Let's go deeper. Natural background radiation and quantum computing
Quantum computers are poised to become the key computing technology of the 21st century for two main reasons. First, they achieve quantum supremacy, an effect that allows a quantum computer to perform calculations in seconds or minutes that would take a classical computer thousands of years. Second, any elementary particles or atoms between which a state arises can be used as the computing units in a quantum computer. quantum entanglement (entanglement). We are also lucky that our computer science is fundamentally binary, and a qubit emerging from an entangled state also ends up in one of two states, which can be labeled as 1 or 0.
It is already known that the effects on which the operation of quantum computers is based, extremely vulnerable to environmental influences. Because of this, modern quantum computers have about one error per 1000 operations, which is unacceptably high. For practical use of such calculations, the number of errors must be reduced by a factor of a billion or more.
A qubit is a logical element of a quantum computer, a two-level quantum system that contains information. Cooper pairs of elementary particles are usually used as qubits. In a qubit, the particles are in superposition, that is, in two states at the same time. Superposition is common in the microworld, and the most famous example explaining how it might look in the macroworld is “Shroedinger `s cat“A superposition always decays into as many states as there are particles entangled in it, and so quantum computing is inherently easily parallelized.
However, before the superposition decays, the operation contained in it must be brought to a result, and this result itself must be extracted and taken into account in subsequent calculations, since when the superposition decays, all the information contained in it is lost. To increase the stability of qubits, as well as to speed up operations on them, these entangled pairs must be kept at temperatures near absolute zero. It is also at such temperatures that the state of superconductivity arises, so an entire scientific discipline arises, engaged in the development of superconducting qubits.
Already in 2021, specialists from Google conducted studywhich showed that superconducting circuits are extremely vulnerable to the high-energy rays that constantly bombard our planet from deep space. Cosmic rays are the main cause of qubit decoherence. At the same time, it is impossible to protect oneself from cosmic rays on the surface of the Earth.
Problem of elimination electromagnetic interference when working with quantum computers not new. Developed error correction methods in quantum computingbut simple electromagnetic noise can usually disrupt the state of only a few qubits, and this situation is easily monitored. Cosmic rays, on the contrary, are capable of throwing an entire chip out of a coherent state at once. This is exactly the situation that Google specialists, led by Qian Xu from the University of Chicago, have undertaken to simulate and try to correct. Below, we will consider how exactly errors occur in qubits.
How Errors Occur in Qubits
To ensure predictable operation of a quantum computer, each pair of qubits must be kept in a state of superposition for as long as possible. Whereas in 1999 such a superposition lasted for a few nanoseconds, in 2020 its duration has already exceeded 200 microseconds. But the longer the superposition lasts, the higher the probability that a cosmic ray, essentially a beam of high-energy electrons, will hit the quantum device. This beam heats the substrate with the qubits by fractions of a degree, but this is enough to disrupt the states of the qubits. Moreover, such a hit sends a wave of electrons through the device, scattering when they collide with atoms. This leads to both heating of the material and an increase in vibrations; both processes contribute to decoherence. The electric field around the qubit also changes unpredictably.
As an experiment, the Google team selected 26 qubits from a test quantum processor and put them all into the same state. They then left the processor in standby mode for 100 microseconds and checked whether the qubits changed state and, if so, how many qubits were affected.
Under typical background electromagnetic noise, 4 of the 26 qubits changed state from excited to ground in that time. But under the influence of a quasiparticle beam similar to a cosmic ray, the state changed for 24 of the 26 qubits.
In this illustration, “g” stands for “ground” and “e” stands for “excited.” To confirm that the error was caused by cosmic rays, scientists observed how the state changed. Cosmic rays are known to be made up of quasiparticlesand the quasiparticle quickly loses energy when it hits the crystal lattice. Therefore, it cannot raise the qubit from the ground state to the excited state. But it can grab some of the energy from the qubit, thereby dropping it from the excited state to the ground. This is exactly what the experiment confirmed: when bombarded with cosmic rays, errors occur much more often if most of the qubits are in an excited state before the beam hits. Xu's group prudently placed the qubits at a distance of 15 millimeters from each other, but recorded that the error still spreads from the affected qubit to neighboring ones and so on, gradually fading. The group came to the conclusion that the error carriers in this case are phonon quasiparticles.
In further experiments, the group tried distributing the quantum computer data across several chips. Firstly, this increases the distance between individual qubits (fewer qubits per chip), and secondly, when a cosmic ray hits one chip, all data on it is erased, but on the other chips, it is preserved.
If cosmic rays hit one of the information chips, there is no need to restart the computer and repeat the calculations, since all the data can be restored based on the auxiliary chip. In Xu's model, the auxiliary chip is additionally shielded and can even withstand the impact of a pair of beams simulating cosmic rays.
Experimenting with topology information chips, Xu's group has achieved that errors related to cosmic rays occur in their computer on average no more than once every 51 days. The achievement is impressive, but here we note: when cosmic rays hit an unprotected qubit all the information in it is erased.
In 2021, another event was held study led by Jonathan DuBois of the Lawrence Livermore National Laboratory in Berkeley, California. The work also involved scientists from the University of Wisconsin-Madison and corporations including Google. DuBois's group set up a test rig consisting of four entangled qubits, which they then blasted with radio-frequency pulses that mimicked cosmic rays. They measured the excitation spectrum of the qubits, watching how quickly a qubit would transition from state 0 to state 1.
