Scientists at the Technion and the University of Bonn present the factors that dictate the maximum speed of quantum computations. This is through an experiment that uses quantum "matter waves".
Researchers from the Technion and the University of Bonn conducted an experiment that answers the following question: What limits the calculation speed of quantum computers? The results of the study were published in the journal Science Advances.
Quantum computers are very sophisticated devices whose operation is based on quantum mechanics, which allows them to process problems that the classical computer cannot handle at all; But even quantum computing is limited in the amount of information it can process in a given time. The information stored in classical computers can be likened to a long sequence of 0's and 1's, the classical bits of information, which constitute the basic calculation elements.
Quantum computers work differently: information is stored in quantum bits, or qubits, which are more like a wave than discrete values. A qubit is not 0 or 1 but a combination of them - a superposition. When physicists talk about the information stored in qubits they are talking about wave functions. This pattern of operation allows the quantum computer to perform many calculations at the same time, which of course speeds up the performance of the system. And yet, even the processing of information in the quantum computer has a speed limit - the quantum speed limit (QSL). What is this limit? This is what the research conducted at the Technion and the University of Bonn dealt with.
Already in the middle of the last century, the Soviet physicists Leonid Mendelstam and Igor Tam, based on theoretical calculations, deduced the speed limit of calculation in a complex quantum system; The present study confirms their theoretical prediction but also adds new findings. According to Dr. Andrea Alberti, who led the research at the Institute of Applied Physics at the University of Bonn, "We allowed individual cesium atoms to move in a controlled manner like marbles in a bowl of light and followed their behavior."
From the quantum point of view, atoms are described as waves of matter. On its journey to the bottom of the bowl of light, the material wave undergoes a change that affects the quantum information it carries. The research group wanted to find out what is the early stage in which a distinct change occurs, that is, a change that can be identified, since this point in time could serve as an experimental view of the Mendelstam-Tam border. If these were ordinary marbles rolling on the side of a real bowl, everything would be simple - according to the position of the marble at given time intervals, it would be possible to reconstruct the moment when it set off; However, in the quantum world, things work differently, since with each measurement of the position of the atom we will get a slightly different result, and this is not an error in the measurement but a result of the properties of quantum mechanics. "That's why," explains Dr. Alberti, "instead of trying to determine the location and shape of the material wave, we developed a new method that directly determines the change we are looking for - the deviation of the quantum particle from its initial state."
"For that," explains Gal Ness, the lead author of the article and a doctoral student under the guidance of Prof. Yoav Sagi in the Technion's Faculty of Physics, "we created a copy of the material wave of the atom so that we actually had two copies of the same state. Using fast flashes of light we achieved for the first time a quantum superposition of the two copies." When one of the two copies of the material waves slides down the light bowl, the other material wave is already at the bottom of its light bowl, so it stays in place and can no longer move or change. At a certain point, the researchers allow the two identical copies to merge with each other and then, by comparing their quantum states at certain points in time, they were able to extract the requested speed limit - the minimum duration of time for a distinct change in the material wave.
By changing the initial height from which the atom descends in the bowl of light, the physicists were able to control not only the average energy but also the energy uncertainty of the atom.
Recall that Heisenberg's uncertainty principle states that it is impossible to determine at the same time the position of a particle and its momentum (from which its velocity is derived); Energy uncertainty means that it is impossible to simultaneously determine the time difference between two quantum events and the energy differences between them. According to Prof. Sagi, "In the present study we were able to show that, as Mendelstam and Tam predicted, the minimum time for the occurrence of that distinct change in the material wave depends on the level of energy uncertainty of the atom; This time shortens as the energy uncertainty increases."
The study confirmed another phenomenon theoretically predicted by Norman Margulos and Lev Levitin in 1998: from the moment the energy uncertainty increases until it exceeds the average energy of the material wave, it is precisely the average energy that dictates the speed limit of the system. According to Dr. Alberti, "this is the first time that both speed limits have been measured in a complex quantum system - and in the same experiment."
The researchers conclude that quantum computers will indeed be able to solve problems at an unprecedented speed, but this speed will also be limited; Now we can know exactly how much it is limited, based on the two limits verified in the current study. The research has implications in many future fields including bosonic quantum computing, quantum simulations and atomtronics (electronics based on quantum gas atoms).
The research was funded by the Reinhard Frank Foundation in cooperation with the Friends of the Technion Association in Germany, the German Research Foundation (DFG), the Helen Diller Quantum Center at the Technion and the German Academic Exchange Service (DAAD).
for the article in the journal Science Advances
More of the topic in Hayadan:
3 תגובות
As long as there is a Planck size limit and the speed of light, the speed of motion will always be limited.
The question of how quantum computation is measured against the biological... that would be interesting
So in the end what is the speed limit?
Does blockchain and all cryptocurrencies have a right to exist if quantum computers become available?