Shining a HOT light on optomechanics

We all use micromechanical devices in our daily lives, although we may be less aware of them than electronic and optical technologies. Micromechanical sensors, for example, detect the motion of our smartphones and cars, allowing us to play the latest games and drive safely, and mechanical resonators serve as filters to extract the relevant cellular network signal from the broadband radio waves caught by a mobile phone's antenna. For applications such as these, micromechanical devices are integrated with their control and readout electronics in what are known as micro-electro-mechanical systems (MEMS). Decades of research and development have turned MEMS into a mature technology, with a global market approaching one hundred billion US dollars.

In recent years, a community of researchers from various universities and institutes across Europe and the United States set out to explore the physics of micro- and nano-mechanical devices coupled to light. The initial focus of these investigations was on demonstrating and exploiting uniquely quantum effects in the interaction of light and mechanical motion, such as quantum superposition, where a mechanical oscillator occupies two places simultaneously. The scope of this work quickly broadened as it became clear that these so-called optomechanical devices would open the door to a broad range of new applications.

Hybrid Optomechanical Technologies (HOT) is a research and innovation action funded by the European Commission's FET Proactive program that supports future and emerging technologies at an early stage. HOT is laying the foundation for a new generation of devices that bring together several nanoscale platforms in a single hybrid system. It unites researchers from thirteen leading academic groups and four major industrial companies across Europe working to bring technologies to market that exploit the combination of light and motion.

One key set of advances made in the HOT consortium involves a family of non-reciprocal optomechanical devices , including optomechanical circulators. Imagine a device that acts like a roundabout for light or microwaves , where a signal input from one port emerges from a second port, and a signal input from that second port emerges from a third one, and so on. Such a device is critical to signal processing chains in radiofrequency or optical systems, as it allows efficient distribution of information among sources and receivers and protection of fragile light sources from unwanted back-reflections. It has however proven very tricky to implement a circulator at small scales without involving strong magnetic fields to facilitate the required unidirectional flow of signals.

Introducing a mechanical component makes it possible to overcome this limitation. Motion induced by optical forces causes light to flow in one direction through the roundabout. The resulting devices are more compact, do not require strong permanent magnets, and are therefore more amenable to large-scale device integration.

HOT researchers have also created mechanical systems that are simultaneously coupled to an electric and an optical resonator. These quintessentially hybrid devices interconvert electronic and optical signals via a mechanical intermediary, and they do so with very low added noise, high quantum efficiency, and a compact footprint. This makes them interesting for applications that benefit from the advantages of analog signal transmission over optical fibers instead of copper cables, such as those requiring high bandwidth, low loss, low crosstalk, and immunity to harsh environmental conditions.

Photo: Henk-Jan Boluijt (AMOLF ) Artist impression of the light circulator. The yellow beam enters at the upper left port and is forced to leave the resonator at the lower left port. The red beam enters at that port (lower left) but cannot follow the reverse path of the yellow beam as it is forced to propagate to the lower right exit.

An example of such a device is a receiver for a magnetic resonance imaging (MRI) scanner, as used in hospitals for three-dimensional imaging inside the human body. In MRI, tiny electronic signals are collected from several sensors on a patient inside the scanner. The signals need to be extracted from the scanner in the presence of large magnetic fields of several tesla, with the lowest possible distortion, to form high-resolution images. Conversion to the optical domain provides a means of protecting the signal. A prototype of a MRI sensor that uses optical readout has been developed by HOT researchers.

Another application of simultaneous optical and electronic control over mechanical resonators is the realization of very stable oscillators. These can function as on-chip clocks and microwave sources with ultrahigh purity. HOT researchers filed a patent application that shows how to stabilize nanoscale mechanical resonators that naturally oscillate at gigahertz frequencies driven by optical and electric fields. Combining all components on a single chip makes such devices extremely compact.