To assess the violations, the researchers bombarded a system of four qubits with radio signals, measuring their excitation spectrum. The experiment showed that when a radio signal hits the system, the energy of the qubit does indeed decrease, but the part of the energy that the affected qubit loses is converted into vibrations and redistributed in a wave-like manner to other qubits. This is how the error propagates.
Radiation of natural radionuclides
Natural background radiation —is the total electromagnetic radiation that consists of the effects of cosmic rays, long-lived natural radionuclides, and in our time — also the effects of man-made radionuclides. The main sources of natural background radiation on Earth are isotopes 40K (half-life about 1.3 billion years), 238U (half-life 4.5 billion years) and 232Th (half-life 14.05 billion years). These elements emit mainly gamma radiation. In turn, cosmic rays, when interacting with atoms (and qubits), generate muons and neutrons. On the scale of a single quantum chip, the natural radiation background can reach an energy of hundreds of kiloelectron volts (keV). In the substrate of a quantum computer, this energy does not form particles, but exists in a diffuse state, which is described as phonon quasiparticles. Phonon — is a quantum of vibrational motion in an atomic lattice, depending on the frequency it can be recorded as heat or sound. The imprint of a phonon in a crystal lattice is much larger than that of an elementary particle; as a rule, a phonon interferes with many qubits in a quantum computer at once. Since a phonon is more like a wave than a particle, and the changes provoked by one phonon in different qubits correlate with each other. Accordingly, it is also possible to correct the errors that arise using a common algorithm. But eliminating the influence of natural radionuclides in a normal environment is even more difficult than shielding a quantum computer from cosmic rays.
Maximum duration of qubit coherence
Already in 2020, a collaboration of employees from several institutes (in particular, MIT, MIT Lincoln Laboratory in Lexington, Harvard, and Pacific Northwest Laboratory) led by William Oliver, Joseph Formaggio, and Antti Vepsalainen published article “Effect of ionizing radiation on the coherence of superconducting qubits.” Formaggio — neutrino physicistso it has experience in setting up shielding against even the most penetrating background radiation. Oliver and Formaggio decided to study how natural background radiation affects superconducting qubits. To do this, it was necessary to select a well-studied radioactive isotope whose radiation slowly decreases and gradually becomes equal in level to natural background radiation. Copper-64 with a half-life of 12.7 hours was chosen as such. Two disks of pure stable copper-63 were irradiated with neutrons and gamma rays for several minutes. After a significant portion of the atoms in the sample were converted to copper-64, the disks were placed in close proximity to two qubits in an entangled state and cooled to a temperature of about 120 millikelvin. Under these conditions, the coherence of the qubits was maintained for no more than 4 milliseconds, which is still twice as long as the above-mentioned figure of 200 microseconds observed with natural background radiation. This experiment demonstrates that the combined effects of cosmic rays, natural background radiation, electromagnetic interference, and any temperatures even a couple of degrees above absolute zero exclude the use of quantum computers in any “field” or “household” conditions. At the same time, the path to high-temperature coherence largely repeats the path of development of high-temperature superconductors. In 2021, a collaboration from the United States and Japan extended the coherence period of solid-state qubits up to 22 milliseconds at 5Kand in 2024, scientists from Kyushu University announced the production of coherent qubits at room temperature (up to 403 K) in a metal-organic compound. Coherence was maintained for about 100 nanoseconds.
Canadian Shield and SNOLAB Laboratory
To try to overcome these challenges, a new collaboration has been launched in Canada, involving representatives from Chalmers University of Technology in Sweden, the Institute for Quantum Computing at the University of Waterloo, and the SNOLAB laboratory near Sudbury, Ontario, Canada. The project is calledCUTE” – literally “cryogenic underground test complex”.
As part of this research, Chalmers is fabricating superconducting qubits that will be tested first on the surface in Sweden and Canada, and then in the abandoned Vales Creighton Mine in Ontario. The mine is buried in 2 km of ancient rock with the lowest natural background radiation, and cosmic rays do not penetrate that deep. The qubits in CUTE are expected to be tested for coherence at temperatures of 300 K, 50 K, 4 K, 1 K, 100 mK, and 10 mK, but at the time of writing, the results of these tests have not been obtained or are not publicly available.
Conclusion
So, at the beginning of the 21st century, quantum computing turns out to be a technology that, on the one hand, fundamentally surpasses the capabilities of classical computers and can lead to the complete obsolescence of Moore's law. On the other hand, the natural electromagnetic background and temperatures of even several kelvins still remain an almost insurmountable barrier, preventing qubits from being kept in a coherent state for at least one second. Nevertheless, prerequisites are already being formed for selecting locations where industrial quantum computers could be placed – apparently, they could work in places like Kamiokande and other neutrino detectors. Earlier, I wrote about such “reuse” of scientific megaprojects in the publication “Chance comes to the aid of those who tirelessly seek.” It would be interesting to read how suitable deep caves on other celestial bodies are for placing quantum computers, like those found on Mercuryor Martian lava tubes.
