Extremely accurate measurements of atom states for quantum computation – ScienceDaily



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A new method allows the quantum state of atomic qubits – the basic unit of information in quantum computers – to be measured with an error twenty times smaller than previously possible without losing atoms. Precisely measuring qubit states, which are analogous to zero states or one of the bits of traditional computing, is a vital step in the development of quantum computers. An article describing the method by Penn State researchers appears on March 25, 2019 in the journal Physics of Nature.

"We are working to develop a quantum computer that uses a three-dimensional array of cesium atoms laser-cooled and trapped as qubits," said David Weiss, a professor of physics at Penn State and a leader in the research team. "Because of how quantum mechanics works, atomic qubits can exist in a two-state superposition, which means that they can be, in a sense, in both states simultaneously. The relative probability of the two results depends on the state of superimposition before the measurement. "The result of a quantum computation is required to perform a measurement on each atom.

To measure qubit states, the team first uses lasers to cool and trap about 160 atoms in a three-dimensional network with X, Y, and Z axes. Initially, lasers intercept all atoms identically regardless of their quantum state. The researchers then spin the polarization of one of the laser beams that creates the X-network, which spatially shifts atoms in a qubit state to the left and atoms in the other qubit state to the right. If an atom starts in a superposition of the two qubit states, it ends up in a superposition of having moved left and moving right. They then shift to an X network with a smaller lattice spacing, which holds the atoms in their new overlapping of displaced positions. When the light is then scattered from each atom to see where it is, each atom is found shifted to the left or shifted to the right, with a probability that depends on its initial state. The measurement of the position of each atom is equivalent to a measure of the initial state of the qubit of each atom.

"Mapping the internal states into space locations is a big step in making that measurement ideal," Weiss said. "Another advantage of our approach is that the measurements do not cause the loss of any of the atoms we are measuring, which is a limiting factor in many previous methods."

The team determined the accuracy of their new method by carrying their networks with atoms in one or another qubit state and performing the measurement. They were able to accurately measure atom states with a fidelity of 0.9994, meaning that there were only six errors in 10,000 measurements, a 20-fold improvement over previous methods. In addition, the error rate was not affected by the number of qubits the team measured in each experiment, and since there were no atom losses, the atoms could be reused on a quantum computer to perform the next calculation.

"Our method is similar to the Stern-Gerlach experiment of 1922 – an experience that is an integral part of the history of quantum physics," said Weiss. "In the experiment, a bundle of silver atoms was passed through a magnetic field gradient with its north poles aligned perpendicular to the gradient. When Stern and Gerlach saw half of the deflected atoms up and down, he confirmed the idea of ​​quantum superposition. of the aspects that define quantum mechanics. In our experiment, we also mapped the inner quantum states of atoms into positions, but we can do so on an atom-by-atom basis. Of course, we do not need to test this aspect of quantum mechanics, we can use it . "

In addition to Weiss, the Penn State research team includes Tsung-Yao Wu, Aishwarya Kumar and Felipe Giraldo. The research was supported by the US National Science Foundation.

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