Insider Brief
- Penn-led researchers used a photonic chip to transfer a winding pattern from one photon to a shared two-photon state, preserving the pattern under noise even after entanglement disappeared.
- The winding pattern remained stable as the quantum state deteriorated, supported by other quantum correlations, but broke down under sufficiently strong noise.
- The findings could one day be used in the development of more resilient quantum networks.
University of Pennsylvania researchers have used a photonic chip to transfer a winding pattern in quantum light that survived noise even after entanglement disappeared, which they say could help develop quantum networks.
Currently, scientists and engineers are exploring ways to create a quantum network that could, for example, connect two quantum computers in separate buildings so they can work together on a calculation. The team suggests this experiment could be an initial step on the path toward more resilient links between those extremely sensitive quantum systems.
The study, published in Nature Physics, demonstrates a way to create and manipulate quantum skyrmions, which are structured patterns in light whose overall winding can remain stable even as their underlying quantum state deteriorates. To picture this, think how a rope can bend and stretch without losing its knot. In this case, these light patterns can retain their overall winding even as noise degrades their quantum state.
The researchers transferred that structure from a single photon to correlations shared between two photons.
According to the study, the silicon nitride chip combined components that generated photon pairs, routed the photons and shaped the light into skyrmions.
Connecting quantum computers and other quantum devices is challenging because noise — which can include anything from heat to cosmic rays — can weaken the delicate relationships that carry quantum information, making it difficult to move that information reliably between systems.
However, this experiment has a tighter scope than demonstrating a working, noise-resistant quantum network. The scientists preserved a particular measurable property of the light, rather than showing that an entire quantum state could be transmitted intact after entanglement was lost. They point out that the experiment does not establish a practical advantage for quantum state transfer.
A Pattern That Holds Its Shape
A photon, which is a particle of light, has a quantum state that can include polarization — the orientation and motion of light’s electric field — and spatial properties associated with how the light’s wavefront is arranged.
In these skyrmions, the orientation of light’s electric field changes from place to place, forming a winding pattern. That pattern is described by a skyrmion number, which counts how many times the polarization arrangement covers the full set of possible polarization states.
The researchers write that the underlying idea comes from topology, the study of properties that remain unchanged when a structure is smoothly deformed. Individual parts of a pattern can move or change without altering its overall winding.
For quantum technology, this is important because a disturbance might degrade many details of a light-based quantum state while leaving its topological number recognizable.
According to the study, earlier research had produced single-photon skyrmions on chips and skyrmions shared between entangled photons using larger optical arrangements. The new work brings preparation and manipulation into an integrated platform and demonstrates a transfer between the two forms.
Entanglement is a quantum relationship where the particles are linked so that measuring one offers information about the other, even when they are far apart. Quantum networks rely on these links, but noise and other disturbances can disrupt them.
The Penn-led team found that the above mentioned skyrmion number could remain preserved over a finite range of noise after a standard measure of entanglement fell to zero. Other quantum correlations also persisted, helping sustain the winding structure, according to the researchers’ analysis.
How the Chip Transferred the Structure
The researchers used short laser pulses to generate photon pairs in two spiral channels on the chip. The photons were separated by wavelength and routed through an array of optical components controlled by tiny electrical heaters.
One photon passed through ring-shaped structures that created the skyrmion pattern. These structures linked its polarization with orbital angular momentum, a property associated with a twisting light wavefront.
The second photon was directed through a component that converted its on-chip paths into different polarizations.
By measuring two properties of the first photon together, the researchers transferred its winding pattern into a pattern shared between the two photons. The structure was then carried by the relationship between the photons, rather than by one photon alone.
The researchers reconstructed the quantum states through repeated measurements, a process called quantum state tomography. This allowed them to assess both how closely the resulting states matched their intended form and whether the winding number survived.
For the principal demonstration, the intended skyrmion number was 2. Measurements of the transferred states yielded values close to 1.98. The four measurement outcomes produced states with fidelities ranging from 87% to 89%, according to the study.
Fidelity measures how closely the quantum state produced in the experiment matches the intended state. When the team added noise, that match worsened, but the light’s overall winding pattern remained largely unchanged.
Testing the Limits of Resilience
To test that stability, the researchers deliberately introduced background noise and adjustable white-light noise. The added noise affected the different measurement settings almost equally, approximating a type of disturbance known as depolarization.
As the noise increased, the measured states became less pure and their fidelity declined. Their skyrmion number, however, remained near its original value even after entanglement vanished.
The team assessed the remaining quantum correlations using a measure called quantum discord. Unlike entanglement, discord can remain present in some noisy states that are no longer entangled.
According to the researchers, its persistence indicates that the surviving topology was associated with quantum correlations beyond entanglement. That does not mean any remaining quantum correlation is sufficient to protect the pattern indefinitely. The study reports that topology can break down before discord reaches zero.
Under sufficiently strong background noise, the skyrmion number collapsed toward zero. The protection therefore had a limit.
The team also examined two more targeted types of error. One changed the relative phase between parts of the quantum state, affecting how they combine. The other combined a phase change with a swap between the encoded alternatives.
For these tests, the researchers used controlled chip operations and numerical mixtures of measured photon counts to represent different error strengths. The winding number again remained stable as fidelity declined, until the disturbance approached the tested critical threshold.
Numerical simulations reproduced the main behavior observed in the experiments. Some measured results deteriorated earlier than an idealized noise model predicted, which the researchers attributed to small additional disturbances that did not affect every measurement setting equally.
Toward Quantum Interconnects
The study suggests that topology could offer an additional property to track or exploit when designing connections between quantum systems. It also provides a chip-based platform for investigating how that property behaves under noise.
Existing approaches such as quantum error correction and entanglement purification aim to protect or restore quantum information, but can require substantial resources and precise control, the researchers write.
The new experiment, however, does not conclusively establish that skyrmions can replace those methods. Its immediate contribution is a demonstration that a quantum light pattern can be prepared, manipulated and transferred while retaining its winding number under conditions that destroy entanglement.
Future work would need to determine whether that resilience can support a useful information-processing task. The researchers identify protocols involving noisy quantum states as a direction for further investigation, alongside efforts to encode and read out topological properties of light.
For quantum networking, the open question is whether preserving this limited but durable feature can help devices communicate more reliably. The chip gives researchers a way to test that possibility.
The team included Haoqi Zhao, Yichi Zhang, Shuang Wu, Yaoyang Ji and Liang Feng of the University of Pennsylvania; Isaac Nape and Andrew Forbes of the University of the Witwatersrand in South Africa; and Chenjie Zhang and Yijie Shen of Nanyang Technological University in Singapore.