Photonics for the second quantum revolution

Lasers and other photonic systems are enablers of the second quantum revolution. Leading suppliers report enormous increases in demand.

Before Professor Theodor W. Hänsch was awarded the Nobel Prize in Physics in 2005 for his work in high-precision laser spectroscopy and optical frequency comb technology, the former director of the Max Planck Institute for Quantum Optics was one of the founders of Menlo Systems GmbH. The company now has more than 100 employees and, with its range of optical frequency combs and quantum laser systems, is a global leader in the future market for quantum technologies. According to Dr. Benjamin Sprenger, an expert for quantum technology and metrology at Menlo Systems, the company’s photonic solutions are currently used on six continents and in all four pillars of the market of the future. “We have customers in quantum communication, quantum simulation, quantum computing and quantum sensors and see great market opportunities for photonic solutions in each of these four areas,” he explains.

In the first quantum revolution, the use of quantum-mechanical processes paved the way for lasers, computer chips, transistors, and other semiconductor technologies and for modern medical and communications technology, a growing understanding and the increasingly precise control possibilities of photonic solutions is now triggering the second quantum revolution. With lasers, it is possible to control quantum effects rather than just using them. Lasers can cool atoms and ions almost to absolute zero (0 Kelvin or -273.15 °C), and, with laser light, they can be captured, moved and positioned. Also based on laser technology, high resolution laser spectroscopy and the aforementioned optical frequency combs enable ultra-precise frequency and distance measurement and determination of proton radii in the nucleus.

In short: As enabling technologies, laser systems provide the necessary precision to make quantum states controllable and, as a consequence, usable for computing, simulation, communication and sensors. But according to Sprenger, to do this they must meet high requirements. “For example, laser systems for quantum computing must be extremely low-noise—and, ideally, all the necessary lasers, each tuned to the particular atomic transitions, should reach the experiment via fiber optics,” he explains. This is still a challenge, especially in the ultraviolet range and requires new developments and improvements in the solutions that are currently available. As in other areas of application, with the complex laser systems used in quantum technologies, it is important to guarantee robust 24/7 operation.

Since it was established in 2011, MuQuans (µQuans), a spin-off from the Institut d’Optique d’Aquitaine in Bordeaux, has grown to become an important pioneer in quantum technology applications. Italian geophysicists use its Absolute Quantum Gravimeter for extensive gravitational measurements of the active volcano Mount Etna. Lasers from µQuans are also used by partner company Pasqal, which is developing a photonic approach for quantum computing that has been widely recognized worldwide. The aim is to develop a quantum platform that can carry out quantum simulations and calculations.

To do this, Pasqal is using quantum manipulation of Rydberg atoms with laser light. According to µQuans founder Bruno Desruelle, the partners are adapting the laser technologies installed in the quantum gravimeter to suit the specific requirements of the Pasqal platform. “This will allow them to access systems that were validated for quantum technologies under operating conditions and that have proved unique performance, robustness, and reliability,” he emphasizes.

Desruelle is convinced of the potential of photonic solutions for quantum technologies. “Lasers can play a key role in various areas, as they enable highly efficient manipulation of quantum objects. This includes quantum sensors with gravimeters or inertial sensors, time and frequency measurement with atomic clocks, or in the areas of quantum communication and quantum information processing,” he explains.

Especially in the last-mentioned applications, lasers are the enabler technology in almost all the approaches under consideration, no matter whether they are based on ions, Rydberg atoms, or trapped photons. This drives demand for quantum-specific laser systems and optical solutions. “We are seeing a lot of activity—in science, with a wealth of exciting research projects, and in the growing quantum industry, which is being driven by dynamic startups,” he says.

One of these startups is Quantum Optics Jena (QUJ), a spin-off from the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) which is also located in Jena. Its founders, Dr. Oliver de Vries and Dr. Kevin Füchsel, worked on various research projects at the IOF and now press ahead with their product development.

The first product is a miniaturized entangled photon source: Entanglement, which Einstein once described as “spooky”, describes the connection of the quantum-mechanical state of a pair of particles that remains even if the particles are separated by a large distance. The founder team is focusing on quantum computers, quantum communication, and quantum sensors and imaging.

“Initially, our priority is quantum communication, because we expect to see the first market-ready applications in this area,” explains Füchsel. With entangled photons, QUJ implements systems for quantum key distribution (QKD), with which communication via fiber optic of laser in space is to be secured. The company’s solution generates several million entangled photon pairs per second, which are exactly assignable thanks to today's nano-second precise resolution and high-precision polarization measurements. The founders are also developing solutions for this.

“We are also implementing software to enable information processing, from detection of quantum states to using the generated key material,” he says.

Now, we are seeing more and more applications of display photonics techniques for imaging and sensing at different scales: including a growing trend towards the use of hybrid refraction/diffraction optics in macro optics, and the possibility of using silicon photonics for micro-optics; In the field of nanophotonics, optical metasurfaces (OM) - surfaces that form patterns on the subwavelength scale - can manipulate light very efficiently; It is also evidence of ongoing technology transfer.

In a paper published this year, a research team has applied these ideas to practical problems and achieved a series of scientific achievements.

For example, a single-shot optical positioning technique has recently been reported that enables observers to detect the position of nanowires with an accuracy of better than 100 picometers. This is achieved by mapping the intensity distribution of topological light diffracted by lines, and then using deep learning techniques to analyze the scattered light and determine the position of the nanowires based on the diffraction pattern.

The picmeter-level accuracy achieved in these optical measurements is a fraction of the typical size of the atom and less than the amplitude of thermal motion of the nanowires. As a result, it far exceeds the spatial resolution of conventional light microscopy.

Another experimental result that marked the rise of photonics showed how light-induced phenomena at the picmeter-scale could lead to the realization of new states of matter - so-called continuous-time crystals.

Wilczek's original idea that time crystals spontaneously break time-translation symmetry was fundamentally opposed at an early stage. However, researchers have implemented a variant of this idea: so-called discrete or Floquet time crystals. These systems break time-translation symmetry by oscillating under the influence of periodic external parametric forces, and have been demonstrated under rigorous experimental conditions in a variety of systems: including trapped ions, qubits, atomic and spin systems, and even all-optical platforms.

"A time crystal is a quantum system composed of particles in repeated motion in the lowest energy state. Because of this, the time crystal is already in its quantum ground state and therefore cannot rest and lose energy to its surroundings, thus representing an energy-free motion in the absence of kinetic energy."

The ability to sustain photonic time crystals in optics could have a profound impact on light science, leading to truly disruptive applications in the future.

In fact, light science has been widely used in our production and life.

In the field of fiber optic communications from satellites to Earth and laser-based data transmission, teams of scientists around the world are working to make quantum technology useful in absolutely safe and reliable optical communications. While this may sound like science fiction, it's surprisingly true. Among the issues being investigated are the effects of dust and clouds on communications, the optimal size of receiving telescopes on Earth and in space, the reliable transmission of quantum keys over tens of kilometers in orbit or hundreds of thousands of kilometers in space......

Another exciting trend is the growing use of artificial intelligence (AI) and machine learning (ML) or deep neural networks (DNN). The application of light science to these areas promises great potential, especially for biomedical applications, and is already a game changer.

At least one thing is already clear: The path to practical answers will be to use lasers and other photon technologies.