Quantum computers are fast. Ridiculously fast. Almost unfathomably fast. So can you actually believe they have a speed limit?
They apparently do. This was previously theorized and finally proven by an international team of physicists who wanted to push the boundaries of quantum physics and see just how fast a quantum computer could process information before it had to put on the brakes. Unlike the your laptop or smartphone, quantum computers process atoms as waves of matter, and the speed limit was determined by how fast the information in these waves transformed.
“We know of two canonical speed limits for quantum evolution,” researcher Gal Ness of Technion (Israel Institute of Technology), who coauthored a study recently published in Science Advances, told SYFY WIRE. “The Mandelstam–Tamm bound says that the state can only evolve slower than the inverse of its energy uncertainty, times some constants. The other relates the maximal speed to the mean energy itself.”
To get why quantum computers have to have a speed limit, you need to understand the Mandelstam-Tamm speed limit that is applied. Quantum computers don’t process bits in the form of endless ones and zeroes that keep appearing in The Matrix. These super-machines instead use qubits, or quantum bits. Atoms are seen as waves of matter in quantum physics. Bits can only have a value of zero or one. Qubits are basic units of information involving something that can exist in two possible states at once. Can you imagine Neo dealing with that?
Qubits can be any type of particle, though cesium atoms, which are known for their controlled way over movement, were used as in this experiment. The researchers let them roll down the sides of a light bowl to observe their movements. As a qubit moves, its quantum information is constantly changing, and determining just how fast a quantum computer can figure something out meant finding the earliest point at which information started to change within the atoms. The only issue was that atoms unpredictably change matter waves. Enter superposition.
“Superposition means that, while a classical bit has either a 0 or 1 value, each qubit can be both 0 and 1 at once,” said Ness. “In contrast to classical memory, the wavefunction continuously evolves since it is preserved in time, so it features an inherent measure of time. This intrinsic tick of time is known as the qubit’s ‘phase’”.
To create atoms that would exist in a state of quantum superposition, or in two states at once, the researchers had to clone them. They used incredibly fast pulses of light to do this. It was as if the same atom was both rolling and sitting at the at the edge of the bowl at the same time. Because one of the atom’s states manages to stay still, the matter wave will not change. Clones were compared using quantum interference, a side effect of superposition in which the wave interferes with itself. This allows precise detection of any inconsistencies in waves. Ness and her team needed this to find out the quantum speed limit. This is why they created two clones of the wave function so one could keep evolving while the other (meant to be a reference) remained frozen in time.
“Interference is a way to leverage the wavy character of a system to highlight differences between waves,” she said. “To probe the quantum speed limit, it is required to have a precise figure of the overlap between the initial wave function at time 0, and the evolved …….
Source: https://www.syfy.com/syfy-wire/quantum-computers-have-not-one-but-two-speed-limits