Research at the Quantum Systems Accelerator has been steadily breaking new ground, quickening the pace toward flexible, stable quantum computers with capabilities well beyond those of today’s classical machines. While the principles underlying quantum systems have been understood for decades, building machines that leverage those ideas requires precision engineering. At this scale, many of the same properties that give quantum computing its unprecedented power also create unique challenges to harnessing it.
Trapped-ion systems have become one of the most established platforms for advancing quantum technology. These systems use electric fields to trap and move ions in a quantum processor, as well as lasers to manipulate their atomic and motional quantum states. This architecture allows for long chains of interconnected qubits that remain in a state of quantum coherence for long periods of time, providing some of the most promising capabilities in quantum research. Recent research at QSA has advanced the capabilities of trapped ion designs to enable holding larger numbers of qubits.
Enchilada Architecture
A group of QSA researchers at Sandia National Laboratories led by Jonathan Sterk, designed, fabricated, and performed preliminary tests on a trap chip capable of storing up to 200 ions. This device, called the “enchilada trap,” incorporated novel features to reduce radiofrequency (RF) power dissipation and multiple operational zones connected via junctions. These features can be used in future traps that may need to store orders-of-magnitude more qubits, as shown in a study published on the arXiv. This work is done in collaboration with other QSA researchers at Duke University and Cornell University, where an enchilada trap has been delivered and is currently in operation.
By raising the RF electrodes and removing insulating dielectric material below them, the design reduces capacitance and subsequently lowers power losses on the device when the necessary 150 V to 300 V are applied. Combined, these innovations allow for scaled-up quantum computers that sidestep power dissipation issues, which can limit the size and complexity of the trap. This enables larger traps, which can be exponentially more powerful than smaller ones. This work is done in collaboration with other QSA researchers at Duke University and Cornell University, where an enchilada trap has been delivered and is currently in operation.
Parallel Power
While trapped ion arrangements offer robust control and long coherence times, one bottleneck is that their physical gate operations are performed sequentially. A QSA team at the University of Maryland, led by Yingyue Zhu, tackled this challenge by performing parallel gate operations in a trapped ion system, as explained in their paper, published in Advanced Quantum Technologies. Previous setups had faced the issue that parallel qubit operations interfered with each other, because all gates were using the same set of motional modes. Zhu et. al. were able to solve this issue by simultaneously controlling qubits along different directions in space, and hence different vibrational patterns, allowing them to be operated on concurrently without interference and with minimal overhead.
This is a key innovation for scaling quantum computing operations because it allows for higher information throughput. This not only improves speed and processing power, with potential for more complex operations, but can also improve stability. Quantum operations are time-sensitive, due to decoherence, in which qubits lose their quantum properties due to unintended interactions with the surrounding environment. By performing more operations in a given timespan, Zhu’s team’s progress could produce more reliable systems.
Squeezing and Scaling
Working on multiple entangled pairs is one method by which quantum processors can scale up. Another method is to entangle more than one ion at once. A QSA group at Duke University, led by Or Katz from Chris Monroe’s team and collaborating with Marko Cetina’s group, adopted this strategy, as detailed in their research published in Physical Review Letters, PRX Quantum, and Nature Physics.
Katz developed a technique in which specific qubits can be controlled and paired efficiently using precise laser pulses. A key innovation in this work was to entangle many qubits in a single group, using a method known as “squeezing.” This tactic alters the scale of ions’ motion or position in a spin-dependent manner, while conceding greater uncertainty in the complementary variable (the product of the uncertainties must remain constant, according to the Heisenberg Uncertainty Principle).
Squeezing allowed the team to entangle the spins of many ions at once, rather than the typical entangled pairs of other systems. By interacting with the ions in a single step, this new technique enabled efficient generation of quantum entangling operations whose structure, using standard pairwise techniques, is rather challenging. This technique opens new avenues for quantum information applications.
Measuring Quantum Advantage
Measuring the performance of quantum machines has typically been done at the end step of their operations. However, mid-circuit measurements, while more challenging in some respects, present unique opportunities to measure a system’s efficacy and control it interactively.
That was the approach taken by Daiwei Zhu et. al. at the University of Maryland’s QSA research group, in a study published in Nature Physics. They spatially separated certain ions to facilitate mid-circuit measurements. Mid-circuit measurements are challenging in many quantum computing architectures because if qubits are not properly isolated, measuring one can unintentionally affect nearby qubits, potentially disrupting the rest of the computations. To overcome this challenge, the group separated certain segments of a chain of ions using precise voltage adjustment. Once isolated, these ions can be shuttled away from the others in their chain to be measured without interfering with the others. This delicate operation allows for measurements and tests more typical of classical systems.
With this procedure in place, the researchers implemented two interactive protocols that provide classically verifiable evidence of quantum advantage: one based on the Learning With Errors (LWE) problem and the other based on a Computational Bell Test. The two problems define tasks that are hard for classical computers to solve alone, but become verifiable when a classical verifier interacts with the quantum computer during the computation via mid-circuit measurements. Zhu’s team used it to show that the quantum computer is indeed acting in a quantum way.
While tests of quantum activity are typically physical in nature, this experiment demonstrated it computationally for the first time, or, as the authors put it, this is the first instance of quantifying quantumness.
Mid-circuit measurements have clear utility in understanding and debugging quantum architectures. They could also be instrumental in making quantum operations more efficient, allowing researchers to perform computations using fewer resources.
The work being developed to make quantum computers more efficient, scalable, reliable, and interactive is accelerating us toward the day when once-intractable problems become solvable. The work at QSA is making some of the most important breakthroughs in advancing this futuristic technology. With each engineering and technical breakthrough, we move closer to a new era of computing.