The physical laws of the quantum world often contradict our everyday experience. How will you explain this world to your audience?
Gerd Leuchs: In many ways, quantum phenomena are similar to experiences in normal life. For example, it has a lot to do with the superposition of waves of different frequencies, which we also experience with water waves, for example. But in the quantum world, a measurement only results in one of these frequencies. This used to be called the “collapse of the wave function”, but today it is more commonly referred to as a projection.
Many listeners will still have problems understanding this.
That’s why I’m going to use a tongue-in-cheek comparison that might help a little: you know those pictures where the brain can switch back and forth – sometimes you see two faces, then a vase. The picture contains both motifs, but when I look at it, I reduce it to one of the two.
What gives many lay people the most headaches is the entanglement of two particles – even if they are far apart, the measurement of one of the particles influences the other.
Again, I can only answer with a comparison: If I know that you always wear one red and one blue sock, and I see one of them I can predict the other. However, quantum physics goes further, it’s difficult to understand in detail – but ultimately, we don’t understand gravity either. Why do two distant masses attract each other? It’s just a concept more familiar to us.
Quantum phenomena will increasingly be used in practical applications. Can you give some examples?
There is a lot of talk about quantum computers, but so far, they only exist as prototypes with limited capabilities. The most progress has been made with gravitational waves – the laser detectors for these only achieve their maximum, almost unimaginable accuracy on a quantum scale and have already produced a number of fantastic results.
You are doing research into quantum communication. Can you explain what this is about?
Quantum communication makes the encryption of data more secure. When two people communicate with encrypted messages, they must somehow ensure that they have the same key. Today, most methods are based on the fact that it is difficult to break down large numbers into their factors. But if I send quantum signals back and forth, I can immediately determine whether someone has intercepted the signal – because in quantum physics you can’t make a measurement without changing the system.
But to do this, the receiver must receive the identical photons that the sender sent. Is this possible via the fibre optic networks that are commonly used today?
Yes, if the distance is not too great. Otherwise, you need “quantum repeaters” to amplify the signals, and these are not yet available in the fibre optic network. Or you can use a direct satellite link.
You have been a Member of the Leopoldina for almost 20 years. What memories do you have of the Academy?
My memories go back even further. My doctoral supervisor, Herbert Walther, became a Leopoldina member in the mid-1980s. The Academy, then still in the GDR, was one of the few institutions open to both East and West Germans, thus linking scientists across the Iron Curtain.
What does the Leopoldina membership mean to you?
If I had founded a company, I could measure my success in terms of profit. In science you don’t create commercial products, so success is measured by citations or prizes. And the Leopoldina membership is a great honour. What I particularly appreciate is that you meet other members who are established scientists from very different disciplines and look beyond the boundaries of your own discipline.
The Interview was conducted by Christoph Droesser