Cryogenic characterization is a central component in facilitating and accelerating scientific and quantum technology breakthroughs. Quantum sensors, quantum communication devices, and future quantum computers all require scalable, efficient cooling for operation.
Quantum computers rely on qubits, which can exist in several states concurrently. These quantum states are exceptionally delicate and vulnerable to disruption from environmental noise, especially thermal energy.
Various quantum computing strategies, particularly those employing superconducting qubits, depend on superconductivity, which occurs at very low temperatures.
Cryogenic temperatures - near absolute zero - reduce thermal fluctuations and allow qubits to sustain coherence for longer durations. In turn, this enables complex quantum computations.
Magnetic refrigeration has gained traction as a promising technology for achieving these conditions. While conventional cryogenic cooling with liquid helium is both costly and complex, magnetic cooling offers an alternative that is potentially simpler, more scalable, and more economical.
Magnetic Refrigeration
Magnetic refrigeration enables extremely low temperatures near absolute zero (sub-Kelvin temperatures below -273 °C). This is accomplished by exploiting the relationship between a magnetic field and entropy in specific materials.
Entropy is defined as the disorder or randomness in a material. In magnetic materials, the entropy is essentially composed of two parts:
a) the entropy of the crystalline structure, where higher entropy corresponds to elevated temperature due to crystal lattice vibrations; and
b) the entropy of the magnetic moments of the crystalline material.
Magnetic moments are vector quantities that represent the magnetic orientation and strength of a magnetic object. A collection of magnetic moments is known as a spin system.
Magnetic refrigeration depends largely on how the entropy of this spin system responds to a magnetic field.
The process is described as follows:
1) Magnetic moments are ordered
2) The application of a magnetic field orders moments and increases temperature
3) The temperature relaxes back to equilibrium
4) The removal of the magnetic field results in cooling
Adiabatic Demagnetization Refrigeration
Adiabatic Demagnetization Refrigeration (ADR) is a specific process in magnetic refrigeration. The term “adiabatic” describes a process in which no heat is exchanged with the surrounding environment.
Demagnetization involves reducing the magnetic field, and refrigeration signifies the cooling process.
In the ADR process, a material, frequently a paramagnetic salt, is pre-cooled to a relatively low temperature, generally using a cryocooler.
The procedure involves two further critical stages:
- Magnetization:
The material is then placed in a powerful magnetic field, causing the magnetic moments (oppositely charged particles) within it to align and so minimizing its entropy. This process generates heat, which is absorbed by a cryocooler thermal bath.
- Demagnetization:
The material is thermally isolated to prevent heat exchange with its surroundings. As the magnetic field is slowly reduced, the magnetic moments/spins attempt to return to a state of higher disorder and entropy. To achieve this, they draw energy from the material’s internal thermal energy, leading to a significant drop in temperature.
ADR is a technique for achieving temperatures close to absolute zero, providing cooling at very low temperatures (milli-Kelvin range).
Cryostats and Continuous ADR
Continuous adiabatic demagnetization refrigeration (cADR) is a variant of adiabatic demagnetization refrigeration that provides continuous cooling at ultra-low temperatures near absolute zero.
cADR is a multi-stage cooling system commercialized by kiutra GmbH (kiutra). Composed of at least two ADR units, the system operates by cooling with one ADR unit while the other ADR unit undergoes regeneration, and vice versa.
It’s a substantial advancement over ADR because it provides continuous cooling, while ADR is a single-shot technique that can only maintain an object at milli-Kelvin temperatures for a limited period.
kiutra’s Continuous ADR Helium-Free Cryogenic Platform
kiutra stands as the sole supplier of continuous cADR cooling solutions capable of generating temperatures as low as 100 mK or -273.05 °C.
Unlike traditional helium-based cooling techniques, kiutra’s helium-3-free magnetic cooling technology is scalable, low-maintenance, and user-friendly.
With reliable and low-temperature systems, kiutra is ideally suited for cooling quantum technologies at an industrial scale. With this platform, ultra-low temperatures can be achieved and maintained without cryogenic liquids.
