adjuncts or alternatives to cryogenics in quantum computing, quantum communication and quantum sensing. Quantum sensing is an advanced technology that detects and senses even the most minimal changes in light, motion, pressure, sound, temperature, and electric and magnetic fields. Quantum sensors analyse data at an atomic level and it is of inestimable use in many disciplines including defence systems, healthcare and medical research, synthesis of new drugs, environmental monitoring, material science, highly sophisticated navigation systems and wide swathes of other scientific research.

One step closer to a global quantum internet

The Toshiba Europe experiment, announced in an article in the British scientific journal Nature, demonstrated that quantum information encoded in light can remain stable over a network of many miles of traditional fibre optic cable at room temperature. The breakthrough was achieved with simple chip-based diodes rather than the application and management of complex cryogenic systems.

Mirko Pittaluga, the lead author of the Toshiba study published in Nature, stated: “Through this trial, we completely re-engineered the way that quantum information is measured and stabilised, with a unique optical configuration that completely eliminates the need for cryogenic lab-grade equipment.” 

Robert Woodward, team leader at Toshiba Europe, added, “The key breakthrough was using semiconductor avalanche photodiodes. This hugely simplifies deployment and enables it to go from the lab to national and international networks.”

A diode is a two-terminal electronic component that allows electric current to flow in one direction and acts as a one-way switch, restricting current flow in the opposite direction. A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, x-rays and gamma rays. It produces an electrical current when it absorbs photons. 

When a photon interacts with a semiconductor material, it can create an electron-hole pair. In essence, an electron-hole is a site, a place, denoting the absence of an electron at a position where it could exist and the hole that is left behind when the electron is removed. The hole is not a physical particle but rather a place where an electron ought to be but isn’t, leaving it positively charged compared to the surrounding electron cloud. Avalanche photodiodes (APD) convert each photon they detect into a large cascade of electron-hole pairs. These pairs can then recombine, emitting a photon, which can be used to encode and transmit quantum information. 

Thus, in quantum communication, electron-hole pairs have a vital role to play in the generation and manipulation of photons, the basic units of light used to transmit quantum information. The holes can be used to create or control the emission of photons, which can be entangled or used in other quantum technologies. In the case of quantum information storage, electron-hole pairs can also be used to store quantum information by encoding it into the spin of the electron or hole.

Electron-hole pairs can be used to generate entangled photons, which are vital to quantum communication protocols, such as QKD and quantum teleportation. Electron-hole pairs can also amplify optical signals and be used to implement quantum logic gates, the basic building blocks of quantum computers and quantum communication systems. What’s more, electron-hole pairs can be used to store quantum information in a semiconductor material, thus acting as a quantum memory.

The demonstration was also marked by another world first: Toshiba Europe’s inaugural real-world deployment (and validation in action) of the company’s twin-field entanglement-based QKD protocol over standard telecom infrastructure. Until recently, coherent quantum communications have usually been demonstrated under very closely controlled laboratory conditions rather than in real-world application, an approach that neatly skirts around the perennial problem of signal attenuation and loss over increasing distance in fibre optic cables.

In its German test, Toshiba used the twin-field QKD protocol (TF-QKD) that the company invented in 2018, in Cambridge, England. It enables quantum-key exchange between two parties with a measurement point located midway between them thus reducing signal loss and allowing longer-distance communication. The demonstration proved that the protocol can greatly extend the range of transmission over that of traditional point-to-point QKD protocols and makes TF-QKD very well suited for secure comms links over long distances. 

Andrew Shields, vice president and head of quantum technology at Toshiba Europe, stated: “We are proud of the world-firsts we’ve achieved in QKD development and commercialisation at Toshiba. This trial represents another significant milestone in scaling QKD even further towards our end goal of building a quantum internet that connects major cities and countries together, with quantum-safe protection at its core.” 

This year, Toshiba celebrates its 150th anniversary. It was founded in 1875 by Hisashige Tanaka, who was honoured throughout Japan as “the genius of mechanical wonders”. In 2025, the company is still doing very well.

– Martyn Warwick, Editor in Chief, TelecomTV