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Enter a quantum lab where scientists trap ions and you’ll find worktops filled with mirrors and lenses, all lasers that focus to hit an ion “trapped” in place on top of a chip. Using lasers to control ions, scientists have learned to exploit ions as quantum bits, or qubits, the basic unit of data in a quantum computer. But this laser setup is holding back research, making it difficult to experiment with more than a few ions and get these systems out of the lab for real use.
Now, researchers at MIT Lincoln Laboratory have developed a compact way to deliver laser light to trapped ions. In a recent paper published in Nature, the researchers describe a block of optical fibers that plugs into the ion trap chip, coupling the light to optical waveguides fabricated in the chip itself. Through these waveguides, multiple wavelengths of light can be routed through the chip and released to strike the ions above it.
“It is clear to many people in the field that the conventional approach, using free space optics such as mirrors and lenses, will only go far,” says Jeremy Sage, author of the paper and senior staff at Quantum Information and Integrated Group of Nanosystems. “If, on the other hand, the light is brought to the chip, it can be directed towards the many positions it needs to be. Integrated delivery of many wavelengths can lead to a very scalable and portable platform. We are demonstrating for the first time that it can be done “.
More colors
Calculation with trapped ions requires precise control of each ion independently. Free-space optics worked well when checking a few ions in a short one-dimensional chain. But hitting a single ion in a larger or two-dimensional cluster, without hitting its neighbors, is extremely difficult. When imagining a practical quantum computer that requires thousands of ions, this laser control task seems impractical.
That looming problem has led researchers to find another way. In 2016, researchers from Lincoln Laboratory and MIT demonstrated a new chip with built-in optics. They focused a red laser on the chip, where the waveguides on the chip directed the light to a grating coupler, a kind of noisy streak to stop the light and direct it towards the ion.
The red light is critical to performing a fundamental operation called a quantum gate, which the team performed in that first demonstration. But up to six differently colored lasers are needed to do everything necessary for quantum computation: prepare the ion, cool it, read its energy state, and perform quantum gates. With this latest chip, the team extended the proof of principle to the rest of these required wavelengths, from violet to near infrared.
“With these wavelengths, we were able to perform the fundamental set of operations required to control the trapped ions,” says John Chiaverini, also author of the paper. The only operation they did not perform, a two-qubit gate, was demonstrated by a team from ETH Zurich using a chip similar to the 2016 work and is described in an article in the same issue of Nature. “This work, coupled with ours, shows you have everything you need to start building larger trapped ion arrays,” adds Chiaverini.
Optic fiber
To go from one wavelength to multiple wavelengths, the team designed a method to tie a block of optical fiber directly to the side of the chip. The block consists of four optical fibers, each specific for a certain range of wavelengths. These fibers line up with a corresponding waveguide modeled directly on the chip.
“Aligning the matrix of fiber blocks with the waveguides on the chip and applying the epoxy was like performing surgery. It was a very delicate process. We had about half a micron of tolerance and had to survive a 4 kelvin recovery time, ”says Robert Niffenegger, who led the experiments and is the first author of the paper.
On top of the waveguides is a layer of glass. On top of the glass are metal electrodes, which produce electric fields that hold the ion in place; holes are cut into the metal above the grating couplers where the light is released. The entire device was manufactured in Lincoln Laboratory’s Microelectronics Laboratory.
Designing waveguides that can deliver light to ions with low loss, while avoiding absorption or scattering, has been a challenge, as loss tends to increase with bluer wavelengths. “It was a process of material development, waveguide modeling, testing, performance measurement and retry. We also had to make sure that the waveguide materials worked not only with the wavelengths of light needed, but also that they didn’t interfere with the metal electrodes that trap the ion, “says Sage.
Scalable and portable
The team can’t wait to find out what it can do with this fully integrated light chip. For one, “do more,” says Niffenegger. “Inserting these chips into an array could bring together many more ions, each capable of being precisely controlled, opening the door to more powerful quantum computers.”
Daniel Slichter, a physicist at the National Institute of Standards and Technology who was not involved in this research, says, “This easily scalable technology will enable complex systems with many laser beams for parallel operations, all automatically aligned and resistant to vibration and conditions. environmental, and in my opinion it will be crucial to make quantum processors of trapped ions with thousands of qubits. “
An advantage of this laser integrated chip is that it is inherently vibration resistant. With external lasers, any vibration at the laser would cause it to lose the ion, as would any vibration at the chip. Now that the laser beams and the chip are paired, the effects of the vibrations are effectively nullified.
This stability is important for the ions to sustain “coherence” or to operate as qubits long enough that they can be calculated with them. It is also important if the trapped ion sensors are to become portable. Atomic clocks, for example, which rely on trapped ions could keep time much more accurately than today’s standards and could be used to improve the accuracy of GPS, which is based on synchronizing atomic clocks carried on satellites.
“We see this work as an example of the link between science and engineering, which offers a real benefit to both academia and industry,” says Sage. Filling this gap is the goal of the MIT Center for Quantum Engineering, where Sage is a principal investigator. “We need quantum technology to be robust, achievable and easy to use for people who are not PhD students in quantum physics,” says Sage.
At the same time, the team hopes this device can help propel academic research. “We want other research institutes to use this platform so that they can focus on other challenges, such as programming and running ion-trapped algorithms on this platform, for example. We see it opens the door to further explorations of quantum physics, “Chiaverini says.
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