New approach transports trapped ions to create entangling gates

Date:
January 28, 2022
Source:
Georgia Institute of Technology
Summary:
Scientists 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 for creating
entangled qubits.



FULL STORY ========================================================================== Trapped ions excited with a laser beam can be used to create entangled
qubits in quantum information systems, but addressing several stationary
pairs of ions in a trap requires multiple optical switches and complex controls. Now, 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.


==========================================================================
In a paper scheduled to be published January 31 in the journal Physical
Review Letters, the researchers describe 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.

"We've shown that ion transport is an interesting tool that can be
applied in unique ways to produce an entangled state using fine control
over the ion transport," said Holly Tinkey, a GTRI research scientist
who led the study.

"Most ion trap experiments have some control over the motion of the ions,
so what we have shown is that we can potentially integrate that existing transport into quantum logic operations." 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.

"We perform some interactions where the ions are trapped together in a
single potential well and where they are very close and can interact,
but then we sometimes want to separate them to do something distinct to
one ion that we don't want to do to the other ion," Tinkey explained.



========================================================================== 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.

"This really opens up the possibility of sharing the light among multiple
sites within a larger structure, without having to have an optical switch
for every pair of ions," said Kenton Brown, a GTRI senior research
scientist who collaborated on the project. "This technique allows us
to literally move the ions physically out of the beam and only leave
those ions we want to gate in the beam." 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, Tinkey said.



==========================================================================
"It basically makes your curves flatter and easier to work with,"
she said.

"That means you could operate your gate at a larger range of de-tunings."
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.

Brown said that quantum researchers had been concerned that moving the
ions and using their motion to create two-qubit gates simultaneously
would create too many complicating factors that might make the whole
approach infeasible. "But it turns out that if you have enough control
of those two things, you can make it work," he added.

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.

========================================================================== Story Source: Materials provided by
Georgia_Institute_of_Technology. Original written by John Toon. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Holly N. Tinkey, Craig R. Clark, Brian C. Sawyer, Kenton R. Brown.

Transport-enabled entangling gate for trapped ions. Physical Review
Letters (accepted), 2022 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220128141324.htm

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