2020 BSJ Blog featured Spring 2020

New experiments can predict occurrences of quantum jumps and may require scientists to reevaluate old theories

By Amar Shah

When the mathematical rules for quantum mechanical theory were first created, Niels Böhr and Werner Heisenberg proposed a way to interpret these rules and explain their physical implications: this became known as the Copenhagen interpretation of quantum mechanics. The idea of superposition is instrumental in this: that until the property of a particle is measured, it can be thought of as in two different states at the same time. The most famous illustration of this is Schrödinger’s cat. If you leave a cat in a box, after a period of time you no longer know whether the cat is dead or alive. Thus, the cat is in a superposition of being dead and alive. If you open the box to find a dead cat, then sometime while the cat was in the box it went from alive to dead.

Böhr also created a model for the movement of electrons rotating around an atom’s nucleus like planets rotate around the sun. In the atomic case, there are specific energy levels that electrons can have. This can be thought of as specific orbitals that electrons follow around the nucleus of an atom(represented by integers n = 1, 2, 3, …). 

Böhr was one of the first to speculate about the changes in these energy levels. He hypothesized that changes in an electron’s orbit are like changes in the “aliveness” of Schrödinger’s cat. He called this change in an electron’s orbit a quantum jump and predicted that they occur with estimable probabilities, but are random and instantaneous unless you are continuously monitoring them. However, Zlatko Minev’s new experiment observes a quantum jump between different energy levels of an artificial three energy level atom and concludes that it is “continuous, coherent, and deterministic.” Not only is Minev’s team able to predict when the jump is about to occur, but the jump itself is not an instantaneous event as Böhr predicted, but a continuous change in the energy level. 

Minev creates an artificial atom with three energy levels: Ground, Bright, and Dark. These energy levels can be thought of as similar to the energy levels or orbitals of an electron. When electrons jump to the lower energy level, the system emits a photon. Minev exploits this in order to make his measurements for quantum jumps between the Ground and Bright Levels. The excitation (increase in the particle’s energy level) to the Bright level is recorded by a photodetector that measures photons emitted. Each time a photon is detected, it registers as a click, which alerts the experimenter that a quantum jump from Ground to Bright has occurred. If there are not many clicks for a period of time, one can infer through the process of elimination that quantum jumps from Ground to Dark are occurring. While the photodetector generally has a poor collection efficiency and oftentimes misses photons or “clicks,” Minev’s experimental set-up minimizes the error in photodetection.

Researchers run experiments to take note of when such “clicks” may stop. Even though they do not directly measure the change from Ground to Dark, researchers use this to detect an advance warning signal for the quantum jump. Researchers test a different version of the experiment in which they wait for the clicks to stop and subsequently suspend all system drives. Doing so freezes the evolution of the system causing all changes in energy level to stop.  From here, they are able to reverse the trajectory of a quantum jump mid-flight. This means that the energy level will return to the Ground state. 

 

These results have large consequences for many fields, namely quantum computing. Quantum computers use artificial atoms, called qubits, that are useful for storing quantum information. Sometimes, there are quantum jumps in the qubits which may cause errors in the calculations of quantum computers. Having an advanced warning of these jumps can help researchers mitigate these errors. Beyond that, these results could cause a large shift in how people think about quantum mechanics. Quantum jumps are not always completely random and spontaneous, but can be predictable and even reversible. 

 

Work Cited

Bigblueboo. “Loop Physics GIF by Bigblueboo.” GIPHY, GIPHY, 19 Jan. 2020, giphy.com/gifs/bigblueboo-physics-atom-bohr-ToMjGplMhvFmZ6GdWCI.

Faye, Jan, “Copenhagen Interpretation of Quantum Mechanics”, The Stanford Encyclopedia of Philosophy (Winter 2019 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/win2019/entries/qm-copenhagen/>.

Minev, Z. K. et al. “To Catch and Reverse a Quantum Jump Mid-Flight.” Nature 570.7760 (2019): 200–204. Crossref. Web.

Gleiser, Marcelo. “The Idea That Changed The World: 100 Years Of Quantum Jumps.” NPR, NPR, 14 Aug. 2013, www.npr.org/sections/13.7/2013/08/14/211650524/the-idea-that-changed-the-world-100-years-of-quantum-jumps.

Pitkanen, M. “Copenhagen Interpretation Dead: Long Live ZEO Based Quantum Measurement Theory!” Research Gate, www.researchgate.net/publication/335882247_Copenhagen_interpretation_dead_long_live_ZEO_based_quantum_measurement_theory

Yale University. “Physicists can predict the jumps of Schrödinger’s cat (and finally save it).” ScienceDaily. ScienceDaily, 3 June 2019. <www.sciencedaily.com/releases/2019/06/190603124621.htm>.