Trapped ions transported by a single laser beam

Transport-Enabled Entangling Gate for Trapped Ions

Scientists at the Georgia Tech Research Institute (GTRI) have demonstrated the feasibility of a new approach that moves trapped ion pairs through a single laser beam, potentially reducing power requirements and simplifying the system.

The researchers described implementing two-qubit entangling gates by moving calcium ions held in a surface electrode trap through a stationary bichromatic optical beam. Maintaining a constant Doppler shift during the ion movement required precise control of the timing.

Measurements showed that the entangled quantum state of the two qubits transported through the optical beam had a fidelity comparable to entangled states produced by stationary gates performed in the same trapping system. The experiment used an optical qubit transition between an electronic ground state and a metastable state of 40Ca+ ions within a surface trap, a setup which allowed both one-qubit and two-qubit gates to be performed using a single beam.

The researchers moved the pair of trapped ions by precisely varying the electrical confinement fields in the trap by controlling the voltages applied to adjacent electrodes. The ions themselves have an electrical charge, a property which makes them subject to the changing electrical fields around them.

Transport operations are used in most ion trap experiments to enable loading, individual detection, and individual addressing. Advances in trap design and electrical potential control have led to improvements in activities such as fast shuttling, fast ion separation, optical phase control, junction transport, and ion chain rotation.

Trapped ions are among the potential platforms being studied for quantum information systems. Other options, such as superconducting qubits, are physically attached to a substrate and would not be amenable to the transport approach used by the GTRI researchers. Quantum computing techniques could help accelerate the discovery of new pharmaceuticals and create advances in materials engineering.

Gating ions via transport had been proposed theoretically a number of years ago, and another experimental group has already created interactions by moving single ions through a stationary beam. The GTRI study is believed to be the first to create a transport-enabled entangling gate with two trapped ions. In their experiment, the GTRI researchers used two tones of red light at slightly different frequencies.

Moving the ions into a single beam has at least three potential advantages. For one, if a single beam can be reflected back and forth across a trap, that one beam could interact with many ions, reducing the need for multiple beams and the power – and control complexity – they require.

Another advantage is that the intensity of the interaction can be controlled by the movement of ions through the beam rather than by adjusting the laser pulses. And because the beam intensity smoothly rises and falls as the ions move through different portions of it, problems of off-resonant coupling can be reduced.

But there are also disadvantages. Because the ions move through the beam, they don’t remain in the most intense portion of it for long, but are exposed to power that ramps up and down as they move. That means a more intense beam must be used to provide a specific amount of power to the ions.

Possible next steps could include extending the transport gate technique to longer ion strings with different transport modes and different ion species. The researchers would also like to use a different laser beam configuration that might further reduce the small error rate they saw in their experiments. (SciTechDaily)

The paper has been published in the journal Physical Review Letters.

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