The formation of quantum entanglement is one of the fastest processes in nature. With a few tricks, this phenomenon can be studied on the attosecond scale. This is what Prof. Joachim Burgdörfer’s team from the Institute of Theoretical Physics at TU Wien has discovered. Together with colleagues from China, the physicists are developing computer simulations that can be used to understand ultrafast processes. This makes it possible to investigate how quantum entanglement arises on a time scale of attoseconds. The results have now been published in the journal “Physical Review Letters”. Joachim Burgdörfer reports on these results here.
Mr. Burgdörfer, can you explain the phenomenon of quantum entanglement?
In quantum physics, systems with several particles are described by a common wave function that treats the system as a single entity. To put it simply: the particles have no individual properties, they only have common properties. They belong together, even if they are spatially far apart. In short: they are quantum entangled with each other.
How do experiments with entangled quantum particles work?
As a theorist, my expertise in this regard is naturally rather limited. In any case, the most important thing is that the system must be disturbed as little as possible in order to preserve quantum entanglement in the experiment. Any interaction with the environment can break quantum entanglement and lead to the so-called “collapse” of the wave function. Preserving quantum entanglement and coherence for as long as possible is one of the challenges one faces if one wants to apply it, for example in the field of quantum cryptography and quantum information.
Now you are a theoretical physicist. How do you approach open questions of quantum entanglement theoretically?
For us, the central task is to calculate the wave function (for experts: the density matrix) of the many-body system as precisely as possible. Then the possible quantum entanglement is also encoded in the wavefunction and its influence on measured observable can be calculated and predicted.
In your latest work, you have now shown theoretically how quantum entanglement works. Could you explain this in more detail?
In time-dependent systems, entanglement, just like other physical quantities, is not constant but can vary as a function of time. Photoionization of atoms by ultrashort attosecond pulses provides a good example of this. The outgoing wave packet of the ionized electron is formed on the attosecond scale. If other electrons in the atom change their state simultaneously during ionization, this can lead to the formation or change of quantum entanglement, and we were able to simulate this in detail.
How fast does such quantum entanglement take place?
For the example of photoionization of helium atoms, we were able to show that quantum entanglement changes on the same ultrafast time scale of attoseconds on which the formation of the wave packet of photoemission itself takes place. In our case, we were able to determine changes in the “birth time” of the wave packet. These variations of the time delay caused by quantum entanglement (more precisely, by its change) amount to around 230 attoseconds.
A few words about your team. What is the focus of your research?
The overarching theme of our research is time-dependent quantum dynamics. In addition to electron dynamics on the attosecond scale, we explore in this context also the interaction of electromagnetic pulses and charged particles with solids as well as the development of new theoretical methods for describing time-dependent many-particle systems.
Are there opportunities to become part of your team?
Of course! Enthusiastic young talents are always welcome.
For anyone who is interested in going to Vienna, what is it like to live in Vienna?
Vienna is currently a fast-growing metropolis that offers a wide range of attractive cultural and leisure activities and ranks among the world's top cities in terms of quality of life. Of course, growth and international appeal have also their downside, e.g. the housing situation for newcomers is not always easy.
Interview: Thorsten Naeser
Original publication:
Time Delays as Attosecond Probe of Interelectronic Coherence and Entanglement
Wei-Chao Jiang, Ming-Chen Zhong, Yong-Kang Fang, Stefan Donsa, Iva Březinová, Liang-You Peng, & Joachim Burgdörfer
Phys. Rev. Lett. 133, 163201
Illustration: Wei-Chao Jiang