“kiutra’s S-Type cryogenic platform is uniquely equipped with two ADR units that enable continuous, cryogen-free sub-kelvin cooling,” said Dr. Steffen Säubert, Senior Product Manager with kiutra.
“It can also operate in one-shot mode to achieve even lower temperatures for a limited duration. In both modes, the cooling process is fully automated and can be managed through an intuitive graphical user interface.
“With a small footprint, the compact S-Type cryogenic platform provides simplified, adaptable, and cryogen-free cooling for a range of uses, including as an operating and testbed system by various research partners worldwide,” added Dr. Säubert.
Optical Access with an Ultra-Low Vibration Decoupling Platform
The S-Type Optical is a variant of kiutra’s S-Type cryogenic platform that features a large, free-beam vertical optical access. This design enables expanded optical research into the sub-kelvin temperature range.
A distinct characteristic of the S-Type Optical cryogenic platform is its ability to eliminate vibrations, which could otherwise disturb both optical results and quantum computing.
Quantum systems, such as qubits, exhibit high sensitivity to their external surroundings. Environmental vibrations, whether produced by the system’s cooling refrigerator or by sound waves in the room, can introduce energy into the qubit. This can disrupt its fragile quantum state and cause it to lose its superposition, an essential feature of quantum computation.
“After careful review, our engineering team selected Negative-Stiffness vibration isolation to support our S-Type Optical cryogenic platform,” continued Dr. Säubert.
“kiutra chose Negative-Stiffness isolators because of their passive capability, 0.5 Hz resonant frequency, and they are very easy to optimize for vibration decoupling without opening the system to adjust during operation.”
Negative-Stiffness Vibration Isolation
Since its introduction in the mid-1990s by Minus K Technology, Negative-Stiffness vibration isolation has gained wide acceptance for vibration-critical applications.
Its popularity is primarily due to its ability to effectively isolate lower frequencies, both vertically and horizontally. The company’s isolators are employed by over 300 universities and government labs across 53 countries.
Negative-Stiffness isolators are unique in that they operate entirely in a passive mechanical mode, requiring no electricity or compressed air. Since they contain no motors, pumps, or chambers, these devices are maintenance-free, as there are no components to wear out.
“Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with a Negative-Stiffness mechanism,” said Erik Runge, Vice President of Engineering at Minus K.
“The net vertical stiffness is made very low without affecting the static load-supporting capability of the spring. Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation. A beam-column behaves as a spring combined with a negative-stiffness mechanism. The result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and high internal structural frequencies.”
Negative-Stiffness isolators deliver a high degree of isolation across multiple directions, offering the flexibility to custom-tailor resonant frequencies to 0.5 Hz vertically and horizontally (with certain versions at 1.5 Hz horizontally)*.
At a 0.5 Hz adjustment, these isolators achieve an isolation efficiency of around 93 percent at 2 Hz, 99 percent at 5 Hz, and 99.7 percent at 10 Hz.
Schematic of kiutra's Negative-Stiffness vibration isolators. Image Credit: Minus K Technology
“Being inherently modular, our S-Type Optical cryostat can be configured and upgraded to match a wide range of user requirements,” explained Dr. Säubert.
“The Negative-Stiffness isolator has been designed to fit perfectly into this modular platform.”
kiutra's S-Type Optical cryogenic platform integrated with Minus K Technology's Negative-Stiffness vibration isolation. Image Credit: kiutra GmbH
In Support of Quantum Technologies
“Due to our many years of hands-on experience working with cryogenic systems, at kiutra we are convinced that magnetic cooling is an extremely versatile and elegant tool to provide research at low temperatures relating to quantum technologies,” said Dr. Säubert.
“And specifically cryogen-free, continuous-cooling magnetic refrigeration systems, which definitely stand out above other cryogenic systems for their superior performance at sub-Kelvin temperatures.”
Repurposed content originally written by Jim McMahon.
This information has been sourced, reviewed, and adapted from materials provided by Minus K Technology.
For more information on this source, please visit Minus K Technology.