Scientists Build a “Quantum Watch” That Tells Time Without a Counter

Imagine trying to measure the exact moment when two flashes of light meet, but you cannot assume your mechanical delay stage is perfect. In ultrafast physics, knowing the true delay between a pump pulse and a probe pulse is essential, yet calibrating the mechanical components and finding a robust time zero can be surprisingly tricky. A 2022 experiment in helium shows a clever alternative: instead of counting oscillations like a clock, let a quantum object produce a one-of-a-kind pattern that acts like a timestamp. The pattern is produced by interference among many Rydberg states in an atom, and because the pattern does not repeat over the measurement window, it can uniquely identify how much time has passed.

The problem: why ultrafast timing is hard

Pump-probe experiments track dynamics that evolve on femtosecond to picosecond timescales. Researchers typically set the delay between pulses using a translation stage, which moves mirrors by tiny amounts to control arrival times. In practice, translation stages can drift, have tiny misalignments, and suffer from systematic errors, meaning that the stage-derived time may not be the true time experienced by the sample. Finding an independent, artifact-free timestamp is therefore valuable. The challenge is to create a time marker that is intrinsic to the quantum system being probed, and which does not rely on external counting or mechanical calibration.

The approach: making and reading a quantum fingerprint

Creating a Rydberg wave packet

The team used helium atoms and an extreme ultraviolet (XUV) pump pulse to create a Rydberg wave packet, that is, a coherent superposition of many high-lying electronic states whose principal quantum numbers range roughly from n = 10 up toward the continuum. Because the XUV pulse had energy centered near helium’s ionization threshold (about 24.59 electronvolts), many Rydberg levels were populated simultaneously, producing a complex, time-evolving quantum wave packet.

You can think of the wave packet like a choir where each singer holds a slightly different pitch. As time passes the different pitches slip in and out of phase, producing a complicated, evolving musical pattern. That pattern is what the scientists read out as a timestamp.

Reading the fingerprint with a probe pulse

To read the evolving wave packet, the researchers used a near-infrared (NIR) probe pulse at a controlled delay to photoionize the excited helium. The emitted photoelectrons were recorded in time and energy with an angle-resolved time-of-flight detector, producing a time-and-energy map of the photoelectron yield. By scanning the probe delay and recording the photoelectron signal, the group obtained a time-dependent interference pattern whose structure is set by the relative phases and energies of the Rydberg states.

A supporting theoretical model

The team developed a model that describes the excitation and subsequent ionization of the coherently populated bound Rydberg states. The model treats the initial superposition, the time evolution of each component (phase evolving with energy), and the probe-induced photoionization. The calculation includes experimentally relevant details such as the XUV spectral shape and an estimated chirp in the NIR probe. Despite simplifying assumptions (for example neglecting dipole phases and direct continuum ionization in parts of the model), the simulation quantitatively reproduced the complex experimental patterns, which allowed the researchers to compare predicted and observed fingerprints robustly.

The breakthrough discovery: quasiunique beat signatures as a timestamp

When many Rydberg levels are coherently excited, their mutual interference produces a complicated oscillatory signal in the photoelectron yield. Because the Rydberg manifold converges to the ionization threshold and the energy spacings vary with n, the combined interference pattern does not simply repeat during the relevant lifetime of the wave packet. The scientists name these patterns quasiunique beat signatures, or QUBS.

QUBS have two crucial properties that make them useful as a watch. First, each pattern is essentially unique over a long time window (the experiment probed delays up to tens of picoseconds and calculations extended much further), so a short measurement window of a few picoseconds can be matched to a single time. Second, because the fingerprint comes from the internal quantum phases of the system, matching experiment to theory provides intrinsic proof that the determined time is correct, independent of the mechanical delay stage.

In the helium experiments, the measured and simulated time-dependent photoelectron yields agreed quantitatively. Using QUBS to assign time, the scientists were able to identify a drift in their delay stage amounting to about 1 femtosecond per picosecond of delay. They also demonstrated that slices of the photoelectron signal as short as 1.7 picoseconds can uniquely determine the absolute time within an assumed 10 nanosecond measurement range, and that for a 100 nanosecond range a 3 picosecond slice is needed. These performance numbers establish QUBS as a practical, artifact-free quantum watch for pump-probe experiments.

