Accentuate The Negative — Scientists Show Photons Can Spend ‘Negative Time’ as Atomic Excitations

Insider Brief

• Researchers demonstrated that photons can cause atoms to experience “negative time” in their excited states, challenging classical ideas of light-matter interactions and suggesting that negative group delays have real physical significance.
• Using a method called the cross-Kerr effect, the team measured atomic excitation times and showed that transmitted photons interact with atoms in such a way that their excitation time corresponds directly to the group delay, even when that delay is negative.
• Although the study doesn’t directly address quantum computing, the findings could have implications for improving quantum memory and communication systems by enhancing the control of photon-atom interactions.

Negative time may sound like the last committee meeting you attended that should have been an email, but researchers from the University of Toronto and Griffith University have reported that photons — particles of light — can spend a “negative” amount of time exciting atoms as they pass through a medium. Published in the pre-print server arXiv, the researchers report this strange phenomenon, confirmed by experiments and theory, challenges traditional views of light-matter interaction and sheds new light on the concept of negative time in quantum systems.

The study, which investigates the group delay experienced by photons, also suggests that this negative time has more physical meaning than previously thought and could have implications for quantum technology, such as quantum computing.

At the heart of the research is the concept of group delay. When light passes through a material, its speed is affected, causing a delay in how long it takes to travel from one point to another. Normally, this delay is positive, meaning that the light slows down as it interacts with the atoms in the material. However, in certain cases, especially when the light is tuned to specific frequencies near the material’s atomic resonance, something strange happens: the group delay becomes negative.

This means the light appears to exit the material before it should, creating a paradox that has puzzled physicists. Essentially, it’s as if the photon caused an effect, like making the atom excited, before it even arrived — something that can happen in the quantum world but doesn’t make sense in our everyday experience of time. (Welcome to the weird, wonderful world of quantum mechanics.)

To better understand this phenomenon, the research team set out to answer a fundamental question: Does this negative group delay correspond to the time photons spend as atomic excitations? The answer, as it turns out, is yes. By using a method called the cross-Kerr effect, the researchers were able to probe the degree of atomic excitation caused by transmitted photons, even when the group delay was negative. The results showed that the time spent by the photons as atomic excitations was directly related to the group delay, suggesting that the negative time observed in the group delay has real physical significance.

Another fundamental question the researchers sought to answer was: how much time do atoms spend in an excited state when a photon is transmitted through a medium?

“We define the average time that the atoms spend in the excited state (τ0), or average atomic excitation time, as the time integral of the expectation value of the number of atoms in the excited state,” the researchers write.

They further explored the quantum nature of this interaction, asking how this time changes when photons are transmitted rather than scattered.

Understanding Negative Time

The idea that photons can cause atomic excitations for a negative amount of time may seem counterintuitive, but it fits within the framework of quantum mechanics. In classical physics, time is always positive—a particle moves forward in time as it travels. However, in the quantum world, time can behave differently. When the researchers tuned their light pulses to specific frequencies close to the atomic resonance of rubidium-85 atoms, they observed that the group delay of the transmitted photons became negative. This implies that the peak of the light pulse exited the medium before it logically should have, based on when it entered.

To explain this, the team used quantum theory and the concept of “weak values,” a formalism that allows certain measurements in quantum mechanics to take on values outside the normal expected range. In this case, the weak value of the atomic excitation time was found to be negative, corresponding to the negative group delay observed. Essentially, the photons were interacting with the atoms in such a way that the atoms were excited before the light even arrived—at least, from the perspective of the group delay measurement.

This strange behavior was measured using the cross-Kerr effect, which allowed the team to detect tiny phase shifts in a secondary beam of light (the probe) caused by the atomic excitations from the transmitted photons. By carefully synchronizing their measurements and using post-selection techniques to focus only on the transmitted photons, the researchers were able to directly measure the atomic excitation time and compare it to the group delay.

Quantum Computing Implications

Although not directly addressed in the paper, the team’s findings may have implications for the field of quantum computing. For example, quantum computers rely on the precise manipulation of quantum bits, or qubits, to perform calculations. Understanding how photons interact with atoms in a medium could lead to better control over quantum systems, particularly in quantum memory and communication networks, where information is stored and transmitted via photons. The ability to manage these interactions, even in cases where the group delay is negative, could help improve the efficiency and stability of quantum processors.

Moreover, the study’s findings about negative time offer new ways to think about how information is transferred in quantum systems. By understanding how photons can interact with atoms for negative amounts of time, researchers may be able to design more efficient quantum circuits that exploit these unusual behaviors to enhance the performance of quantum computers.

Experimental Methodology

The researchers conducted their experiments using a cloud of cold rubidium-85 atoms, which were illuminated by two beams of light: a strong “signal” beam that caused atomic excitations and a weak “probe” beam that measured the resulting phase shifts.

The researchers write in their paper: “When atoms are illuminated by a pulse of resonant light—which we will refer to as the ‘signal’ beam—they become polarized and have some probability of being found in the excited state at any given time. For a single photon input, the number of excited atoms is represented by the operator Ne, i.e., ⟨Ne⟩(t) is the single-atom excitation probability multiplied by the total number of atoms.”

By comparing the phase shifts caused by transmitted photons to those caused by the average photon, the team was able to calculate the atomic excitation time. They found that for narrowband pulses, which are light or electromagnetic waves that have a limited range of wavelengths or frequencies, the mean atomic excitation time was negative, directly corresponding to the negative group delay.

The experimental setup required extreme precision, with the team using a single-photon counting module to detect individual transmitted photons. Each measurement cycle lasted several milliseconds, and the researchers collected data over many hours to ensure the accuracy of their results. The data were then compared to theoretical models, which predicted that the atomic excitation time should match the group delay, even when it was negative.

Speculations on Future Directions

Because practical implications were introduced in this article that weren’t directly addressed in the paper, it’s important to note some limitations. The experiments required a highly controlled environment, including ultra-cold atoms and precise timing of the light pulses. Scaling these findings to larger systems or different types of media may be challenging. Additionally, the negative group delay observed in this study is closely tied to quantum interference, which can be sensitive to noise and disturbances. Further research is needed to explore how these effects might manifest in more complex quantum systems or in practical quantum computing applications.

The work could also open up a range of new research directions. Looking ahead, researchers might want to investigate whether similar behaviors occur in other atomic systems or with different types of light, such as entangled photons. Scientists could also explore how these findings could be applied to improve quantum memory and communication systems, where the precise control of photon-atom interactions is crucial for maintaining the integrity of quantum information.

Research Team

This research was conducted by a team from the Department of Physics and the Centre for Quantum Information and Quantum Control at the University of Toronto, including Daniela Angulo, Kyle Thompson, Vida-Michelle Nixon, and Andy Jiao. The study also involved contributions from Howard M. Wiseman at Griffith University’s Centre for Quantum Computation and Communication Technology in Australia, with leadership from Aephraim M. Steinberg at the University of Toronto.

For a deeper, more technical explanation that cannot be provided in this summary, check out the team’s arXiv paper. The pre-print paper is not technically an official peer-reviewed paper, but does serve as a way for fellow scientists to review the findings informally.