Novel Microwave Isolator Points the Way to New Radio Cameras and Quantum Computers

Credit: NAOJ
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Researchers at the National Astronomical Observatory of Japan (NAOJ) invented a novel microwave isolator and demonstrated for the first time its basic principle that enables small isolators that will be essential for future quantum computers. This isolator consists of two frequency mixers, which are widely used in receivers for radio telescopes, and paves the way for the development of large-scale multi-pixel radio cameras as well.

An isolator is an electronic component that controls the direction of signal propagation and is used in various instruments, including quantum computers and radio receivers for astronomy. Conventional isolators utilize magnetic materials, which in principle make it difficult to construct an isolator smaller than a few centimeters. This is an obstacle to the realization of large-scale quantum computers with one million qubits. Currently realized quantum computers have only about 100 qubits, and to increase this number by a factor of 10,000 would require a large number of components, including isolators, each of which would have to be miniaturized. The same problem exists in the development of large-format cameras for radio astronomy observations.

In this new study, the research team utilized two sets of frequency mixers for an isolator, instead of magnetic materials, and has demonstrated the principle both theoretically and experimentally. “This is based on a completely new principle,” says Sho Masui, a Project Researcher at the NAOJ Advanced Technology Center (ATC). “We have created a novel circuit device in the microwave band. It is also significant from an electrical engineering point of view, as the new isolator can provide highly scalable circuits.”

One of the major scalability benefits is the fact that the new isolator can be configured entirely within a flat circuit on a printed circuit board. This means that an isolator with integrated circuits can be miniaturized to the millimeter scale, which is 1,000 times smaller in volume than conventional isolators.

The research team is aiming higher; they are looking at developing isolators that can also amplify signals. The frequency mixers widely used in radio observation equipment are superconductor-insulator-superconductor mixers (SIS mixers), which have the function of amplifying signals. In this development, a commercially available semiconductor mixer is used as a frequency mixer, but if an SIS mixer is used instead, a new device that both amplifies signals and functions as an isolator will be realized.

“Radio wave observation equipment and quantum computers share some common development elements,” says Yoshinori Uzawa, the NAOJ Director of Engineering. “NAOJ has accumulated knowledge in the development of observational instruments for radio waves and visible and infrared light. Recently we established the Social Implementation Program in the NAOJ ATC and formed a project team to promote technological development for the application of quantum computers. We will continue to develop breakthroughs in these two fields.”

These research results were presented in Sho Masui et al. “A Novel Microwave Nonreciprocal Isolator based on Frequency Mixers” in the IEEE Microwave and Wireless Technology Letters published on March 15, 2023.

Related Link

• Novel Microwave Isolator Points the Way to New Radio Cameras and Quantum Computers (Advanced Technology Center)

Source: National Astronomical Observatory of Japan (NAOJ)/Science

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Gigapixel radio image of the Universe using Europe as a radio telescope

An illustration of the difference in resolving power between using only the Dutch LOFAR stations and all of the international stations throughout Europe, arranged in the formation of an HBA core stations. The animation fades from an angular resolution of 6" to a resolution of 0.3". Credit: Frits Sweijen

An international team of astronomers has created one of the largest and most detailed radio maps at megahertz frequencies thanks to Dutch supercomputers. This research, led by Frits Sweijen at Leiden University, has been published in Nature Astronomy on Thursday. Using the International LOFAR Telescope they have, in the wake of tremendous progress late last year, mapped an area of the sky the size of 25 full moons in great detail, with a resolving power comparable to optical telescopes on Earth. The resulting radio image contains nearly seven billion pixels and contains just shy of 2500 radio galaxies.

LOFAR

The sky is filled with radiation invisible to the naked eye, including radio waves at frequencies ten million times lower than red light. With tens of thousands of antennas across Europe, the LOFAR telescope listens to those cosmic radio waves at a frequency of 144 MHz, just above the FM radio band. Through these antennas the European continent transforms into an almost 2000 km big radio telescope. This tremendous size means that LOFAR can see exquisite and unprecedented detail at such low radio frequencies, with a resolving power high enough to make out the Great Pyramid if it were on the Moon. The combined area of all the antennas make it sensitive enough to detect a mobile phone ringing all the way out on Mars.

