From Theory to Experiment | A New Way to Push Quantum Measurement Boundaries

In a collaborative effort between teams in Australia and the United Kingdom, Dr Tingrei Tan of the University of Sydney and Dr Christophe Valahu, among other coauthors, have introduced a new measurement scheme that reframes the constraints of the Heisenberg uncertainty principle. Their work, reports the first experimental implementation of a grid-state / modular observable technique to achieve improved simultaneous sensitivity of position and momentum.

Valahu, C. H., Stafford, M. P., Huang, Z., Matsos, V. G., Millican, M. J., Chalermpusitarak, T., Menicucci, N. C., Combes, J., Baragiola, B. Q., & Tan, T. R. (2025). Quantum-enhanced multiparameter sensing in a single mode. Science Advances, 11(39). https://doi.org/10.1126/sciadv.adw9757

Since Werner Heisenberg formulated his uncertainty principle in 1927, it has served as a guiding limit: the more precisely one measures a particle’s position, the less precisely its momentum can be known, and vice versa. In laboratory settings, this manifests as a trade-off in measurement noise or disturbance. Over time, quantum engineers and physicists have developed tools such as squeezed states, weak measurement, and back-action evasion to push precision boundaries in one parameter or in specially constrained configurations. But a universal, practical route to simultaneous high precision in both canonical variables has remained elusive.

Dr Tingrei Tan of the University of Sydney stated,

“This work highlights the power of collaboration and the international connections that drive discovery.”

In their new work, Tan, Valahu, and colleagues propose shifting the focus: rather than trying to measure absolute position and momentum directly, they adopt modular variables; essentially, position and momentum modulo some periodic scale. This allows them to trade off ignorance of coarse information (which cell or which grid region a particle may occupy) in exchange for enhanced precision in tracking fine deviations within that region.

The team implements this idea in a trapped-ion system. They confine an ion in a trap and engineer its quantum state into a grid, or lattice, pattern in phase space. In such a “grid state,” the ion’s wavefunction is arranged as repeated peaks at regular intervals. If an external force or perturbation shifts the ion slightly, the grid shifts in position and momentum space in a correlated way. Because the grid is periodic, one need not resolve which exact “cell” the ion ends up in; only how much relative shift has occurred.

By focusing measurement resources on the fine shift (the deviation modulo the grid period), the experiment gains sensitivity to small changes in both position and momentum, while sacrificing the knowledge of the coarse location or momentum. The approach effectively confines the quantum uncertainty into the coarse degrees of freedom, leaving the fine degrees more sharply measurable.

In practice, the experimenters used calibrated laser pulses and careful control to prepare the grid states, then applied weak forces to induce shifts, and read out both modular position and momentum sequentially in a way that avoids strong back-action on the fine degrees. Their results show measurable gains in metrological sensitivity relative to classical or conventional simultaneous measurement schemes.

In particular, the authors calculate a metrological advantage of approximately 5.1 dB over the simultaneous standard quantum limit in their configuration, demonstrating that the method is more than a theoretical curiosity. (This is drawn from the ratio of signal to noise in their modular measurements.)

What this scheme achieves is not a violation of Heisenberg’s principle, but a reassignment of where the “uncertainty budget” is borne. The coarse uncertainties (which grid cell, large jumps) remain ambiguous, but the smaller variations become more sharply resolved. In many real sensing tasks, being able to detect tiny deviations matters more than knowing absolute position or momentum.

Comparisons can be drawn to analogous ideas in quantum metrology and quantum control, where one tailors what to measure and what to let float rather than attempt full control over everything. The grid-state / modular observable method joins a toolbox of advanced measurement techniques that include parameter estimation strategies, adaptive measurement, and engineered entanglement.

Also relevant is the use of grid states in quantum error correction (e.g. the Gottesman Kitaev Preskill code), where appropriately structured wavefunctions are leveraged to detect and correct small displacement errors. The techniques developed in that area (for creating and manipulating grid states) can inform the experimental implementation of modular sensing. Indeed, prior theoretical proposals for grid states and continuous variable error correction have explored how grid states respond to displacements and how information scales with number of photons or energy in the system.

While the conceptual leap is compelling, translating it into widely useful devices will demand careful engineering. First, preparing high-fidelity grid states with minimal decoherence is technically demanding. The discrete periodicity must be preserved over the measurement duration. Any noise, dephasing, or coupling to uncontrolled environmental modes threatens the delicate superposition.

Second, the method trades away the coarse information. For some use cases, knowing exactly where a particle is (or its absolute momentum) is necessary. The method is most beneficial when what matters is the change or deviation from a reference point, not the absolute reference itself.

Third, scaling the method beyond a single trapped ion to multi-ion systems, or to other physical platforms, introduces complexity in coupling, crosstalk, and readout. Extending the modular scheme into continuous fields or many degrees of freedom will require further theoretical refinement and control engineering.

Fourth, measurement readout fidelity and repeated application noise must be carefully managed. Any error in reading modular observables could propagate or degrade the gain. Ensuring that sequential modular measurements commute (or can be accessed without undue disturbance) is part of the experimental design challenges

Despite the challenges, the new method may offer a path toward ultra-sensitive quantum sensors in applications where detecting small shifts in position or momentum is essential. Potential domains include navigation (e.g. inertial sensing), gravimetry, magnetic resonance detection, force microscopy, and even biological measurement at the nanoscale. In many of these applications, the absolute baseline may be less important than high-fidelity detection of small changes.

For engineering communities working at the quantum frontier, the message is that the limits we accept may still have room for creative reinterpretation. Rather than pushing harder to measure everything, one may gain more by choosing what to leave unspecified and focusing effort on the variables that matter most.

In coming years, integrating modular observable schemes with robust control, error suppression, and scalable architectures will be crucial. If successful, devices built on these principles could push performance in quantum sensing farther than previously thought possible — not by defying quantum mechanics, but by working with it more judiciously.

Let’s see how the next generation of experiments extends these ideas across systems, and whether this new direction becomes part of the standard toolkit for quantum engineering.