A somewhat more exotic application of hybrid transducers, but one with potentially far-reaching implications, is the interconnection of quantum computers. Quantum computers hold the promise of tackling computational problems that our current classical computers will never be able to solve. The leading contender as the platform for future quantum computers encodes information in microwave photons confined in superconducting circuits to form qubits. Unlike the bits used in conventional computers that take on values of either 0 or 1, qubits can exist in states representing both 0 and 1 simultaneously. The qubits however are bound to the ultracold environment of a dilution refrigerator to prevent thermal noise from destroying their fragile quantum states. Transferring quantum information to and from computing nodes, even within a quantum data center, will require conversion of the stationary superconducting qubits to so-called flying qubits that can be transmitted between separate locations. Optical photons represent a particularly attractive option for flying qubits, as they are robust at room temperature and thus provide one of the few practical means of transmitting quantum states over distances greater than a few meters. In fact, the transfer of quantum information encoded in optical photons is now routinely achieved over distances of hundreds of kilometers.

A key prerequisite for quantum networking is therefore quantum-coherent bidirectional conversion between microwave and optical frequencies. To date, no experimental demonstration exists of efficient transduction at the level of individual quantum states. However, many research groups around the world are diligently pursuing various possible solutions. The approaches that have come the closest so far utilize a mechanical system as an intermediary, and this is where the technologies pursued by the HOT consortium come into play.

HOT researchers have created compact chip-scale devices on commercially available silicon wafers that are fully compatible with both silicon photonics and superconducting qubit technology. The unique optomechanical designs developed by the HOT consortium exploit strong optical field confinement, producing large optomechanical coupling. As a result, electrical signals at the gigahertz frequencies typical of superconducting qubits can be coherently converted to optical frequencies commonly used for telecommunication. Such integrated photonic devices employing optomechanical coupling are often plagued by the deleterious effects of heating due to absorption of high-intensity light. The thermal problems can be circumvented by optimizing the device design and using alternative dielectric materials, and internal efficiencies exceeding unity have been achieved for ultra-low optical pump powers.

Photo: STMicroelectronics Image of a packaged transducer with cap removed. The ceramic housing is built with a backside hole sealed with a glass optical window. The sensing die with its wire bonding is placed on top of the window, while the input and output optical signals are transmitted through a fiber connected via a focusing lens on the back side. Photo: STMicroelectronics Image of the packaged transducer with cap on.

With the capabilities provided by such transducers, the power of quantum information processing could be brought to a whole new class of tasks, such as secure data sharing, in addition to creating networks of quantum devices.

As hybrid optomechanical systems enter the quantum regime , new challenges emerge. One particularly important consideration is that the very act of measuring the state of a system must be rethought. Contrary to our everyday experience, quantum mechanics requires that any measurement exerts some inevitable backaction onto the system being measured. This often has adverse effects; the response of an optomechanical sensor to a signal of interest, for example, can be washed out by the backaction caused by reading out the sensor. Luckily, these effects are well understood today, and can be corrected for using advanced quantum measurement and control techniques.

HOT researchers have pioneered the application of such techniques to mechanical sensors. They have shown how quantum state estimation and feedback can help overcome the measurement challenges. The classical counterparts of these approaches are widely used in many areas of engineering and are familiar to consumers in such products as noise-cancelling headphones.

In the setting of optomechanics, they have been used to measure and control the quantum state of motion of a mechanical sensor. For example, HOT researchers have managed to limit the random thermal fluctuations of a vibrating drum to the minimal level allowed by quantum mechanics. This provides an excellent starting point to detect even the smallest forces exerte by other quantum systems like a single electron or photon.

Photo: University of Copenhagen An image of the optical table at Prof. Albert Schliesser's laboratory at the Niels Bohr's Institute at the University of Copenhagen together with his graduate students who worked on the quantum drum . From left to right in the photo: Albert Schliesser, David Mason, Massimiliano Rossi, and Junxin Chen.

With its focus on real-world technologies, the HOT consortium also considers such practical matters as device packaging and large-scale fabrication. Optomechanical devices require electronic and optical connectivity in a package that also keeps the mechanical element under vacuum. Whereas such demands have been met separately before, consortium member and industry giant STMicroelectronics is addressing their combination in a single device package as well as the potential for mass production.

This project is financed by the European Commission through its Horizon 2020 research and innovation programme under grant agreement no. 732894 (FET-Proactive HOT).

For more information about Hybrid Optomechanical Technologies, visit our website , like our Facebook page , and follow our Twitter stream. You can watch our videos on YouTube , including an explanation of the optical circulator built by the network and how to silence a quantum drum.