Time is one of the most familiar features of everyday experience. We perceive it as flowing smoothly and continuously, marked by the ticking of clocks and the steady progression of cause and effect. Yet at the quantum scale—the realm of atoms, electrons, and photons—time behaves in ways that challenge classical intuition. Events unfold on unimaginably short timescales, often measured in femtoseconds or attoseconds, where one attosecond is a billionth of a billionth of a second. Understanding how long such ultrafast quantum events actually last has long been one of the most profound challenges in physics. Recent research has now taken a major step forward by showing how the duration of quantum transitions can be measured without relying on any external clock.
In classical physics, measuring time is straightforward. A stopwatch or atomic clock provides a reference against which events can be compared. In quantum mechanics, however, time occupies a peculiar role. Unlike position or energy, time is not represented by an operator in the same way. Instead, it appears as an external parameter that governs the evolution of quantum systems. This has led to deep conceptual debates about whether time can be directly observed at the quantum level or only inferred indirectly through changes in other physical quantities.
One of the most intriguing quantum processes is photoemission, in which electrons absorb energy from incoming light and escape from a material. This phenomenon lies at the heart of technologies ranging from solar cells to photoelectron spectroscopy. For decades, physicists assumed that once an electron absorbed a photon with sufficient energy, its emission was effectively instantaneous. However, advances in ultrafast laser technology have made it possible to probe these processes on attosecond timescales, revealing that electron emission takes a finite, measurable amount of time.
The challenge has been determining how to measure this duration without introducing an external clock that could disturb the system. Traditional approaches rely on precisely timed laser pulses, using one pulse to trigger an event and another to probe it. While powerful, these methods still depend on an external time reference. The new approach takes a fundamentally different route by allowing the quantum system itself to serve as its own clock.
Researchers achieved this by closely tracking subtle changes in electrons as they absorb light and transition from being bound within a material to becoming free particles. As electrons interact with light, their energy, momentum, and phase evolve in ways that encode information about how long the transition takes. By analyzing these changes with extreme precision, physicists can reconstruct the duration of the quantum event without measuring time directly.
Crucially, the results show that quantum transitions are not instantaneous. Instead, their duration depends strongly on the atomic and electronic structure of the material involved. In some materials, electrons escape more quickly, while in others they experience delays due to interactions with surrounding atoms, electronic bands, or collective excitations. This demonstrates that time at the quantum level is not a universal constant for a given process, but an emergent property shaped by the environment in which the process occurs.
This discovery has profound implications for our understanding of time in quantum mechanics. It suggests that time can be inferred from internal dynamics rather than imposed externally. In this sense, time becomes relational: it emerges from correlations between different parts of a quantum system. This idea resonates with broader efforts in theoretical physics to rethink the nature of time, particularly in attempts to reconcile quantum mechanics with general relativity, where time is intertwined with space and gravity.
Beyond its conceptual significance, the ability to measure ultrafast quantum durations has practical consequences. Many modern technologies depend on controlling electron dynamics on extremely short timescales. In semiconductor devices, for example, the speed at which electrons move and respond to electric fields determines how fast information can be processed. In photovoltaic materials, the efficiency of converting light into electricity depends on how quickly excited electrons can be separated and collected before they lose energy.
By revealing how atomic structure influences the timing of electron emission, the new findings provide valuable insights for material design. Engineers may be able to tailor materials with specific electronic properties to optimize ultrafast processes. This could lead to more efficient solar cells, faster electronic components, and improved photodetectors.
The research also has implications for emerging quantum technologies. In quantum computing and quantum communication, precise control over quantum states is essential. Understanding how long quantum transitions last can help reduce errors caused by unwanted delays or decoherence. Moreover, the concept of using internal dynamics as a time reference could inspire new approaches to synchronizing quantum systems without relying on classical clocks.
Another important aspect of this work is its contribution to the ongoing debate about whether time is a fundamental entity or an emergent phenomenon. If time at the quantum level can be inferred from changes in physical systems rather than measured directly, this supports the idea that time may not be a basic building block of nature. Instead, it may arise from the interactions and correlations between quantum systems. This perspective aligns with certain approaches in quantum gravity, which suggest that spacetime itself emerges from more fundamental quantum relationships.
At the same time, these findings remind us that quantum mechanics does not abolish time altogether. Rather, it reshapes our understanding of what time means. The fact that ultrafast events have measurable durations shows that time remains a meaningful concept, even if it behaves differently from our everyday experience. Quantum time is subtle, context-dependent, and deeply connected to the structure of matter.
In conclusion, the discovery that ultrafast quantum events have measurable, material-dependent durations marks a significant advance in both experimental technique and conceptual understanding. By measuring time without an external clock, physicists have shown that quantum transitions are governed by internal dynamics rather than instantaneous jumps. This insight deepens our understanding of time at the smallest scales, bridges fundamental physics with real-world applications, and opens new avenues for controlling matter on ultrafast timescales. As research continues, these findings may ultimately reshape how we think about time itself—transforming it from a background parameter into a dynamic feature emerging from the quantum world.
Source: Ecole Polytechnique Fédérale de Lausanne
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