This resolving power is challenged by Earth's atmosphere however. Ultraviolet radiation from the Sun creates a layer of charged particles in the upper atmosphere. This so-called ``ionosphere'' distorts radio waves from space before they reach the telescope. For LOFAR it therefore is like looking at the sky from the bottom of the ocean. With advanced techniques these distortions can be corrected, focusing the telescope across its entire field of view, thus allowing it to be mapped.

Super computers

Determining these corrections and subsequently converting radio waves into an image, requires modern algorithms and a lot of compute power. Thanks to super computers this was not an issue. Locally, the Academic Leiden Interdisciplinary Cluster Environment (ALICE) lent its power to the scientists. Nationally, SURF in the Netherlands, provided early access to the new platform data processing platform named Spider. This platform is specifically designed for data-intensive projects like this. Lengthy calculations could be run in a massively parallel fashion thanks to these supercomputers. The final image was too large to be made in one go. To image the full field of view, it was processed in 25 smaller chunks each covering an area the size of a full moon. Each of these chunks was turned into an image over seven days, using software recently developed at ASTRON. Piece by piece, on a single computer, this process would have taken more than 175 days to complete. Thanks to the large scale compute infrastructure at Leiden and SURF, however, it only took seven days effectively.

The sharp eyes of the International LOFAR Telescope allow scientists to study the evolution of black holes and their host galaxies in more detail than before. Galaxies in the early Universe, for example, that would otherwise be too small to resolve due to their distance or young age, can have their spatial structure studied. The published results enable this for thousands of sources at once. With its near seven billion pixels this single image contains almost as many pixels as radio surveys from the past did covering the entire sky. These new results explore a tip of the ice berg with a detailed map of the entire Northern sky as the future goal.

This work made use of the Dutch national e-infrastructure with the support of the SURF Cooperative using grant no. EINF-251; the ERC Starting Grant ClusterWeb 804208; the Medical Research Council grant MR/T042842/1; the UK STFC ST/R000972/1 and ST/V000594/1 grants and the Academic Leiden Interdisciplinary Cluster Environment (ALICE) provided by Leiden University.

Article in Nature

Source:ASTRON-Netherlands Institute for Radio Astronomy/News

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Algorithmic improvements for radio interferometry

The 27 antennas of the Very Large Array (VLA) in New Mexico observe the sky simultaneously, forming a single virtual telescope with a gigantic diameter. Credit: NRAO/AUI/NSF

Schematic illustration of an interferometer: A larger distance of two sources in the sky leads to an increased difference in travel time (marked red) as does a larger distance between antennae. Antennae placed at larger distances are therefore able to resolve smaller structures, while antennas placed closely together are more sensitive to larger structures.© MPA 

Central region of a typical radio interferometric coverage. The colours of the individual data points indicate the observed strength of spatial fluctuations of the flux density. © MPA

Radio telescopes observe the sky in a very indirect fashion. Sky images in the radio frequency range therefore have to be computed using sophisticated algorithms. Scientists at the MPI for Astrophysics have developed a series of improvements for these algorithms, which help to improve the telescopes' resolution considerably.

Optical telescopes produce data, which are a direct representation of the observed object's brightness distribution, i.e. an image in the conventional sense, which can be used for further analysis without additional processing. In radio interferometry (i.e. the high-resolution observation of the sky at radio frequencies), the situation is more complicated: here one does not obtain the sky brightness at a specific location, but rather data points indicating the amount, frequency and direction of brightness fluctuations.

If these data points were arranged on a regular two-dimensional grid, converting them to a normal image would be straightforward, but unfortunately no radio telescope design exists which could produce this arrangement. Realistic data point distributions often exhibit complex, inhomogeneous patterns (see fig. 3). Further complications arise if the individual antennae are not placed perfectly on a single plane, and/or if the observed sky region is too large to be approximated by a flat surface. In this case, additional correction terms have to be applied during the image generation, which further increases the computational cost.

The operation described above is called "gridding" and in practice various different implementations exist. Some are not particularly accurate (usually because they date back to the early times of radio astronomy, when computational power was very limited and many approximations and simplifications had to be made). Others provide good accuracy, but often are not fast enough to process the huge amounts of data produced by contemporary radio telescopes in an acceptable amount of time.

