Why Qubits Need Millikelvin Temperatures

A qubit must be far colder than its own transition energy so that thermal photons do not randomly excite it out of a well-defined quantum state.

Temperature as an energy scale

Temperature is really an energy scale: thermal energy is roughly Boltzmann's constant times temperature. A quantum device has definite energy levels, and whether those levels are cleanly defined or smeared by thermal excitation depends on how the level spacing compares with thermal energy. For a qubit to sit reliably in its ground state, its transition energy must be much larger than the thermal energy of its environment.

The number that matters

Superconducting qubits have transition frequencies of a few gigahertz. Converting a frequency to a temperature via Planck's and Boltzmann's constants, a 5-gigahertz qubit corresponds to about 0.24 kelvin. To keep the probability of thermal excitation low, the environment must be several times colder than this, which lands squarely in the tens-of-millikelvin range. At 10 millikelvin the thermal occupation of a 5-gigahertz mode is negligibly small, so the qubit stays in its ground state until deliberately driven.

import math
h = 6.626e-34; kB = 1.381e-23
f = 5e9  # 5 GHz
T_equiv = h * f / kB
print(round(T_equiv, 3), 'K')  # ~0.240 K
# thermal occupation at 10 mK
n = 1 / (math.exp(h*f/(kB*0.010)) - 1)
print(f'{n:.2e}')  # extremely small

Beyond the ground state

Cold does more than keep the qubit in its ground state. Thermal photons in control lines and the environment cause dephasing and unwanted transitions; keeping everything cold reduces this photon bath. It also keeps the superconductor deep in its superconducting state, minimizing the quasiparticles that cause errors. The millikelvin requirement is thus overdetermined: several distinct effects all demand it.

The consequence

Because the millikelvin requirement follows directly from the qubit's own energy scale, it is not something clever engineering can wish away for superconducting and spin platforms. It is why the dilution refrigerator and its helium-3 mixture are foundational to this style of quantum computing, and why the field's growth is tied to cryogenic capacity and isotope supply.

• Thermal energy is Boltzmann's constant times temperature
• A 5 GHz qubit corresponds to about 0.24 K
• Environment must be several times colder, hence tens of mK
• Cold also suppresses dephasing photons and quasiparticles