The first part of this FAQ series looked at “electronics that operate in extreme heat” (up to 800°C). This FAQ delves into the opposite end of the temperature spectrum and heads toward absolute zero. There are several existing cryogenic electronics applications, including superconducting quantum interference devices (SQUIDs) for measuring extremely subtle magnetic fields, microwave preamplification, superconducting power cables, and setting measurement standards. Emerging applications for cryogenic temperatures, including quantum computing and quantum compasses.
One of the largest uses of SQUIDs is to read out superconducting transition-edge sensors. Hundreds of thousands of multiplexed SQUIDs coupled to transition-edge sensors are presently being deployed to study the cosmic microwave background, for X-ray astronomy, to search for dark matter made up of Weakly interacting massive particles, and for spectroscopy at Synchrotron light sources. Advanced SQUIDS called near quantum-limited SQUID amplifiers form the basis of the Axion Dark Matter Experiment (ADMX) at the University of Washington. A potential military application exists for use in anti-submarine warfare as a magnetic anomaly detector (MAD) fitted to maritime patrol aircraft.
Another successful ongoing area of cryogenic electronics is microwave preamplification. The cooling of amplifiers to reduce noise is well established. For many years in the scientific community, this has been employed for receivers used in radio astronomy and deep-space communications with distant spacecraft. Cooling transistors greatly reduces their thermal noise, which is the dominant noise at microwave frequencies.
Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic field. The NMR signal is produced by excitation of the nuclei sample with radio waves into nuclear magnetic resonance, which is detected with sensitive radio receivers.
Modern NMR spectrometers have a very strong, large, and expensive liquid helium-cooled superconducting magnet because resolution directly depends on magnetic field strength. Less expensive machines using permanent magnets and lower resolution are also available, giving sufficient performance for certain applications such as reaction monitoring and quick checking of samples. NMR has largely replaced traditional wet chemistry tests such as color reagents or typical chromatography for identification.
Magnetic resonance imaging (MRI) is a complex application of NMR where the geometry of the resonances is deconvoluted and used to image objects by detecting the relaxation of protons that have been perturbed by a radio-frequency pulse in the strong magnetic field. This is most commonly used in health applications.
Superconducting power transmission
In large cities, it is difficult to transmit power by overhead cables, so underground cables are used. But underground cables get heated, and the wire’s resistance increases, leading to a waste of power. Superconductors could be used to increase power throughput, although they require cryogenic liquids such as nitrogen to cool special high-temperature superconductor based cables to increase power transmission.
There is already a small number of superconducting cables operating in AC networks. However, the EU-funded ‘Best Paths’ project has focused on investigating HVDC solutions for bulk power transmission with a modular design that is easily adaptable. The rated current and voltage can be matched to any power grid specification. Nexans recently completed successful qualification testing of a ‘Best Paths’ superconductor cable for HVDC power links. The Nexans qualified the 320kV direct current superconducting cable for currents up to 10kA with a 3.2GW power transmission capability.
Superconductivity sets the voltage standard
The Superconductive Electronics Group at the U.S. National Institute of Standards and Technology (NIST) utilizes the quantum effects of Josephson junctions in specialized superconducting integrated circuits to improve measurement techniques and standards for fundamental metrology, such as for dc and ac voltage, waveform synthesis, and primary thermometry, and for applications that require high-performance, such as energy-efficient advanced computing and RF communications. The Quantum Voltage and Noise Thermometry Projects develop and disseminate standard reference instruments and measurement best practices for dc and ac voltage metrology, RF metrology, and primary thermometry. The Flux Quantum Electronics Project develops cryogenic superconductive circuits and measurement techniques for advanced, energy-efficient computing, RF communications, and electrical metrology. The group uses the NIST’s Boulder Microfabrication Facility (BMF) to fabricate all of their devices.
HYPRES all-Nb voltage standard chips were developed through close collaboration with NIST and define the standard volt worldwide. HYPRES is the sole commercial supplier of both 1-volt and 10-volt standard chips and systems made with its all-refractory Nb technology.
HYPRES is the only commercial manufacturer of the superconducting integrated circuit used in Primary Voltage Standard Systems. The HYPRES Josephson Junction Array Voltage Standard circuits provide the ultimate accuracy for realizing and maintaining the SI Volt. The HYPRES’ offers systems that use liquid helium cooling or a cryocooler system that enables continuous operation without liquid helium and is designed for laboratories where liquid helium is not readily available or is cost-prohibitive.
Barely above absolute zero to define the ampere
In the redefined SI, the ampere is based on the elementary charge of a single electron — an extremely small quantity that is a constant of nature. One amp is now defined as the amount of charge carried by 6.24 billion electrons past a given point in one second. Devising a device that conforms to the new definition is a demanding task. But progress is being made. It is now possible to count individual electrons with a tiny device called a single-electron transistor (SET).
A SET uses the same basic structure as an ordinary silicon transistor. It contains a source of electrons, a voltage “gate” that controls their flow, and a drain where the electrons exit and are measured. The difference is that the SET also includes an “island” made from a microscopic quantum dot that enables researchers to move electrons one at a time from the island to the drain, where they are counted.
The distance from the source to drain is about one-tenth the width of a human hair, and the electron channels are 10 times smaller. The energies involved are so tiny that that device must be cooled to about 10 thousandths of a degree above absolute zero. The output of a single SET pump is about one trillionth of an amp.
Work is underway on prototype designs that can ultimately boost that amount by 10,000 times through an optimal choice of materials, increasing the pump rate to around a billion electrons per second. By running 100 of these SET pumps in series and amplifying the result, researchers could achieve larger currents.
Combining the electric current from large numbers of SETs may ultimately provide a quantum-accurate measure of the larger amounts of current present in real-world electronic equipment. Eventually, researchers hope to reach about 1 microampere — within the range needed to develop a practical, working standard for electric current.
In addition to its use as an electric current standard, a high-throughput SET pump with low measurement uncertainties could be combined with ultra-miniature standards for voltage or resistance, both of which are now defined with quantum exactitude in terms of fundamental constants of nature. The result would be a single, compact, completely quantum-based measurement suite for all three elements of the “metrology triangle” that could be delivered to factory floors and laboratories — a primary goal of “NIST on a Chip.”
Quantum computing and quantum compasses
Quantum computing and quantum compasses are currently under development. Quantum computing is the use of quantum phenomena such as superposition and entanglement to perform computation. Quantum computers are believed to be able to solve certain computational problems, such as integer factorization (which underlies RSA encryption), substantially faster than classical computers. While there are numerous groups developing quantum computers and related technologies, significant obstacles remain.
In addition to improvements in the stability of the Qbits, Sourcing parts for quantum computers is also challenging. Like those constructed by Google and IBM, many quantum computers need Helium-3, a nuclear research byproduct, and special superconducting cables that are only made by the Japanese company Coax Co.
The next generation of inertial sensors may be based on a compact gyroscope and a three-axis accelerometer using new techniques in atom interferometry. Called a quantum compass, it is an instrument that measures relative position using the technique of atom interferometry. It includes an ensemble of accelerometers and gyroscope based on quantum technology to form an inertial navigation unit.
A quantum compass contains clouds of atoms frozen using lasers. By measuring the movement of these frozen particles over precise periods of time, the device’s motion can be calculated. The device would then provide a tamper-proof accurate position in circumstances where satellite navigation is not possible.
NIST Superconductive Electronics Group, National Institute of Standards and Technology
Nuclear magnetic resonance, Linde
Nuclear magnetic resonance spectroscopy, Wikipedia
Quantum ampere standard, National Institute of Standards and Technology