In electronics, cryoelectronics or cryolectronics is the study of superconductivity under cryogenic conditions and its applications. It is also described as the operation of power electronic devices at cryogenic temperatures. Practical applications of this field is quite broad, although it is particularly useful in areas where cryogenic environment exists such as superconducting technologies and spacecraft design. It also became a special branch of cryophysics and cryotechnics and plays a role in operations that require high resolution and precision measurements.
Cryogenic electronics is important for a growing number of applications, including superconducting classical computing, superconducting quantum computing and quantum annealing, and superconducting single-photon detector arrays. Cryoelectronic devices include the SQUIDs or the superconducting quantum interference devices, which represent magnetic sensors of highest sensitivity. They serve as the backbone of applications that range from materials evaluation, geological and environmental prospecting, and medical diagnostics, among others.
Implementing electronics at temperatures as low as 4 K or −269◦C is not trivial, as most electronic components are not supposed to be operated outside the industrial temperature range down to −55◦C. Various technologies have been investigated to implement cryogenic electronics, such as GaAs high-electron-mobility transistors (HEMTs), SiGe heterojunction bipolar transistors (HBTs), rapid single flux quantum (RSFQ) devices, and custom semiconductors. However, the most reliable and common technology for fabricating integrated circuits is CMOS (complementary metal-oxide-semiconductor) . It is the only technology that allows the integration of billions of transistors and at the same time ensures a low power consumption and extremely high reliability. Although CMOS transistors have been shown to operate at temperatures as low as 100 mK
Cryogenic Electronics Applications
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.
Electronics for cryogenic space exploration missions
Electronic components and systems capable of low-temperature operation are required for many future NASA space exploration missions where it is desirable to have smaller, lighter, and less expensive spacecraft. Presently, spacecraft operating in the cold environment
of deep space carry on-board a large number of radioisotope heating units (RHUs) to maintain an operating temperature for the electronics of approximately 20 Degrees C. This is not an ideal solution because the radioisotope heating units are always producing heat, even when the spacecraft may already be too hot, thus requiring an active thermal control system for the spacecraft. In addition, RHUs are very expensive and require elaborate containment structures.
Electronics capable of operation at cryogenic temperatures will not only tolerate the hostile environment of deep space but also reduce system size and weight by eliminating radioisotope heating units and associated structures. The exclusion of heating elements will result in reduced system development and launch costs, improved reliability and lifetime, and increased energy densities.
In addition to deep space applications, low temperature electronics have potential uses in terrestrial applications that include magnetic levitation transportation systems, medical diagnostics such as magnetic resonance imaging (MRI) that uses superconducting magnets, cryogenic instrumentation, and super-conducting magnetic energy storage systems. The utilization of power electronics designed for and operated at low temperature is expected to result in more efficient systems than room temperature systems. This improvement results from better electronic, electrical, and thermal properties of materials at low temperatures. In particular, the performance of certain semiconductor devices improves with decreasing temperature down to liquid nitrogen temperature.
Cryogenic isolator array for use in quantum computer research, radio astronomy, sensors, and research in Bose-Einstein condensation.
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.
At low temperatures, majority carrier devices demonstrate reduced leakage current and reduced latch-up susceptibility. In addition, these devices show higher speed resulting from increased carrier mobility and saturation velocity. An example is the power MOSFET that has lower conduction losses at low temperature due to the reduction in the drain-to-source resistance RDS(on) resulting from increased carrier mobility.
Superconducting QUantum Interference Device (SQUIDs)
Superconducting QUantum Interference Device (SQUIDs) is one of the most sensitive detectors of magnetic flux and field known, with an equivalent energy sensitivity that approaches the quantum limit. Due to their unique properties, SQUID devices are widely used in several applications like biomagnetism, magnetic microscopy, non-destructive evaluation, geophysics, quantum information, and nanoscience.
Magnetometers have been used to detect submarines since the second world war. Magnetic Anomaly Detection (MAD) employs magnetometers to detect very small changes in the earth’s magnetic field- like one caused by a massive hunk of metal. . They are used for geophysical mineral and oil exploration, ordnance and weapons detection (UXO), maritime intrusion detection, and Anti-Submarine Warfare (ASW).
