A micromechanical qubit might last a full second

Researchers at the University of Surrey and Northwestern University are proposing a new type of qubit built not with superconducting circuits, but with quantized oscillations in superfluid helium. The device, consisting of a superfluid weak link and a mechanical element, is predicted to function as a charge-neutral quantum bit with micron-sized dimensions and exhibit millisecond-scale coherence time. This approach leverages both dissipationless mass flow and Josephson tunneling demonstrated in superfluid helium, offering a distinct path toward scalable quantum information processing. The work shows this quantum regime is within reach for a range of device designs.

Superfluid Helium Weak Link as Josephson-like Element

A qubit capable of maintaining quantum coherence for millisecond-scale times, despite being built with micron-sized components, is proposed by Priya Sharma of the University of Surrey and Jens Koch of Northwestern University. This potential advancement differs from prevalent qubit research centered on superconducting circuits, instead leveraging the unique properties of superfluid helium. The device, detailed in recent work, relies on quantized oscillations within a superfluid weak link coupled to a mechanical element, offering a charge-neutral alternative to traditional qubit designs.

Researchers envision a cylindrical cell containing superfluid helium-3, incorporating a nanoaperture acting as a weak link; this link connects the cell to a superfluid reservoir. The design incorporates an elastic plate, responsive to pressure changes induced by superfluid motion, functioning as a fluidic capacitor within an equivalent circuit.

This arrangement allows for the creation of discrete, resolvable energy levels at millikelvin temperatures, a prerequisite for maintaining the superfluid state and enabling quantum behavior. The proposed Superfluid Helium Oscillator Quantum (SHOQ) device operates by establishing a phase difference across the weak link, initiating a Josephson current and subsequently a pressure difference that displaces the elastic plate. This displacement, in turn, drives mass current through the weak link, creating sustained superfluid oscillations.

Calculations suggest the energy stored within the device oscillates between the elastic plate and the weak link, with the kinetic energy of superflow dominating bulk superfluid flow. The researchers assume a sinusoidal current-phase relation for the weak link, a characteristic enabling its function as a nonlinear element essential for qubit operation.

Appropriate engineering of the device, the authors suggest, can yield the necessary nonlinearity to realize qubit functionality, potentially operating as a charge-neutral quantum bit. This approach offers a pathway toward compact, long-coherence qubits, potentially circumventing challenges posed by charge and flux noise commonly encountered in superconducting systems.

Superfluid 3He-B Phase and Quantized Oscillations

The proposed device, termed the Superfluid Helium Oscillator Quantum (SHOQ), relies on a carefully engineered weak link within the superfluid to establish and maintain these quantized states. These two phenomena, previously observed independently, are combined in this design to create a system where energy oscillates between an elastic plate and the superfluid weak link. The researchers note that while they have focused on a pristine superfluid system, further investigation into the effects of surface roughness, disorder, and the specific superfluid phase could reveal additional phenomena beyond the scope of this initial work.

SHOQ Device Design: Cell and Elastic Plate

Unlike conventional qubits, the SHOQ device leverages the unique properties of superfluidity, specifically, dissipationless mass flow and Josephson tunneling, to establish and maintain quantum states. The superfluid coherence length varies from approximately 10 to 80 nanometers for the range of hydrostatic pressures in the B-phase, and this sets the required size of the weak link.

Crucially, the cell also features an elastic plate acting as a mechanically responsive element; changes in pressure induced by the superfluid motion cause the plate to displace, creating a dynamic system where energy is exchanged between the superfluid and the mechanical component. The researchers assume the weak link exhibits a sinusoidal current-phase relation, a characteristic essential for nonlinear behavior and qubit functionality.

The team predicts that this design can achieve millisecond-scale coherence time, a significant advantage for practical quantum computing, despite the device’s micron-sized dimensions. The ability to maintain coherence for this duration, combined with the small physical footprint, positions the SHOQ device as a promising candidate for scalable quantum technologies. They state, “We consider the simple case of a pristine superfluid enclosed by smooth surfaces, noting that the role of textures, surface roughness and the choice of superfluid phase may lead to additional fascinating effects beyond the scope of this paper.”

Superfluid Coherence Length Defines Weak Link Size

This potential quantum bit, dubbed the Superfluid Helium Oscillator Quantum (SHOQ) device, relies on the precise dimensions dictated by the superfluid coherence length within helium-3, a property that defines the size of a critical weak link in the system. The interplay between these components generates quantized oscillations, creating discrete energy levels resolvable at millikelvin temperatures.

The team assumes smooth and uniform textures of superfluid helium-3 on either side of the weak link, simplifying the model while still capturing essential physics. This dominance allows for a more streamlined model, focusing on the core interactions driving the qubit’s behavior.

Non-Sinusoidal Current-Phase Relations in 3He

Unlike electronic Cooper pairs in superconductors, superfluid helium-3 offers charge neutrality, potentially shielding qubits from disruptive noise sources like flux and charge fluctuations. The design incorporates an elastic plate, whose displacement is driven by pressure changes resulting from the superfluid current, creating a mechanical element analogous to a capacitor in traditional superconducting circuits. While non-sinusoidal current-phase relations can arise due to the internal spin structure of Cooper pairs, the team notes that these relations are inherently nonlinear, a key characteristic for realizing qubit functionality.

The device can potentially maintain phase coherence for times on the millisecond scale, coupled with the device’s small physical footprint, position it as a promising candidate for practical quantum computing. This simplification allows for a focused examination of the core physics governing the device’s operation.

Modeling Assumptions: Pristine Superfluid & Smooth Surfaces

The researchers specifically chose to disregard factors like surface roughness, disorder, and variations in the superfluid phase, acknowledging these could be investigated in future work. This simplification stems from a need to isolate the fundamental mechanisms at play. The device’s functionality depends on the interplay between a superfluid weak link, a tiny constriction allowing mass flow, and a mechanical element, an elastic plate.

To accurately predict the quantized energy levels necessary for qubit operation, the modeling assumes a sinusoidal relationship between mass current and phase across the weak link, mirroring observations in similar superconducting systems. This assumption, however, acknowledges that the internal spin structure of the superfluid helium could lead to deviations from this simple relationship, potentially creating more complex, but also potentially useful, quantum behaviors.

The team’s modeling also assumes that kinetic energy from superflow through the weak link significantly outweighs the energy associated with bulk superfluid flow within the device, allowing them to neglect the latter in their calculations. The choice of a pristine superfluid is not merely a computational convenience; it’s rooted in the understanding that imperfections can introduce unwanted noise and decoherence. This focused approach, they believe, is crucial for demonstrating that the predicted quantum regime, with potential millisecond-scale coherence time, is realistically achievable with current technology and design parameters.

Potential for Millisecond Coherence in SHOQ Devices

The functionality of the SHOQ device hinges on the interplay between Josephson tunneling and dissipationless mass flow, both experimentally demonstrated in superfluid helium. This simplification, they contend, is crucial for demonstrating the potential for millisecond-scale coherence time, a duration considered significant for practical quantum computation despite the device’s diminutive size.