Quantum Computing Research Using Free-Electron Lasers and Megawatt Magnets

"NI equipment made communication between different control and acquisition systems possible."

- Laurence Stant, University of Surrey

The Challenge:

Researching a method for quantum computing at the High-Field Magnetic Laboratory (HFML) and the Free Electron Lasers for Infrared Experiments (FELIX) facility in Nijmegen, Netherlands, scientists held a sample within a 33 tesla resistive magnet which consumes 18 MW of electricity and requires adaptive cooling. To align with the leading experimental standard, the system requires precise control of more than 40 kA at 500 V. In addition, a THz free–electron laser called FLARE provided light at the adjacent FELIX facility. Because it is highly resonant, all components must have high stability and precision.

The Solution:

Using a wide range of NI equipment, from a USB data acquisition device to high-density I/O on PXI real-time targets, we could reliably control and manage all components. In addition to using NI hardware to directly control and monitor process variables, we used various serial interfaces within the LabVIEW control programs to communicate with programmable logic controllers (PLCs) and build automation systems. Because we also chose LabVIEW as experiment control software, integrating it with facility systems was fast and painless.

In 1965, the cofounder of Intel, Gordon E. Moore, predicted that the number of components on an integrated circuit would double every year. For 50 years, the trend has held, and Intel will soon release 14 nm-processor technology. However, heat dissipation and miniaturization difficulties mean that this expansion is beginning to slow, and technology must mitigate this to continue to develop. One candidate that has recently attracted media attention is quantum computing. Instead of using classical bits to store information, these systems use quantum bits, or qubits, which power a new generation of computers and provide a dramatic increase in speed and efficiency. This makes a big difference in the field of optimization algorithms, which are becoming increasingly important in the modern world, as they underpin many applications such as cryptography, economic planning, and network infrastructure management.

Basic quantum computer operation requires three steps:

First, the qubit registers must be reset and programmed with the starting values (similar to a classical computer). This requires the qubits to couple well with their environment so that the controlling signals can reach them.
The second step is computation, and the power of quantum physics and the superposition principle are harnessed so that the system can far outperform a classical machine. For it to work correctly, however, the qubits must be as isolated as possible from any environmental effects. This ensures that the superposition phases interact with each other in a controlled way, without interference.
The last step is to read out the finished result. Like the first step, this requires good coupling to access the qubit state information.

This presents an obvious challenge—whatever physical form the qubits take, they must be able to either switch their environmental coupling or provide a suitable compromise. It is for this reason that there are many different technologies currently being researched. They range from photonic (purely light-based) to nuclear, and each has its own problems and benefits. One option being researched stores quantum information in electrons bound to Group V impurities held in a silicon lattice. This system can potentially store two qubits per impurity atom, as well as harness some of the $300 billion semiconductor industry’s current manufacturing infrastructure, making it a very attractive option, if found effective.

The COMPASS project, led by Professor Ben Murdin at the Advanced Technology Institute (University of Surrey), including the London Centre for Nanotechnology and the FELIX free-electron laser facility in the Netherlands, is currently researching this technology. The project has previously demonstrated electronic state control in high fields and is now experimenting with spin control. For maximum control, facilities such as the HFML at Radboud University Nijmegen provide fields nearly a million times the strength of the Earth’s own. Under these conditions, spin control required the FLARE free-electron laser (part of the adjacent FELIX facility). To make our measurements, we collected data from all of these systems, as well as from sensors mounted to our sample. Therefore, we needed easy communication between all components.

HFML Facility

The HFML is home to some of the world’s strongest magnets. This experiment used one of its two 33 tesla Bitter magnets, which consume 18 MW of DC power delivered through high-precision, high-stability power supplies built by Imtech Vonk. Current control accuracy, which reaches up to 40,000 amperes, is 0.002 percent. The control scheme is PLC-based; however, the user interface is written using LabVIEW, with additional measurement tools utilizing LabWindows™/CVI software. This not only provides scientists with a simple and familiar way to interact with the system, but also offers control and measurement using their own experimental virtual instruments (VIs).