Scientists at MPA have now used various approaches – both from both radio astronomy itself and from unrelated scientific areas – to implement a new version of the gridding operation. This new implementation produces very accurate results while at the same time consuming considerably less CPU time and computer memory.

Gridding, however, is only one of several components necessary to produce realistic images from interferometric observations. To suppress noise in the data and eliminate the directional changes in antenna sensitivity, sophisticated iterative algorithms are employed. Traditionally, very often a variant of the so-called CLEAN algorithm is used, which is comparatively quick but does not provide an uncertainty estimate for the resulting image. In contrast to CLEAN-based methods, the MPA scientists developed an (admittedly significantly slower) approach, which delivers physically motivated results including error bars, making use of Information Field Theory and Bayesian statistics.

Three different image reconstructions of the radio galaxy Cygnus A from VLA interferometry data. Two small regions (a bright and a dark one, respectively), have been enlarged for easier comparison. Top panel: naive Fourier transform without optimization. Middle panel: reconstruction using the CLEAN algorithm. A higher resolution is reached, but some unphysical structures are generated, especially in the darker regions. Bottom panel: reconstruction using Bayesian imaging and Information Field Theory. Resolution in the bright areas is further improved, yet no artefacts are visible in the dark areas.For a larger view of image reconstructions B and C, please see below. © MPA; reconstruction middle image: NRAO, Klasse Richard A. Perley

Fig. 4 shows a comparison of the two methods. The observed astrophysical source is the radio galaxy Cygnus A with a supermassive black hole (weighing more than a billion solar masses) at its centre. Two jets, observable at radio wavelengths, leave this centre and at some point encounter the intergalactic medium, where they are reflected and start to emit very brightly. The back-flows create large volumes of radio-bright gas, which can be observed with radio interferometers like the VLA.

The image obtained with the new algorithm exhibits significantly better spatial resolution in the bright image regions, since the signal-to-noise ratio in these areas is much higher. At the same time, it does not show the pronounced structures in the darker regions from the CLEAN results; presumably these structures are artefacts of the CLEAN algorithm which do not correspond to real features on the sky.

The methods presented here essentially open up two new possibilities for radio astronomy: they allow re-processing of already existing data sets, in order to gain additional insights from the improved images. For future observations, there may now be an option to reach the desired image quality with shorter observation times, thanks to the improvements in the algorithms, allowing for a higher overall number of observations. Source: Max Planck Institute for Astrophysics

Other scientific disciplines such as medical imaging, especially magneto-resonance tomography (MRT) employ imaging techniques that are closely related to those in radio interferometry. It is therefore possible that the insights gained from these developments will be beneficial in these areas as well.

 Additional information:

The two last images from Fig 4 can be enlarged separately below.

Image reconstruction with the CLEAN algorithm.
© MPA; reconstruction: NRAO, Klasse Richard A. Perley

Image reconstruction with the new algorithm developed at MPA.
© MPA

 

Source: Max Planck Institute for Astrophysics

 

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Authors

Arras, Philipp Arras
PhD student
Tel: 2034
Martin Reinecke
Scientific Staff

Enßlin, Torsten Enßlin
Scientific Staff
Tel.:2243

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Original publications

1. P. Arras, R.A. Perley, H.L. Bester, R. Leike, O. Smirnov, R. Westermann, T.A. Enßlin Comparison of classical and Bayesian imaging in radio interferometry. Cygnus A with CLEAN and resolve A&A, Forthcoming article
Source/ DOI 

 

2. Philipp Arras, Martin Reinecke, Rüdiger Westermann, Torsten A. Enßlin
Efficient wide-field radio interferometry response A&A, accepted 

Source/DOI

3.Ye, H.
Accurate image reconstruction in radio interferometry
Doctoral thesis
Source/DOI

4. Alex H. Barnett, Jeremy F. Magland, Ludvig af Klinteberg A parallel non-uniform fast Fourier transform library based on an "exponential of semicircle" kernel SIAM Journal on Scientific Computing, 41, 5, C479--C504, 2019 

Source 

 

5. Enßlin, Torsten A. 

Information theory for fields Annalen der Physik 2019, 1800127

DOI

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