German Researchers Observe Shapiro Steps in an Ultra-Cold Atomic Josephson Junction

In a vacuum chamber colder than deep space, researchers in Germany persuaded ultra-cold atoms to reproduce a crucial component of quantum technology.

Rather than using wires and superconductors, their circuit relied on light and matter waves, taking quantum engineering towards an unusual new form of “electronics.”

How a quantum circuit can form without wires

In most laboratories, a Josephson junction has an unremarkable appearance: two minuscule superconducting sections divided by an extremely thin insulating layer. It has no moving components and produces no spark. Even so, this microscopic layered structure underpins many quantum computers, highly sensitive medical scanners and voltage standards.

Within the junction, electrons form pairs and tunnel through the insulator without electrical resistance. When microwave radiation is applied, the voltage across the junction settles on exact plateaux called Shapiro steps. Their values depend solely on universal constants: the electron charge and Planck’s constant. Metrology laboratories exploit this phenomenon to define the volt with remarkable precision.

Normally, this process takes place inside pieces of metal cooled to almost absolute zero. It occurs across only a few nanometres and is concealed beneath manufactured layers. The electrons themselves cannot be watched; their behaviour must be deduced from electrical measurements.

In Kaiserslautern, a team replaced electrons and metal with ultra-cold atoms and a laser barrier, and watched a Josephson junction unfold in real time.

Published in Science as “Observation of Shapiro steps in an ultracold atomic Josephson junction,” the result is the first realisation and direct imaging of this defining effect in an atomic gas.

Why Josephson junctions matter beyond a single experiment

Josephson junctions are central to a range of technologies:

• Superconducting qubits in many prominent quantum computers use them to set energy levels.
• Magnetometers known as SQUIDs employ them to measure exceptionally weak magnetic fields, including those relevant to brain imaging.
• National standards institutes connect thousands of them to produce extremely stable voltage references.

In each application, a junction functions as a quantum valve for a superfluid of charge. Cooper pairs - the bound pairs of electrons found in a superconductor - tunnel coherently through the barrier. A phase difference between the two sides then generates a current without resistance.

Shapiro steps show that the junction can synchronise with an outside rhythm. As microwaves strike the device, tunnelling becomes locked to the radiation. The current–voltage curve develops evenly spaced steps, with their separation carrying information about the microwave frequency. This response demonstrates that the device is following quantum laws rather than untidy classical dynamics.

Why solids rarely allow direct observation

Investigating these processes in metals presents a major practical difficulty. They unfold on nanometre scales, while electrons travel through a crystal lattice behind layers of material. Directly probing them without disrupting them is almost impossible.

For numerous problems, condensed-matter physicists instead use quantum simulation. Rather than opening up a device to observe its electrons, they construct another system governed by the same quantum rules, but with elements that can be more readily controlled and imaged.

Quantum simulators trade microscopic charge carriers for larger, slower atoms that still obey quantum mechanics, but move on length scales visible under a microscope.

Ultra-cold atomic gases are especially well suited to this task. When dilute atom clouds are cooled to temperatures extremely close to absolute zero, they become Bose–Einstein condensates. In this state, the atoms act as one coherent matter wave. Scientists can confine them with lasers, use light to create barriers, and monitor their distribution using high-resolution cameras.

Building an atomic Josephson junction in Kaiserslautern

A vacuum chamber creates two condensates

For the Kaiserslautern experiment, Herwig Ott’s group began with an enclosed vacuum chamber. They cooled an atomic gas to roughly −273.12 °C, just a fraction of a degree above absolute zero. Thermal movement virtually vanishes at this temperature, allowing the gas to become a Bose–Einstein condensate: a superfluid made from matter waves.

Importantly, the researchers produced not one condensate but two adjacent atom clouds, each acting as an independent quantum fluid. In superconductivity terminology, these condensates correspond to the two superconducting electrodes of a Josephson junction.

Lasers replace insulators and microwaves

The team recreated the slim insulator between superconductors with a sheet of light. A tightly focused laser beam formed a narrow barrier whose properties could be adjusted between the condensates. Atoms were able to tunnel through this wall of light, in the same way that Cooper pairs pass through the insulating layer of a solid-state junction.

They then introduced the equivalent of microwaves. By periodically varying either the height or the position of the laser barrier, the scientists effectively “shook” the junction. This repeating drive matches the function of microwave radiation in a conventional Josephson junction.

As the barrier moved back and forth, atoms travelled between the two condensates in both directions. Changes in their number imbalance and phase relationship over time provided a direct picture of the atomic current.

When the laser barrier vibrated at the right frequencies, the atomic junction locked into distinct transport plateaus – the matter-wave version of Shapiro steps.

The effect is more than a striking image. It agrees closely with theoretical Shapiro-step predictions, including the locations of the steps and how they vary with driving strength.