In addition to the user interface, we chose NI products for the coil protection system. Each magnet is comprised of several laminated coils which are subject to intense electrical and thermal stresses during operation. It is crucial that these are safely monitored to ensure a fast shutdown at any sign of failure. The system needed to be time-critical and integrate easily into the existing Supervisory Control and Data Acquisition interlocking and alarm system. An obvious choice was an NI PXI system with the LabVIEW Real-Time Module. Using high-density multifunction cards, we could build a cost-effective system to monitor the complete range of sensors fitted to the magnet, without worrying that the OS would cause watchdog trips.

The facility also employs a host of NI GPIB interfaces to read a variety of equipment, ranging from oscilloscopes to lock-in amplifiers. A particularly important instrument is the nanovoltmeter, which is attached to the magnet power supply and measures the delivered current from which to calculate the actual field strength. These values are recorded for all experiments using the magnets and a GPIB-ENET/100 bridge that offers access anywhere within the experimental halls.

FELIX Facility

The FELIX facility is also located on the campus of Radboud University in Nijmegen and contains two world-class lasers, FELIX and FLARE, for cutting-edge research. Free-electron lasers work by generating light from electrons travelling at relativistic speeds. To provide these electrons, however, a particle accelerator is required, which makes facility requirements rather complex.

Due to the high speeds and deterministic nature of particle accelerators, all aspects of electron transport must be controlled from a series of timers. To tune the machine, component duration and start offset—including in the klystron microwave amplifier, electron source, and steering magnets—all need to be easily adjusted, so the timing system must be configurable via computer. Because other projects have used NI products with high-speed timing applications in accelerators (the Large Hadron Collider at the European Organization for Nuclear Research, for example), an NI PXI system was an excellent solution to this critical challenge.

Another area of the machine with important control criteria is motion, required for many parts of the electron transport as well as optical assemblies. A prime example is the FLARE 4.5 m-long undulator, which contains two rows of alternating magnets which cause the electrons travelling between them to bend and emit the desired light. The gap size between the rows can be adjusted, controlling the magnetic field strength and thereby the frequency of light that the laser produces. It is therefore very important to have an accurate and precise control system—otherwise, the experimental data could be affected. The UMI-7774 and associated PXI control devices provided a straightforward way to interface to these devices with the advantage of good cable management and modular connectivity.

At the heart of most large-scale machines lies user interface and control software. As with any application in which the hardware is continually in development, it was essential that we could easily reconfigure and add to the solution. We required a high degree of customization, as the machine is built from a mixture of commercial and in-house parts for which we needed to write new drivers. LabVIEW provided all of these features, in addition to being very accessible for the host of scientists and students working at the facility.

The Experiment

To measure the spin state of the electrons in our sample, we needed to use a second laser to excite them and create quasiparticles called excitons. These would collapse after a short time and release charge, which we could measure. The amount of charge released would correspond to the spin state. Finally, we measured this with a digital signal-processing lock-in amplifier—a tool used extensively in science for sampling AC signals. As the laser used here was not a commercial product and was assembled from components, we needed to write a custom closed-loop control system for that, too, and also provide real-time diagnostics from a USB spectrometer. LabVIEW was the perfect choice, as we could use it to sample data from the facility systems, as well as quickly change anything while visiting the facility: Writing powerful software in C++ is fine, until you need to make a change abroad, and your compile environment is back in your office! With LabVIEW, we avoided that situation.

The experimental software supports a modular plug-in-based architecture, with a replacement object-oriented system currently under development. Using built-in VIs, such as proportional integral derivative control loops, makes it swift to code and clear to debug.

Summary

This project was a very challenging experiment requiring cooperation between two major physics facilities—and it was a success. We achieved communication between different control and acquisition systems by extensively using NI hardware and software, chosen separately by both facilities and the experimental user as the best for their applications. This collaboration provided a seamless experience when working between system components.