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In a swap gate, neighboring qubit states (blue and beige) are exchanged. The qubits are made of cold atoms trapped inside an artificial crystal created by laser light.(Image credit: Mika Blackmore-Esslinger / ETH Zurich)Share this article 0Join the conversationFollow usAdd us as a preferred source on GoogleSubscribe to our newsletter
Scientists have developed a novel “quantum operation” exhibiting significantly enhanced stability compared to prior methodologies. This breakthrough propels one specific hardware configuration — neutral-atom qubits — closer to enabling practical quantum computers.
Quantum computers employ qubits, which can embody a state of 0, 1, or a combination of both simultaneously. Essential for their computational prowess are “gates,” mechanisms capable of transitioning qubits between states, facilitating parallel calculations. A crucial gate type, the swap gate, enables information routing by exchanging the states of two qubits.
Many quantum systems depend on highly energized electronic states or atomic interactions, as well as the quantum tunneling phenomenon, where particles traverse barriers insurmountable in classical physics. However, swap gates utilizing these approaches (especially tunneling) are contingent on the speed and intensity of lasers — which confine neutral atoms to form qubits.
Consequently, minor deviations in laser timing or power can introduce inaccuracies and diminish system fidelity, rendering a gate unreliable.
This exacerbates the primary obstacle to scaling quantum computing for superior performance over the fastest supercomputers: qubits are highly prone to errors and decoherence during computations. Their error rate is approximately 1 in 1,000, contrasting with 1 in a trillion for conventional bits.
To address this challenge, researchers at ETH Zurich have engineered a method to dramatically improve the stability of qubits within neutral-atom quantum computers. Their findings were detailed in a study published on April 8 in the journal Nature.
Opening the gateway to more stable quantum computers
Instead of relying on conventional gates, the team leveraged a more subtle physical phenomenon known as a geometric phase. Unlike other techniques for implementing quantum gates for neutral atoms or trapped particles, which are sensitive to the speed and force applied to atoms, their swap gate utilizes the trajectory atoms follow within an artificial “crystal of light” formed by intersecting laser beams (termed an optical lattice).
Neutral-atom platforms offer the potential for thousands of qubits in a single device. This particular setup employs tens of thousands of potassium atoms cooled to near absolute zero and held in position by laser light. Yann Hendrick Kiefer, a postdoctoral researcher at the ETH Zürich Institute for Quantum Electronics and lead author of the study, elaborated on its mechanism to Live Science.
“Laser light is essentially monochromatic electromagnetic radiation,” Kiefer explained via email. “When a neutral atom is placed within this electric field, a dipole moment is induced, generating a force that allows us to anchor atoms in place.”
When two such potassium atoms approach each other closely enough for their quantum wave functions to overlap, their combined state undergoes a transformation dependent solely on the geometry of their movement, rather than their velocity or laser intensity. This renders the swap operation considerably less susceptible to experimental noise.
“Quantum mechanics is characterized by wave functions,” Kiefer stated. “The manipulation of these wave functions typically introduces a phase, which can stem from either dynamical or geometric origins.”
“Practical-scale quantum computing still necessitates considerable advancements.”
Yann Hendrick Kiefer, postdoctoral researcher at the ETH Zürich Institute for Quantum Electronics
Dynamical quantum methods generate this phase by exerting highly precise control over parameters such as energy levels, timing, and laser intensity, meaning even minor errors can lead to inaccuracies. The geometric approach operates differently: rather than depending on exact timing or force, it relies predominantly on the overall path the system traverses from its initial to its final state. Consequently, it is inherently less vulnerable to external disturbances or minor imperfections, resulting in more stable and dependable quantum operations.
Building machines that will need far fewer qubits than we thought
Employing this methodology, the research group achieved a highly robust swap gate with an accuracy exceeding 99.91%, executing within a millisecond (one-thousandth of a second) across a system comprising an impressive 17,000 qubit pairs. While certain superconducting or trapped-ion gates can operate in sub-microseconds (one-millionth of a second), these systems typically perform such gates on only a limited number of qubit pairs concurrently.
The team also demonstrated their capability to generate “half-swap” gates, which are indispensable for executing actual quantum algorithms. Half-swap gates — a quantum operation that partially exchanges two qubits rather than completing the exchange — are crucial because entanglement is the distinguishing feature of quantum computing. A full swap primarily redistributes information, whereas a half-swap can both partially transfer information and establish correlations between qubits that are unattainable with classical bits. The researchers aspire to eventually integrate these resilient swaps with a quantum gas microscope — capable of imaging and targeting individual atom pairs — to construct a more adaptable and programmable quantum computing architecture.
Nevertheless, Kiefer acknowledges that a functional quantum computer remains a distant prospect. “Practical-scale quantum computing still necessitates considerable advancements,” he commented. “The most significant limiting factors are twofold: scale and fidelity.”
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However, Kiefer maintains an optimistic outlook. He referenced a recent investigation exploring the potential to solve complex problems like Shor’s algorithm with a system requiring as few as 10,000 qubits, a significant reduction from the millions previously estimated as necessary.
Shor’s algorithm is a quantum procedure adept at rapidly deciphering specific types of contemporary encryption by efficiently identifying the prime-number factors of large numbers, a task beyond the capacity of classical computers. It continues to serve as a widely recognized benchmark in quantum computing research.
“There remains substantial work ahead before Shor’s algorithm can be practically solved,” Kiefer remarked, “but we are entering an era where the vision of quantum computing may gradually transition into tangible reality — these are exciting times!”
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