Extremely accurate measurements of atomic states for quantum computing

A new method makes it possible to measure the quantum state of atomic "qubits" – the basic unit of information in quantum computers – with twenty times fewer errors than before without losing atoms. The precise measurement of qubit states, analogous to the one or zero state of bits in classical computing, is a crucial step in the development of quantum computers. An article describing the method by Penn State researchers appears on March 25, 2019 in the newspaper Physical Nature.

"We are working on the development of a quantum computer using a three-dimensional matrix of laser-cooled, qubit-quenched cesium atoms," said David Weiss, professor of physics at Penn State and head of the company. Research Team. "Because of the mechanics of quantum mechanics, atomic qubits can exist in a" superposition "of two states, which means that they can be, in a sense, simultaneously in both states. A quantum computation, it is necessary to perform a measurement on each atom.Each measurement finds each atom in only one of its two possible states.The relative probability of the two results depends on the state of superposition before the measurement. "

To measure the qubit states, the team first used lasers to cool and trap about 160 atoms in a three-dimensional lattice with the X, Y, and Z axes. Initially, the lasers capture all the atoms identically, regardless of their quantum state. The researchers then rotate the polarization of one of the laser beams that creates the X-array, spatially displacing the atoms of one state to the left and those of the other state to the right. If an atom begins with a superposition of the two states of qubits, it ends up superimposed to have moved left and right. They then switch to an X network with smaller network spacing, which tightly traps the atoms in their new superposition of shifted positions. When the light is then dispersed from each atom to observe where it is, each atom is either moved to the left, or moved to the right, with a probability that depends on its initial state. Measuring the position of each atom is equivalent to measuring the initial qubit state of each atom.

"The mapping of internal states to spatial locations greatly contributes to making it an ideal measure," Weiss said. "Another advantage of our approach is that measurements do not cause the loss of any of the atoms we measure, which is a limiting factor in many previous methods."

The team determined the accuracy of their new method by loading their atomic arrays into one or other of the qubit states and performing the measurement. They were able to accurately measure the states of atoms with a fidelity of 0.9994, which means that there were only six errors out of 10,000 measurements, or twenty times more than the previous methods. In addition, the error rate was not affected by the number of qubits that the team measured in each experiment and, as there was no loss of money. Atoms could be reused in a quantum computer to perform the next calculation.

"Our method is similar to the Stern-Gerlach experiment of 1922 – an experience that is integral to the history of quantum physics," Weiss said. "In the experiment, a beam of silver atoms crossed a magnetic field gradient whose north poles were aligned perpendicular to the gradient.When Stern and Gerlach saw half of the atoms fall and fall back to half, In our experience, we also mapped the internal quantum states of atoms on positions, but we can do it atom by atom, of course, we do not need to test this aspect of quantum mechanics., we can just use it. "

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