A world-first observation of Shapiro steps

This research is the first unambiguous observation of Shapiro steps in an ultra-cold atomic Josephson junction. Previous cold-atom experiments had demonstrated Josephson oscillations and associated effects, but the quantised response to a periodic drive had not been achieved.

The Kaiserslautern researchers addressed that missing result. Their atomic setup reproduced both the form and the numerical details of Shapiro steps familiar from solid-state devices. The match reinforces the principle that Josephson physics is not determined by the particles’ microscopic identity, provided that a coherent quantum fluid tunnels across a barrier.

It also offers an advantage generally unavailable in solid-state systems: direct, spatially resolved access to the “current”. Cameras record the atom clouds in situ, enabling scientists to observe, frame by frame, how the density profile changes while atoms tunnel.

Conventional Josephson junction	Atomic Josephson junction
Carriers are Cooper pairs (paired electrons)	Carriers are ultra-cold atoms in a condensate
Barrier is a solid insulator	Barrier is a tunable laser beam
Driven by microwaves	Driven by periodic modulation of light
Measured via voltage and current	Measured via images of atom number and phase

Atomtronics: circuits made from matter waves

The study belongs to the expanding field often termed atomtronics. Its key premise is to construct circuit-style networks using directed flows of ultra-cold atoms rather than metals and semiconductors. Within these networks, coherent matter waves perform the role normally played by electric current.

Potential atomtronic components include:

• Atomic Josephson junctions that operate as quantum switches or components of interferometers.
• Ring-shaped traps that work in a similar way to superconducting loops in SQUIDs.
• Condensate networks arranged into artificial lattices with adjustable geometry.

By linking several atomic junctions, the Kaiserslautern group intends to create complete circuits that emulate complicated superconducting devices. Instead of depending only on abstract models, physicists could use a tabletop platform to “replay” quantum electronics in slow motion and at single-pixel detail.

These circuits may also become exceptionally sensitive sensors. Because condensates react strongly to minute changes in magnetic fields, gravity and rotation, carefully designed atomtronic loops could compete with or supplement existing quantum sensors in geophysics and navigation.

Implications for quantum computing and fundamental physics

Quantum processors based on superconductors, whether made by technology giants or start-ups, depend on Josephson junctions as their principal non-linear components. A substantial challenge remains in understanding the loss of coherence and the routes through which noise enters these circuits. Many influences are obscured by manufacturing flaws or material defects that cannot easily be isolated.

Atomic junctions avoid those complications. Their atoms are suspended in a near-perfect vacuum, and both their interactions and surroundings can be precisely controlled. Recreating Josephson behaviour with atoms gives researchers a clean reference system, where interactions can be turned on or off, barrier shapes can be altered almost freely, and disorder can be introduced deliberately.

Cold-atom junctions act like a cleaned-up version of a quantum chip, where theorists can test ideas about coherence, noise and control before confronting messy solid-state hardware.

Beyond practical uses, the experiment strengthens the conceptual link between distinct areas of physics. Superconductivity, superfluid helium and Bose–Einstein condensates are often covered in separate textbooks, yet Josephson effects connect them all. Observing Shapiro steps in an atomic gas makes that shared relationship particularly tangible.

Extra context: Bose–Einstein condensates in practice

“Bose–Einstein condensate” may sound like an abstract term, but it results from a straightforward sequence. A dilute gas is first placed in a vacuum. Laser cooling combined with magnetic or optical trapping then removes energy from the atoms. Once the temperature reaches the nanoKelvin range, each atom’s thermal de Broglie wavelength expands until it starts to overlap with those of neighbouring atoms.

At that point, the gas stops behaving as a collection of separate particles and enters one quantum state. A single wavefunction represents the whole cloud. This collective state permits effects including frictionless flow, quantised vortices and Josephson tunnelling between distinct condensates.

In many ways, a condensate has the same function as the Cooper-pair superfluid within a superconductor. This parallel makes it a natural substitute for charge carriers in model circuits built from atoms.

The next possibilities for atom-based circuits

Further research could take atomic junctions into conditions that today’s solid-state devices find difficult to access. Scientists may explore more powerful interactions, drive protocols far from equilibrium, and designed noise patterns that put theoretical models under pressure.

Hybrid methods are another possibility. One research direction seeks to connect cold atoms with superconducting microwave circuits, combining the advantages offered by each platform. Atomic Josephson junctions already fluent in the “language” of Shapiro steps and driven phase dynamics could fit naturally into these approaches.

For engineers and students moving into quantum technology, this work creates another experimental environment. Developing an instinct for phase, tunnelling and coherence can be challenging when the physics is hidden inside chip packages. Seeing atoms perform the same processes on camera provides that understanding with a concrete, almost tactile foundation.