Consider you’re watching a magic show where a magician makes a coin disappear only to reappear in a different spot before you even blink. Now, picture this on a quantum scale — where light, or photons, can seemingly spend less than zero time interacting with atoms. This isn’t magic; it’s a groundbreaking discovery from researchers at the University of Toronto and Griffith University in Australia.
The researchers, including Daniela Angulo, Kyle Thompson, and Howard M. Wiseman, worked to understand what happens when a pulse of light travels through a medium. Specifically, they focused on how long a photon spends exciting atoms in a cloud of cold rubidium (85Rb) atoms. The traditional view would have you think that when light passes through a material, it slows down due to interactions with atoms. But what happens when this interaction time appears to be negative?
In their experiment, the team used a technique known as the cross-Kerr effect. Here’s how it works in layman’s terms: Imagine you have two beams of light. One is a ‘signal’ beam, which is a short pulse of light tuned to resonate with the atoms. The other is a ‘probe’ beam, a continuous wave of light that doesn’t resonate but can still interact with the excited atoms. When the signal beam passes through the atom cloud, it slightly excites the atoms, changing their state. This change can be detected by the probe beam as a phase shift, which is like a twist in the light’s path.
What they found was astonishing. For certain pulse durations and optical depths of the atom cloud, the time spent by photons in atomic excitation — which they measured by integrating this phase shift over time — ranged from -0.82±0.31 times τ₀ for the narrowest pulses to 0.54±0.28 times τ₀ for the broadest, where τ₀ represents the time an atom would typically be excited by an average photon. Negative time? How does that work?
In the quantum world, “negative time” doesn’t mean time travel or reversing the clock. Instead, it’s about how the group delay of light — how long it takes for the peak of a light pulse to pass through the medium — can become negative. This happens when the light is very close to the resonance frequency of the atoms, causing an unusual interplay of quantum effects where the light seems to “leave” before it “arrives” in terms of excitation.
The researchers used pulses with root mean square (rms) durations of 10, 18, 27, and 36 nanoseconds, examining these in clouds with optical depths of around 2 to 4. They measured phase shifts in the probe beam that were as small as 10 to 20 microradians, which is like detecting a whisper in a bustling crowd. Collecting this data over tens of millions of atom cycles, they provided a clear picture of this quantum paradox.
Why should we care? Well, this isn’t just an abstract curiosity. Understanding these interactions can revolutionize fields like quantum computing and quantum communication. For instance, if we can control how light interacts with matter at this level, we could potentially develop faster, more efficient quantum memories or even manipulate light in ways we’ve only theorized before.
Reference
Angulo, D., Thompson, K., Nixon, V., Jiao, A., Wiseman, H. M., & Steinberg, A. M. (2024). Experimental evidence that a photon can spend a negative amount of time in an atom cloud. ArXiv. https://arxiv.org/abs/2409.03680
