Near Absolute Zero Temperatures are Essential to Their Proper Function
Quantum computing can solve problems beyond the most powerful classical computers. But it faces many hard engineering challenges to evolve. Among the toughest is providing an extremely cold environment. [1]
Qubits or Bust
Qubits, or quantum bits, differ significantly from classical computing bits. A qubit can be both 0 and 1 simultaneously, a property known as superposition. Another fundamental trait is entanglement, where the state of one qubit is intrinsically linked to another, regardless of distance.
These properties enable exponential scalability, allowing quantum computers to solve complex problems much faster than classical computers. Years of processing time by conventional computers are replaced by just minutes using quantum computing. [2]
Working with qubits is challenging. Made from superconducting circuits, photons, and other methods, they are extremely susceptible to noise, which here includes background radiation, phone and wi-fi signals, and the slightest bits of heat energy. Tiny disturbances can lead to calculation errors.
One way of preventing these effects is to keep qubits near absolute zero temperatures where thermal vibrations are minimized. Here, they can be encoded in two distinct states: grounded and excited. Each qubit can exist in one or the other of these two states, or in a quantum superposition of the two. Putting qubits into superposition states allows a quantum processor to simultaneously examine many potential solutions to a problem, a dramatic improvement in computational power over a classical computer.
How powerful is quantum computing? Google has stated that their state-of-the-art processors can solve a computing problem in under five minutes, which, by comparison would take today’s fastest supercomputers about 10 septillion years (1025 ) or equal to “more time than the history of the universe.” [5]
Cooling the Qubits
Liquid and air systems cool conventional electronics, but cooling quantum chips brings thermal engineering into the very cold world of cryogenics. While quantum chip power consumption is very low, they need to be kept at extremely low temperatures. Even tiny temperature increases can make a system unworkable.
Much engineering goes into providing the cryogenic-level low temperatures that support qubits. The chandelier-like structures associated with quantum computers are part of these cooling systems, with the chips installed at the bottom. Nearby are the tanks, electronics and many connections that feed and power these intricate cooling systems.
The Very Cold World of Cryogenics
Kelvins are the standard for coldness measurements in cryogenics. Zero Kelvin is equal to absolute zero (-273.15°C or -459.67°F). Theoretically this is the lowest temperature possible. At absolute zero, atoms have no kinetic energy and are at rest. There is no motion or heat. By comparison, the average temperature of outer space is a balmy 2.7 K (-270.45°C or -455°F).
Quantum computer cooling from dilution refrigerator systems, the most common cryo technology, can bring qubits to about 50 millikelvins above absolute zero. Millikelvins (mK) are a unit of temperature measurement, i.e., a thousandth of a Kelvin (1/1000th of a Kelvin), used in the realm of cryogenics and quantum physics.
Note that cryogenics deals with temperatures from near absolute zero up to -150°C (or 123.15 K). The higher end of this range, from −150°C to -190°C is where naturally occurring biological processes are halted. Here is where many biologicals, e.g., cell and gene therapies, are kept for long term storage.
Dilution Refrigerator Systems
Google, IBM, Amazon, and others have been building quantum computers using large, complex, expensive systems known as dilution refrigerators with multiple stages of cooling to chill circuits to 1 kelvin or below. The complexity of these refrigerators is greatest at the coldest stage, which involves mixing different isotopes of liquid helium.
Dilution refrigeration leverages properties from mixing helium-3 and helium-4 isotopes. Basically, helium-3 is continuously diluted into the helium-4 in the cooling system, causing the system to extract heat from the surroundings, down to near absolute zero temperatures.
This function is preceded by precooling and evaporative cooling stages in the cylinder, the hanging, chandelier-shaped structure. Each stage downward reduces the temperature until the near zero temp is reached at the bottom stage, which connects to the chip. [8]
In Figure 5, A is the highest and warmest stage of the cooling system. This initial pre-cooling may use standard refrigeration techniques. At B, evaporative cooling via liquid helium may be used to cool the environment further. At this stage, the system is cooled to just a few degrees above absolute zero. At C actual dilution refrigeration occurs. Helium-3 and -4 are mixed and circulated to draw away heat from the system. Finally, at D, qubits in the quantum chip are maintained at temperatures near absolute zero. [10]
The numerous wires and connectors in quantum computers are used for controls and readouts, power, and cryogenics.
Developing Technology
As more resources are provided for quantum computing, all aspects of the technology will improve. Useful quantum applications, including drug design, cyber security, and battery development will further cultivate and commercialize this exciting computing field.
Improved processes are already easing the cryonic temperature requirements, though only very slightly at this point. Alternative cooling systems now include the use of magnets, high pressure waves, and tuned laser light.
Other areas of quantum computing are seeing improvements and innovations. These include advanced error correction techniques, and expanded cloud-based quantum computing platforms that will bring quantum computing to more users
References
- https://www.ibm.com/think/topics/quantum-computing
- https://www.bluequbit.io/quantum-volume#:~:text=The%20property%20of%20superposition%20grants,problem%2Dsolving%20and%20technological%20innovation.
- https://en.wikipedia.org/wiki/Sycamore_processor#:~:text=Sycamore%20is%20a%20transmon%20superconducting,ordered%20state%2C%20with%2031%20qubits
- https://azure.microsoft.com/en-us/solutions/quantum-computing/
- https://cybernews.com/tech/googles-quantum-chip-willow-achieves-once-elusive-benchmark-researchers-say/
- https://exoswan.com/quantum-computer-visual-guide
- https://www.datacenterdynamics.com/en/analysis/cooling-quantum-computers/
- https://blog.google/technology/research/behind-the-scenes-google-quantum-ai-lab/?utm_source=tw&utm_medium=social&utm_campaign=og&utm_content=&utm_term=
- https://www.sharetechnote.com/html/QC/QuantumComputing_HW_Structure.html#Reference
- https://idstch.com/technology/quantum/the-cool-path-to-quantum-computing-dilution-refrigeration-technology-and-superconducting-qubits/#:~:text=Dilution%20refrigerators%20are%20the%20unsung,stability%20and%20functionality%20of%20qubits.
- https://epjquantumtechnology.springeropen.com/articles/10.1140/epjqt/s40507-019-0072-0/figures/3
![Figure 3 - Vapor Compression Cycle Thermodynamic State Point Diagram [2]](http://qats.com/cms/wp-content/uploads/2011/07/Leading_Edge_Strategies_to_Cool_Laptop_Computers_figure_31.png "Leading_Edge_Strategies_to_Cool_Laptop_Computers_figure_3")
