Efforts to build a useful quantum computer are underway, with startup companies competing alongside the big leaders in information technology. Other quantum technologies are already part of our everyday lives, from MRI scanners in hospitals to atomic clocks, which are used to keep precise track of time on GPS satellites and inform Coordinated Universal Time. The word “quantum” itself has become almost synonymous with cutting-edge technological advancement.
Many, but not all, quantum technologies require cryogenic cooling to operate. Take the examples above. The magnets in MRI machines are cooled to 4 K, but atomic clocks run at room temperature. Quantum computers based on superconducting qubits use dilution refrigerators to reach temperatures of 10 mK, but quantum computers based on atoms or photons don’t need to be in a cryostat or refrigerator. Why is that? Let’s first look at quantum technologies and what unifies them.
What is quantum technology?
The outlines of what is and isn’t a quantum technology are fuzzy, but broadly, a quantum technology is one that leverages the aspects of quantum mechanics that do not usually manifest themselves in our daily lives. These aspects include quantum phase coherence, superposition, entanglement, quantum tunneling and quantum measurement. The fact that we don’t usually experience these phenomena is already hinting at the reason why many quantum technologies require cryogenic cooling: in order to observe effects not often encountered in our daily lives, we might need to subject a system to more extreme conditions.
Why do quantum computers need cryogenics?
There are a number of reasons why some quantum computers need cryogenics. The most fundamental reason is the need to preserve quantum phase coherence. Phase coherence is a prerequisite for other useful quantum behaviors such as superposition and entanglement.
One example of a quantum object is a photon, or a single particle of light. If you think of a photon as a wriggling electromagnetic field moving through space, preserving phase coherence means that the photon remembers to wiggle on beat, so that the field traces a neat sinusoidal wave over time. This remembering is key for quantum computing because the phase coherence is used to store information, and computers don’t work if they forget the information they’re processing. Maintaining coherence in any quantum object is largely achieved by making sure the system doesn’t interact with any other object. For photons, this is pretty easy, since they don’t interact very much. They almost never interact with other photons (although they can be made to interact through a nonlinear intermediary), and they can maintain their coherence when traveling through glass or when reflecting off metal surfaces. Quantum computers that use photons as their fundamental components don’t need any cooling to maintain coherence. However, single-photon quantum computers may use cryogenic cooling for other reasons. Detecting single photons reliably requires exquisitely sensitive and low-noise detectors, and the highest-performing detectors are based on superconductors that need to be cryogenically cooled.
When a photon interacts with something in its environment, like an atom, it can lose its phase coherence. A single atom is another type of quantum object. An atom can undergo its own kind of periodic transformation of a property called spin, and coherence means remembering to transform on beat. Atoms, however, easily interact with other nearby atoms through electric or magnetic fields, leading to a loss of coherence. To mitigate this, quantum computers based on neutral atoms or ions use electric fields, magnetic fields and lasers to isolate and cool individual atoms and to keep them trapped in a vacuum, far away from other atoms. The atoms in these computers are quite cold, with temperatures on the order of 1 mK. However, the techniques used to cool the atoms involve slowing down their vibrations using lasers, so cryogenics are not needed to cool the atoms. However, cryogenic cooling might still be useful in atomic/ionic quantum computers. For example, the atoms comprising a quantum computer might be housed in a cryogenic chamber in order to create a better vacuum. A better vacuum means fewer stray atoms around, which is important since stray atoms can lead to collisions that knock a computing atom out of its trap.
Perhaps the most well-known quantum computing effort uses superconducting qubits. Superconducting qubits are electronic circuits made of superconducting metals. These qubits need to be cryogenically cooled for two reasons. The first is that they are made of superconducting metals. Superconductivity is a special kind of electrical conductivity, where charge carriers are not constantly colliding with phonons or nuclei in the conductor’s atomic lattice. If the circuit were not superconducting, such collisions would lead to almost instant phase decoherence. Superconductivity only occurs at temperatures below the critical temperatures of the superconducting metal, which for aluminum, a common material for this application, is around 1 K. The reason temperatures as low as 10 mK are needed is that, unlike trapped atoms, it is impossible to completely isolate a superconducting qubit from its environment. Superconducting qubits are built on semiconductor wafers using nanofabrication techniques. They are quite large by quantum standards, usually several tens of micrometers across. The quantum circuits are therefore surrounded by many atoms. Atoms with thermal energy vibrate, creating variable electric and magnetic fields. The extremely low temperatures keep the atoms in the environment frozen in place and thereby quiet the electric and magnetic field fluctuations that would otherwise lead to decoherence in the qubit.
There are other notable quantum computer platforms in the solid state, such as quantum dots or defects in silicon and Majorana fermions for topological quantum computing, and these also require cryogenic cooling to the sub-100 mK level.
What other quantum technologies need cryogenics?
There are many other quantum technologies that require cryogenics, such as superconducting magnets, which are used in MRI scanners and the Large Hadron Collider, single-photon detectors for quantum information and astrophysics, quantum memories and transducers for quantum networks, and fundamental science inquiries such as searching for dark matter.
In this series, I plan to explore the intersection of quantum technologies and cryogenics, focusing on the developments in quantum technologies at cold temperatures and the progress in cryogenic technologies enabling them.
The views expressed are the author’s own and do not necessarily reflect those of NASA or JPL.
Image: When a photon interacts with something in its environment, like an atom, it can lose its phase coherence. Credit: Ioana Craiciu, Jet Propulsion Lab
