NIST team manipulates qubits

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Physicists at the National Institute of Standards and Technology (NIST) say they have devised a viable way to manipulate a single bit in a quantum processor without disturbing the information stored in its neighbours. The approach, which makes use of polarised light to create effective magnetic fields, could bring quantum computing a step closer.

The research team says one of the big challenges in creating a working quantum computer is maintaining control over the 'switches' in a quantum processor, while isolating them from the environment. One approach to quantum computer development aims to use a single isolated rubidium atom as a quantum bit, or qubit. Each rubidium atom can take on any of eight energy states, so the design goal is to choose two of these energy states to represent the 'on' and 'off' positions. Ideally, say the researchers, these two states should be completely insensitive to stray magnetic fields that can destroy the qubit's ability to be simultaneously on and off. However, choosing such states is said to make the qubits less sensitive to magnetic fields used intentionally to select and manipulate them. "It's a bit of a Catch 22," said NIST scientist Nathan Lundblad. "The more sensitive to individual control you make the qubits, the more difficult it becomes to make them work properly." To solve the problem, the NIST team has used two pairs of energy states within the same atom, with each pair suited to a different task. One pair is used as a 'memory' qubit for storing information, while the second 'working' pair comprises a qubit to be used for computation. While each pair of states is field insensitive, transitions between the memory and working states are sensitive and amenable to field control. When a memory qubit needs to perform a computation, a magnetic field can make it change without disturbing nearby memory qubits. The NIST team demonstrated this approach in an array of atoms grouped into pairs, using the technique to address one member of each pair individually. Grouping the atoms into pairs, said Lundblad, allows the problem to be simplified from selecting one qubit from many to selecting one of two. This is achieved by creating an effective magnetic field using a beam of polarised light. "If a working quantum computer is to be built," Lundblad continued, "these problems need to be addressed and we think we've made a good case for how to do it."