**April 2, 2015**

Today we have a new paper out on the arXiv, on entanglement generation in a quantum thermal machine: https://arxiv.org/abs/1504.00187

Entanglement is a phenomenon which is at the heart of quantum physics and distinguishes it from the everyday world of classical physics. Particles that are entangled behave, in a sense, as if they are a single entity even when separated and manipulated independently. Entanglement gives rise to correlations that are stronger than any which can be found in classical physics (as I have mentioned before, e.g. here), and it is a prerequisite for many applications such as quantum enhanced sensing (which I have also written about before) and of course quantum computing.

In our everyday experience, we do not notice these curious correlations. Quantum entanglement and coherence is fragile – when small quantum systems interact with a noisy environment, they are quickly messed up and become impossible to observe. Because of this, we usually think of entanglement as something that has to be carefully protected by isolating our quantum systems as much as possible from the surroundings and, in many cases, keeping them very cold. This is one of the reasons that building a quantum computer is really tricky. Much of the hard work that go into experiments is about achieving this high degree of isolation from noise, to keep the systems coherent long enough to do something fun with them.

Could it be, however, that there are hot, noisy systems where entanglement nevertheless survives? Maybe even where the noisy environment helps generating the entanglement? There are indications that quantum effects play a role in biological processes such as photosynthesis, where the systems are complex and definitely not well isolated from the environment. So perhaps it should be possible.

It turns out that the answer is yes: there are indeed noisy system where entanglement is stable. In our paper we describe such a system. We are not the first to ask these questions, or to show that entanglement can be generated by noisy processes. But we do have a very neat example, where the system is really stripped down to the bare essentials. It consist of just two quantum two-level systems (qubits), coupled to each other and to two thermal baths at different temperatures. This setup can be understood as a small thermal machine operating out of thermal equilibrium. Heat flows in from the warmer bath, through the qubits, and out into the cold bath. There is no other external control or sources of coherence, but nevertheless we show that the steady state of the qubits is entangled, when the couplings and temperatures are right. So our thermal machine maintains entanglement just by interaction with a noisy, thermal environment. Perhaps similar processes take place in biological systems?

Because the machine is so simple, it looks promising to realise experimentally as well. In the paper we suggest implementations using either superconducting flux qubits, or a double quantum dot. Both of these are systems which have already been studied a lot, and where high degree of control has been achieved. So we hope some of our experimentalists colleagues will be interested in giving it a try. Anyone?

**Published paper:**** **https://iopscience.iop.org/article/10.1088/1367-2630/17/11/113029