According to the latest research from technion, quantum computers do have speed limits.
Compared with traditional computers, the computing speed of quantum computers can be increased exponentially, but quantum computers are subject to speed limitations, theoretically, more than one.
Recently, the Technion team tried to break through the boundaries of quantum physics and propose and prove the speed limit of quantum computers. The findings were published in Science Advances.
Unlike laptops or smartphones, some quantum computers process atoms as waves of matter, and their speed limits depend on the speed at which information is converted in these waves of matter.
According to the paper, quantum mechanics sets a fundamental limit on how quickly a quantum state changes over time. Two well-known theories of quantum velocity limits are the MT Bound proposed by Mandelstein and Tam and the Margoras-Levitin Law (ML Bound). Using interferometry of fast material waves, the research team tracked the motion of individual atoms in a light trap while testing these two velocity limits in a plurionic system.
"We know that there are two recognized speed limits for the development of quantum states." Gal Ness, a researcher at the Technion-Israel Institute of Technology, said, "Mandelstein and Tamm argue that quantum states develop at a slower rate than their reciprocal (multiplied by certain constants) of energy uncertainty. The other limit (ML Bound) relates the maximum speed of quantum state development to the average energy itself. ”
To understand why quantum computers have speed limits, it is necessary to understand the field in which speed limit theory applies. Quantum computers do not run binary systems of 0 and 1, i.e. bits, but instead operate using qubits, or qubits.
In quantum physics, atoms are seen as fluctuations in matter. The bit value of the bit can only be 0 or 1. As a basic unit of information, qubits can exist in two possible states at the same time, 0 and 1.
Qubits can be any type of particle, and Technion used cesium atoms in this experiment because the way cesium atoms move is controllable. The researchers had cesium atoms roll down the side of a thin bowl and observe their movements. As a qubit moves, its quantum information is constantly changing. Determining how fast a quantum computer can calculate means finding the first point where information begins to change in atoms. That's why at the beginning of the experiment, it's necessary to put atomic or material waves into a superposition state to see how they change.
"Superposition means that when a conventional bit has a value of 0 or 1, each qubit can be both 0 and 1 at the same time." "Unlike traditional storage, which is kept in time, the wave function (the amplitude of a material wave) is constantly changing, so it has a fixed time measure," Ness said. This fixed time period is called the 'phase' of the qubit. ”
To create atoms that exist in a quantum superposition state or in both states, the researchers need to clone them. They cloned with very fast pulses of light, as if the same atom could roll and stand still at the same time. Because one state of the atom remains stationary, the waves of matter do not change. Clones are compared using quantum interference, which is the additive effect of the interference of the matter wave itself, so that the difference between the two material waves can be pinpointed. The research team needed to figure out the speed limit of the quantum, so they created clones of two wave functions, so that one can continue to change, while the other serves as a reference, remaining stationary in time.
"Interference is a way to use the wave-like nature of a system to highlight the differences between waves." Ness describes, "In order to probe the speed limit of a quantum, we need to have precise numbers that overlap between the initial wave function and the changed wave function at a certain time. Through quantum interference, we explored the difference between these two clones. ”
The limit of quantum speed in a multilevel quantum system
The team found that the velocity limits of Mandelstein and Tam always limit the speed at which quantum states develop, and that the crossing of the two speed limits occurs after a longer period of time.
Because a particle's energy can never be accurately discovered, it always averages. As Predicted by Mandelstein and Tam's speed limits, the fastest speed a qubit can process depends on the uncertainty of its energy, and higher energy uncertainty will cause the speed limit to arrive faster. But in quantum physics, if the uncertainty of energy is high enough to reach the average energy of an atom, then matter stops accelerating, and the speed limit remains at average energy. So even a quantum computer is not infinitely fast. These research results are of great significance for understanding the final performance of quantum computers and related quantum technologies.
But this still does not negate the fact that quantum computers are still super fast compared to the electronic devices we use today. There is still a long way to go before the quantum smartphone really arrives.