Two qubits now map how energy spreads through quantum systems

Mapping Energy Flow with Two‑Qubit Quantum Sensors

Researchers at Johannes Gutenberg‑Universität Mainz have shown that a pair of coupled qubits can act as a miniature microscope for the way energy spreads through complex quantum materials. By tailoring pulse sequences within a T₂‑spectroscopy framework, the two‑qubit device isolates the symmetric (noise) and antisymmetric (response) correlation functions of the surrounding many‑body system. This separation lets the sensor visualize spatio‑temporal correlation fronts—light‑cone‑like profiles—that map how low‑energy excitations propagate, and it can differentiate between ballistic, diffusive, and crossover transport regimes.

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1. Two‑Qubit T₂ Spectroscopy: From Noise to Response

Traditional single‑qubit probes measure a combined effect of environmental fluctuations and the system’s linear response, making it difficult to untangle the two. The new approach applies distinct pulse protocols to each qubit, enabling independent extraction of:

• Symmetric correlation function – quantifies environmental noise.
• Antisymmetric correlation function – captures the system’s linear response.

Because the method does not rely on equilibrium fluctuation‑dissipation relations, it remains valid even when the probed material is driven far from thermal equilibrium.

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2. Visualizing Light‑Cone‑Like Correlation Fronts

When an excitation is introduced into a many‑body system, correlations spread outward at a characteristic speed, forming a light‑cone boundary in space‑time. The two‑qubit sensor records a correlated dephasing signal that vanishes beyond the product of this speed and elapsed time, directly mapping the front. External driving can modify the profile, producing additional fringes outside the nominal cone—signatures of non‑equilibrium dynamics and altered interaction landscapes.

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3. Distinguishing Transport Regimes

By analyzing the shape and decay of the correlated dephasing signal, the sensor discriminates between:

• Ballistic transport – sharp, linear light‑cone edges indicating coherent propagation.
• Diffusive spreading – broadened fronts with Gaussian‑like tails, reflecting random scattering.
• Crossover behavior – intermediate profiles where ballistic and diffusive features coexist.

The diffusion length and propagation speed can be extracted quantitatively, offering a granular view of energy flow that was previously inaccessible.

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4. Outlook: Scaling and Applications

The demonstrated protocol is compatible with existing solid‑state defect platforms such as nitrogen‑vacancy (NV) centers, and it can be extended to more qubits or richer pulse sequences. Potential future directions include:

• Momentum‑resolved noise spectroscopy for deeper insight into microscopic interactions.
• Probing quantum materials, simulators, and engineered lattices where transport properties are critical.
• Integrating the technique into quantum‑information workflows to monitor decoherence pathways in real time.

By providing a unified framework that bridges noise spectroscopy and linear‑response measurement, two‑qubit T₂ spectroscopy opens a new window onto the dynamics of many‑body quantum systems, promising advances in both fundamental physics and emerging quantum technologies.