Trapped ion quantum technology gets smaller

Scaling down for scaling up: Team members Robert Niffenegger (left) and Christopher Caron in the trapped ion quantum computing and photonics lab at UMass Amherst. (Courtesy: Derrick Zellmann for UMass Amherst) A new integrated photonics platform can perform precision quantum experiments that were previously only possible with multiple table-top lasers and other bulky apparatus. According to its US-based developers, the new chip-scale device could find applications in quantum computing and portable optical clocks based on trapped ions. Today’s quantum computers and optical clocks depend on a range of equipment that typically includes some combination of lasers, cryogenic coolers, vacuum chambers and optical reference cavities. The last of these can take up more than half the device’s total volume, and they are crucial for stabilizing laser frequencies to the high precision required for controlling the quantum states of trapped ions. Such ions can serve as quantum bits (qubits) in quantum computing and can also be used for precision timekeeping in optical clocks. In the latter case, each clock “tick” is defined by the frequency of the light the ions absorb and emit as they undergo a specific, sub-Hz transition (the so-called “clock transition”) between atomic energy levels. Miniaturizing large laser systems Researchers led by Daniel Blumenthal of the University of California Santa Barbara (UCSB) and Robert Niffenegger at the University of Massachusetts Amherst have now shown for the first time that these large, stabilized laser systems can be replaced with small photonic chips. They used these chips to prepare and control the quantum state of strontium ions at room temperature as well as driving the clock transition. Though the fidelity of the system is not yet high enough to compete with the best traditionally-constructed devices, Niffenegger describes it as a critical first step for producing next-generation clocks and future quantum computers with millions of qubits. “Reaching such a goal will only be possible with such integrated quantum systems on a chip,” he explains. Blumenthal, Niffenegger and colleagues used two components to create their chip-based stabilized laser: an integrated Brillouin laser with a wavelength of 674 nm, connected to an integrated 674 nm, 3 m long coil resonator cavity.

The team characterized the stability of this laser and coil by measuring the 0.4 Hz quadrupole optical clock transition in strontium-88 (88Sr+) ions trapped at an electrode located on a single surface electrode trap (SET) chip. This transition is one of the most precise used by quantum researchers today, and its narrow linewidth makes it relatively easy to measure using high-resolution trapped ion spectroscopy. “The fact that these results were achieved with the SET at room temperature is remarkable given the precision of the transition, and is a major step forward in realizing portable versions of this quantum technology,” Blumenthal says. Making optical clocks more portable and robust As well as being smaller than traditional lasers, the chip’s 674-nm Brillouin laser light also removes the need for bulky frequency conversion equipment. A further advantage is its reduced high-frequency noise, which is important for clock acquisition and qubit state preparation fidelity, and which cannot be achieved using standard electronic feedback loops. The coil, for its part, reduces mid- and low-frequency noise, stabilizing the laser’s carrier frequency even further so that it can be locked to the precision sub-Hz trapped-ion clock transition. According to Niffenegger, this combination of improvements enabled the team to achieve a frequency noise profile and so-called Allen deviation (a measure of stability) of just of 8.8 × 10–13 – an unprecedented figure for a room-temperature chip. “We can therefore prepare qubit states with high fidelity and interrogate the clock transition, which is essential for quantum computing applications,” he says. Ion-clock transition could benefit quantum computing and nuclear physics Read more As optical clocks become more portable and robust, they become more feasible for a greater variety of applications. The ultimate goal, says Blumenthal, is to reach a stability range of 10-14 to 10-16, which would allow optical clocks to replace GPS-based navigation on missions to the Moon and Mars. “Such clocks could also help advance fundamental science – for example, by mapping gravity and measuring orbit time around Earth for climate science, detecting gravitational waves and dark matter/energy and for general relativity measurements, to name just a few,” he explains. To achieve this improvement in stability, Niffenegger says it would be necessary to scale the team’s integrated platform to a grid of 100 or more ions. He and his colleagues are now working to integrate other experimental components (including the ion trap chip, the optical cavity chip and other photonics) onto a single, full-architecture chip that builds on their current designs. “Preliminary results already show improved performance, with further exciting developments anticipated soon,” they tell Physics World. The present work is detailed in Nature Communications. Want to read more? Registration is free, quick and easy Note: The verification e-mail to complete your account registration should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder. If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org. E-mail Address Register