Superconducting qubits operate at temperatures colder than outer space. How dilution refrigerators are built and why extreme cold is so critical.
❄️ The Coldest Technology on the Planet
Deep space between galaxies has a temperature of about 2.7 Kelvin — that is -270.45°C. It is the temperature corresponding to the Cosmic Microwave Background, the afterglow of the Big Bang. However, in the quantum computing laboratories of Google, IBM and other companies, superconducting chips operate at temperatures below 15 millikelvin — roughly 180 times colder than space itself.
This is not an exaggeration. It is a physical necessity. The superconducting quantum circuits that form qubits — the basic units of information in a quantum computer — cannot function without these extreme conditions. And the device that makes this possible is a marvel of engineering: the dilution refrigerator.
🔬 Why Do We Need Such Low Temperatures?
Superconducting qubits are circuits made from superconducting materials — mainly aluminum on a silicon substrate, but also niobium or tantalum in newer designs. Superconductivity is a phenomenon where electrons form Cooper pairs and move through the material without resistance, as a single quantum wave. This only occurs at temperatures close to absolute zero.
But superconductivity alone is not enough. Even below the transition temperature, thermal quasiparticles exist — quasi-particles that run through the superconducting circuit and interfere with qubit operation. These quasiparticles are exponentially suppressed at lower temperatures. That is why the circuits are cooled to 10-15 mK — far below the temperature required for superconductivity alone.
There is also a second issue. The energy levels of a qubit are separated by roughly 5 GHz. A temperature of 1 Kelvin corresponds to thermal fluctuations of 20 GHz — four times the qubit's energy separation. This means thermal energy would immediately destroy the quantum information. At 15 millikelvin, thermal fluctuations drop to 0.3 GHz, low enough for the qubit to maintain its coherence for milliseconds or more.
⚙️ How a Dilution Refrigerator Works
The idea of the dilution refrigerator was first proposed by Heinz London in the early 1950s and was experimentally realized in 1964 at the Kamerlingh Onnes Laboratory of Leiden University. The basic principle exploits the properties of a mixture of two helium isotopes: helium-3 (³He) and helium-4 (⁴He).
When this mixture is cooled below approximately 870 millikelvin, it undergoes spontaneous phase separation. Two layers form: the concentrated phase (nearly 100% ³He) and the dilute phase (about 6.6% ³He in 93.4% ⁴He). The key to cooling lies in the fact that transferring ³He atoms from the concentrated to the dilute phase is an endothermic process — it absorbs heat from the environment, cooling the mixing chamber.
The process is analogous to the evaporation of a liquid: just as evaporation absorbs heat, so does the “dissolution” of ³He into ⁴He cool the system. The fundamental difference is that even at absolute zero, the dilute phase retains 6.6% ³He — a quantum phenomenon that ensures the refrigerator can operate continuously, with no moving parts in the low-temperature zone.
🔄 Dry Refrigerators and Modern Evolution
Classic ("wet") dilution refrigerators require external supplies of liquid nitrogen (77 K) and liquid helium (4.2 K) to precool the mixture. This made them complex and expensive to operate. In the 1990s, the development of commercial cryocoolers — particularly pulse tube coolers — enabled the creation of “dry” dilution refrigerators that need no external cryogenic liquids.
Modern dry dilution refrigerators operate with a high degree of automation. The Finnish company Bluefors holds a significant market share — its systems are used by Google, IBM and many university laboratories. Each system resembles a tall cylinder of golden plates, successive cooling stages: the top is at room temperature, the base at 10-15 mK.
📐 The Challenge of Scale
One of the biggest obstacles to building large quantum computers is not the qubits themselves — it is cooling them. Each qubit requires microwave control lines and readout lines that must reach from room temperature to 15 mK. These cables introduce heat — and the cooling power of a dilution refrigerator at millikelvin temperatures is measured in microWatts.
A future system of millions of qubits would require new architectures: cryogenic electronics (cryo-CMOS) that would operate inside the refrigerator, reducing the number of cables. Already, research teams at Intel, Google and academic institutions are developing such circuits. Oxford Instruments and Bluefors are designing next-generation refrigerators capable of cooling hundreds of thousands of qubits simultaneously.
There is also the so-called Kapitza resistance — a thermal resistance at the interface between liquid helium and the solid surface of the heat exchanger. It is inversely proportional to T⁴: to achieve the same thermal conductivity with a 10-fold temperature reduction, you need 10,000 times more surface area. In practice, very fine silver powder is used to increase the contact surface.
🏆 From Onnes to Google
The history of cryogenic physics begins in July 1908, when Heike Kamerlingh Onnes managed to liquefy helium and reach a temperature of 2 Kelvin. Three years later, he discovered that mercury becomes superconducting at 4.2 K — a discovery that earned him the Nobel Prize in Physics in 1913.
Today, cryogenic technology is the unseen pillar of every superconducting quantum computer. Google's Sycamore processor, which claimed “quantum supremacy” in 2019, operated in a dilution refrigerator. IBM's Condor with 1,121 qubits (2023) and Google's Willow (2025), which achieved independently verifiable quantum advantage, are cooled with the same technology. Yasunobu Nakamura, who built the first superconducting qubit in 1999, and John M. Martinis, awarded the 2025 Nobel Prize in Physics for the Sycamore processor, relied on the same cryogenic foundations.
This method has no theoretical lower temperature limit. The practical limit, around 2 mK, arises from geometric constraints: each halving of temperature requires 16,384 times larger tubing volume. Beyond 2 mK, nuclear demagnetization takes over.
Dilution refrigerators are perhaps the most impressive yet least known technological achievement behind the quantum revolution. Without them, no superconducting quantum computer could function. They are the bridge between quantum theory and the real world — and the hidden challenge for every company that dreams of millions of qubits.