Advantages of Colder Temperatures

There are many advantages to cooling of quantum systems, whose importance depends on the specific qubit modality and the device scale. The most important include the ability to: 

• Suppress unwanted excitations: Consider the discrete energy levels of a quantum system. Thermal fluctuations can ‘kick’ a qubit out of its ground state, spoiling the ongoing quantum computation. A cold enough system has a temperature which cannot produce an energy large enough to bridge this gap, and these kicks are exponentially rare. 
• Shrink Randomness- Johnson Noise: The amplitude of this noise drops with dropping temperature. This is essential for small signal readout, especially where microwave pulses are present. 

• Quiet Supporting Electronics: Control hardware required for quantum systems will also exhibit random Johnson noise dependent on T. For some systems, this noise has to be minimized to the extreme to allow for practical signal readout. Consider the electrical control hardware necessary for superconducting qubit systems. In many cases, some front-end electronics must be placed within the dilution refrigerator, as these devices require millikelvin temperatures to achieve the resolution often required. 
• Exotic Properties: Some quantum systems rely on properties only revealed at low temperatures. A popular example are superconducting materials, which must be held below their critical temperature in order to remain superconducting. 

These examples, among many others dependent on the system setup, are among the many reasons why cryogenics for quantum computing are required. 

Methods for Quantum Cryogenics

Cooling requirements vary significantly across qubit modalities. Superconducting qubits, together with their some front-end control hardware, must operate at millikelvin temperatures. Achieving these conditions typically necessitates the use of a dilution refrigerator, a large and costly piece of machinery.

Dilution refrigeration relies on vacuum pumping of helium isotopes: pure ⁴He can cool a system to a ~ 1 kelvin, whereas a ³He/⁴He mixture enables temperatures ~ 10 mK in commercial systems. A continuing challenge is the limited global supply of ³He, which increases relevant system operating costs by the year as supply diminishes.

Neutral atoms, on the other hand, do not require comprehensive system cooling. Instead, the qubits (or atoms) are cooled via a method called laser cooling, while the remaining system hardware can remain ‘room temperature.’ It should be noted that laser cooling often requires use of a stronger ultra-high-vacuum (UHV) which other cryogenic systems do not.

What is Laser Cooling? Why is it Better?

Laser cooling cools atoms and ions with light instead of cryogenic liquids. By tuning a laser just below an atomic resonance, atoms that move toward the beam pick up photons that push back against their motion, giving off kinetic energy and forcing them into the micro-kelvin, or even nano-kelvin,  range. This method earned the 1997 Nobel Prize in Physics. Its extension, optical tweezers, which trap these ultracold atoms with tightly focused beams to extreme precision, won again in 2018 for the ability to isolate individual biological components. There are many advantages to laser cooling compared to other dilution fridge-based cryogenic cooling methods. Some of these include: 

• Lower Temperature Range: Laser cooling can achieve micro/nano kelvin temperatures unreachable by dilution refrigerators (Note: atomic, not system, temperatures)
• Hardware Footprint: Dilution refrigerators are extremely large. Optical tables, and other engineering setups required for other modalities, often require less space, especially as systems move towards integrated photonics. Current optical tables can still prove quite large, but have no required cryogenic plumbing, saving space. 
• Focused Cooling: Laser cooling systems only cool the atom or focused object. Dilution refrigerators must cool the whole system, including auxiliary hardware
• Cost: The cost is often less, especially as the price of Helium isotopes only grows with time. Laser cooling is pricey, but by avoiding expensive Helium isotopes, costs can often be saved.

The advantages of the cooling mechanism are one of the many reasons why neutral atoms are such a promising modality for qubit design, and why we at QuEra are so excited about the technology!