Not so fast! Quantum computers have a speed limit

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 your laptop or smartphone, some 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 limit 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 realm in which the Mandelstam-Tamm speed limit 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.  This is why the atoms, or matter waves as the computer interprets them, were put into superposition at the beginning of the experiment to see how they would change.

"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 that is preserved in time, the wavefunciton continually evolves, 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 his 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," he 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 wave function at some later time t. With quantum interference, we probed the difference between those two clones."

Because a particle’s energy can never be exactly found out, it is always seen as average. Just as the Mandelstam-Tamm limit predicted, the fastest a qubit could possibly be processed depended on the energy uncertainty, with higher energy uncertainties leading to faster speed limits. But. There is always a “but” in quantum physics. If the energy uncertainty went high enough that it ran into the average energy of the atom, that was it. Things stopped accelerating and the speed limit stayed at the average energy. Even quantum computers aren’t infinitely fast.

This still doesn’t negate the fact that quantum computers can process things at what seems like warp speed to us, especially compared to the gadgets we use now. We are still a long way from holding quantum smartphones in our hands. However, weird physics might someday make it happen.