Why it matters: practical and conceptual impacts

An intrinsic timestamp for ultrafast experiments

The quantum watch gives experimentalists an intrinsic timestamp that does not require independently finding time zero or trusting a mechanical delay line. In experiments where time zero is ambiguous, for example when excitation proceeds via complex multi-step processes or in certain condensed-matter systems, having an internal clock could simplify interpretation and improve reliability.

A diagnostic for apparatus drift and systematic error

Because the QUBS-time can be compared against the time set by the delay stage, it can reveal misalignment or drift in the stage. The scientists used the method to detect a small but measurable drift in their delay stage, showing the watch’s value as a diagnostic tool to improve experimental fidelity.

Versatility across systems and timescales

While the demonstration used helium Rydberg states, the principle is general. Any system that supports a large, long-lived manifold of coherently excited states may produce a QUBS-like fingerprint. The scientists suggest that other noble gases such as neon, argon, krypton, and xenon, or even ions and x-ray-driven schemes using fluorescence to populate valence states, could be adapted to build quantum watches tailored to different spectral ranges and lifetimes. This opens possibilities from femtoseconds up to microseconds, depending on the lifetimes of the states used.

A new way to validate atomic theory

The sensitivity of the fingerprint to small energy shifts is striking. The scientists used the time-dependent patterns to test the known quantum defect values, which capture how real atoms deviate from the ideal hydrogenic energy ladder. By fitting the experimental QUBS to theory, they recovered a quantum-defect amplitude factor of 0.98 ± 0.08 relative to high-precision computed values, offering a complementary route to benchmark atomic structure with time-domain interference rather than direct spectroscopy.

How precise is the quantum watch?

Two limits set the watch accuracy. First, the experimental data quality determines how tightly a measured pattern can be fit to theory. The scientists show that, with their data, fitting can pinpoint the fitted time to roughly 1 femtosecond. Second, the intrinsic correspondence between pattern structure and time determines how narrowly the matching identifies one time versus another. From their comparisons, the team estimates that the QUBS-time uncertainty due to quantum-defect amplitude uncertainty is on the order of 8 femtoseconds in their setup. In practice, the overall accuracy will depend on the species, excitation bandwidths, detector performance, and environmental perturbations.

Caveats and future directions

The demonstration is compelling, but some caveats are worth noting. The theoretical model intentionally simplified certain effects: for example, the dipole phase in the photoionization matrix elements was neglected and strong-field AC Stark shifts were not included. Despite these simplifications, the excellent agreement with experiment suggests those approximations were justified for the conditions used. However, in other experimental regimes with stronger fields or different pulse shapes, those effects could matter and would need to be included in model fits.

The QUBS method also relies on having many long-lived, coherently excited states. That requirement is met naturally for Rydberg manifolds near the ionization limit, but may be harder in systems with rapid decoherence or dominant competing decay channels. Extending the idea to ions or to core-excited states accessed via x rays would be exciting, but may present experimental challenges because of competing decay mechanisms.

Looking ahead, worthwhile experiments include applying QUBS-based timing in condensed-matter pump-probe studies where time-zero is ambiguous, building quantum watches in heavier noble gases and ions to explore different timescales, and integrating a QUBS monitor as an in-line diagnostic in ultrafast beamlines. Improving theoretical models to include more effects would also help push accuracy further, and exploring automated pattern-matching algorithms could make QUBS an easy-to-use tool for experimental labs.

Conclusion: a quantum pattern that keeps time

The quantum watch is an elegant fusion of atomic physics and practical experiment. Instead of counting cycles, it reads a unique interference fingerprint imprinted by many Rydberg components and uses that fingerprint as an absolute timestamp. The technique offers artifact-free assurance of timing, sensitivity to fundamental atomic parameters, and adaptability across experimental platforms. For scientists who need to know the true time in ultrafast experiments, this new quantum approach promises an intrinsic, reliable, and surprisingly intuitive solution.

The research was published in Physical Review Research on October 18, 2022.