However, these sensitive instruments based on molten potassium or cryogenic superconducting quantum interference devices (SQUIDs) require bulky insulation and significant resources to maintain their operating temperature. SQUID-based magnetometers still suffer from major disadvantages: they require extreme cooling and can be challenging to set up. Together with their detection range, this currently makes it unlikely that SQUIDs will be put on satellites anytime soon. While cryogenic cooling is already used in space for astronomy missions, it remains overly expensive.
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.
Nuclear magnetic resonance is based upon the measurement of absorption of RF radiation by a nucleus in a strong magnetic field created with superconducting magnets. 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.
In 1911, a Dutch scientist discovered a class of materials which, at temperatures near absolute zero, could conduct electricity with no resistance and therefore zero loss of power. These materials called Superconductors have unique properties including, Zero resistance to direct current; Extremely high current carrying density; Extremely low resistance at high frequencies; Extremely low signal dispersion; High sensitivity to magnetic field; Exclusion of externally applied magnetic field; Rapid single flux quantum transfer; and Close to speed of light signal transmission.
Conventional superconductors consist of simple metals, such as niobium, lead, or mercury, which become superconducting when cooled to below a characteristic “critical temperature” close to absolute zero—4.2 K in the case of mercury. These became known as Low Temperature Superconductor (LTS) materials.
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 sensors
Quantum computers promise an exponential speed-up over conventional computing, thus providing the resources to solve complex problems such as climate models, to breach conventional security algorithms and to search more quickly in extremely large databases.
Unfortunately, these quantum phenomena only become ‘visible’ at extremely low temperatures. Moreover, the qubits are very sensitive to disturbance and their coherence time is usually limited to several microseconds for the best qubits currently available. Therefore, the quantum device has to be operated close to absolute zero, ideally below 100 mK, and quantum error correction is required to track and repair qubit states. Both requirements impose severe constraints on the electronics employed in the quantum-classical interface. This interface controls the qubits, reads the quantum information and processes this data to track and correct errors. It should be able to produce very accurate signals for qubit operations and to read tiny signals from the quantum device. Furthermore, the error-correction cycle should be faster than the qubit coherence time to enable fault-tolerant quantum computing.
Currently, this electronic system is implemented at room temperature, which is about 300◦C warmer than the qubits. To interface these environments, interconnects are implemented between cryogenic and room temperature. The number of interconnects is limited, as heat injection into the qubits will disturb the quantum states. For now, this is not a problem per se, but millions of qubits are required for large-scale quantum operations and all of them need to be interfaced with electronics. To reduce heat injection, one solution is to operate qubits at higher temperatures, but their performance is currently limited due to thermal noise. A second solution is to operate the electronics
at lower temperatures. The ultimate solution is to operate both quantum device and electronics at the same temperature, but this is not predicted to happen in the very near future.
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.
RF components that are commonly used in Cryogenic applications
i) Circulators and Isolators
The components like isolators and circulators are used in low power applications. You can use them in scientific research, measurement & test, and communications! Mainly such applications need excellent RF performance within an economical and compact package. You can also get their custom designs, higher load rating, and input power options.
All unique and popular RF coaxial connectors need different kinds of fixed attenuators in various frequency and power ranges. You can get them from DC to 65HZ, and ranging from 1 to 300 watts. They are also available in a different body and mounting styles, mixed connector types, and sex.
The operating frequency of pin diode switches ranges in between 0.02 to 18 GHz. They are available in different varieties. It comprises multi-way having TTL control, single pole two throw, single pole single throw. Generally, these switches support both low insertion loss and high isolation features.
iv) Cryogenic RF Cable
An RF cable mostly belongs to either test or coaxial cable. The former type of Flexible Cryogenic Cable is made to be used as laboratory equipment where low-loss, flexibility and high performance matters the most.
In the case of coaxial cryogenic RF cables, they are not necessarily of semi-rigids detailed design. Additionally, these cryogenic cables can be used as a versatile solution to subassembly and equipment cabling.
References and Resources also include: