From the dawn of agriculture until the industrial revolution, all over the world, human beings have been facing the problem of food preservation. We are now quite familiar with many techniques, most of which utilised in our own kitchens, to reach this fundamental goal for our existence on Earth. Efficient and very employed procedures are, for instance: drying, salting, smoking, cooling and freezing. Let us focus on the last one, which works well for a very wide variety of foods. We are aware that, in order to preserve foods for long times by freezing, our freezers and refrigerators must be able to maintain temperatures well under zero Celsius degrees, typically −18° C or below (0° Fahrenheit or below). Air at the poles of our planet would be an extremely efficient freezer for foods, although it is not a very pleasant environment where to live (the average temperatures at North Pole and South Pole are, respectively: 0° C (32° F) and −28.2° C (−18° F) during summer; −40° C (−40° F) and −60° C (−76° F) during winter). There is therefore a continuous technological development in engineering efficient and eco-friendly freezing machines to assure a trustable and lasting food preservation. If someone comes and tells us that it is possible to preserve food by freezing at room temperature we wold not believe them, unless we are in front of Marvel’s Iceman (see picture aside).
Nevertheless, as we highlighted in a recent experiment (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.160402) following a theoretical prediction (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.210401), there is something very precious at the very fundamental level of matter that is possible to freeze, in principle, even at room temperature: this is quantum coherence. Quantum coherence represents the wavelike nature of matter and the essence of quantum parallelism, that is the possibility for a very little system (at atomic scale, nanometers, and below) to be simultaneously in different states. During the last years, quantum coherence has been shown to constitute the primary ingredient enabling the development of quantum technologies for commercial applications to networked secure communication, computing, imaging, sensing, and simulation. The performance of quantum devices in making these tasks greatly supersedes the ordinary classical technology currently available. There is thus great interest in the practical use of quantum coherence other than its fundamental value. But, as often happens when one wants to get an ambitious goal, there are challenging obstacles to be overcome. In particular, one of the biggest problems towards the reliable exploitation of this quantum resource is the unavoidable adverse environmental condition. This is not to be meant as “bad weather” but rather as the destruction of quantum coherence due to the interaction between the quantum system and its surrounding environment, a phenomenon known as decoherence.
This decoherence is one of the interpretations scientists give of the fact that we do not observe quantum parallelism at macroscopic level. Putting it in a Schrödinger-like fashion, the cat in the box is either alive or dead, it is not alive and dead: the interaction with the environment destroys the possibility of the simultaneous existence of the two “states” for the cat in extremely small and imperceptible times. This is applicable to all macroscopic “classical” objects. At the microscopic level, one can think of an assembly of quantum bits (qubits), which are two-state systems (e.g., atoms, photons, nuclear spins) which can live in their 0 and 1 states at the same time thanks to the quantum mechanical laws. Although quantum coherence between the possible collective states is observable at this scale, it typically lasts for only a fraction of a second before decoherence destroys it. This means that if we want to use quantum coherence for next generation inviolable communications and super-fast computing, we must design and engineer efficient strategies to maintain it as long as possible. Researchers around the world have developed methods to slow down or correct the effects of decoherence but these methods are generally very demanding, requiring external precise controls of the quantum evolution, adjunctive quantum devices and particular structured environments. Differently from these approaches, our study shows a natural mechanism of compound quantum systems to contrast decoherence without any external intervention such that, under suitable conditions, quantum coherence remains unaltered, frozen, during the evolution. Great Scott!, Dr. Emmett “Doc” Brown would say.
Let us now go a little deeper, but shortly, into the story which led us to obtain this result. A story of a group of people meeting in different places at different times to “coherently” discuss the main aspects of the theory and device the experiments. As usual in science, collaboration among people is essential: a sort of human coherence, linking people from Nottingham (UK), São Carlos and Rio de Janeiro (Brazil), Munich (Germany) and Palermo (Italy).
The hard core of the theoretical study and prediction about the phenomenon of frozen quantum coherence was established in Nottingham. Previous works on universal freezing of quantum correlations in compound quantum systems and the introduction of suitable quantifiers of coherence opened the way to the discover that quantum coherence in a system made of an even number of qubits can be in principle maintained constant and equal to its initial value during a non-dissipative evolution. This is thus achievable, once prepared a suitable initial state, whenever there is no energy exchange between the system and the environment, provided that the quantum coherence is measured along a direction which is perpendicular to the direction where the decoherence naturally acts. In fact, the nature of the system-environment interaction creates a given way of action for decoherence, which destroys every coherence it finds along this way, let us say along a z axis. To give a pictorial representation, one can see the system made of qubits, where each qubit has two basis states represented by two arrows (spins) pointing up
or down along the z direction. The quantum coherence of the system is then observed along the x direction, by aligning detectors sensitive to arrows pointing along the x axis. And, very importantly, the coherence is predicted to be frozen independently of the measure employed to quantify it: in this sense, the phenomenon is universal. In the picture, you can see Gerardo (right) close to me, along a street of Nottingham, making a call to the labs having this successful “observation gimmick” in mind: our tricky happy faces are quite evident!
It is now the turn of our visit to the labs in Brazil and the planning of the experiment there. It was August 2014 when Gerardo and I went to the Institute of Physics in São Carlos (Universidade de São Paulo) to meet our colleagues there, who soon became great friends. In the picture below, you can see Diogo (right), Gerardo (center) and me (left) smiling during a discussion about the main theoretical aspects of the experiment in nuclear magnetic resonance, who would have reproduced the predictions on freezing coherence, with the background company of the magic music by Vinícius de Moraes and Antônio Carlos Jobim. This picture represents, in my opinion, a very good synthesis of the lovely and funny atmosphere of that time which certainly contributed to the success of the story. The experiment involved people from São Carlos but also from Rio de Janeiro, working at Centro Brasiliero de Pesquisas Físicas (CBPF). Therefore, we spent an entire week in Rio to meet our colleagues at CBPF and follow the work in the lab.
Rio de Janeiro: what an amazing and unique city! I will never forget the landing by airplane there. The pilots introduced Rio to us by a slow spectacular flight above the city, allowing us to immediately appreciate the beauty of houses nested in a lush green nature and vegetation gently fading out into the blue Atlantic ocean, from which small islands and green mountains emerge like sweet fingers greeting the people looking at them. The statue of Christ The Redeemer on the top of Corcovado really gives the feeling of embracing all the city, giving an eye to the Sugarloaf mountain (Pão de Açúcar). It was August, which means summer in Europe, winter in Brazil. But it is universally known that Rio has only two seasons: summer and hell! Therefore, we enjoyed summer at that time, moving from Copacabana beach to downtown samba events. I had the opportunity to go to the Pedra do Sal (Rock of Salt), a historical site where samba is played and sang by musicians around a table with the company of hundreds of people, Carioca and tourists as well: amazing, moving and poetical. Thank you Rio for that night! Of course, being those days mainly dedicated to our scientific research, we had the luck to understand that also nucleuses of carbon and hydrogen danced bossa nova subject to nuclear magnetic resonance (NMR) to assembly themselves into the desired quantum states. During our visit to CBPF, where you can see a picture aside with Roberto (right), Gerardo (left) and me (back), the guys of the lab gave us the good news that they would have been able to implement the theoretical method using a setup made of a simple Chloroform sample labelled with Carbon-13, to encode a two-qubit system in 1H (hydrogen) and 13C (carbon) nuclear spins. The systems are naturally affected by dephasing noise (the most common type of non-dissipative environmental noise), so that once the desired initial quantum states are prepared, the freezing of quantum coherence can be automatically observed, without external control.
The German contribution then enters the game with the aim to go further and prove the occurrence of this counterintuitive phenomenon in larger quantum systems. Thanks to our “globetrotter” researcher Isabela, who traveled a lot to connect all the involved people by an effective “human coherence”, the lab at the Technische Universität München was successful in increasing the number of qubits from two to four. This four-qubit system was a heteronuclear sample developed in Steffen Glaser’s group, manipulated by a prototype NMR probe. In particular, the four qubits were encoded in 1H, 13C, 19F (fluorine) and 31P (phosphorus) nuclear spins. Both Brazilian and German setups demonstrated long-lived quantum coherence involving room-temperature liquid-state NMR quantum simulators. Thus, freezing coherence at room temperature is possible, opening up further research on exploiting this resource and understanding its possible role in biological complexes! Thank you Isabela, Alexandre, Tom, Marco, Raimund, Roberto, Ivan, Steffen, Eduardo, Diogo and Gerardo for the great collaboration.
I would like to conclude this story by pointing out the following message: it is often by changing the viewpoint that striking results may surprisingly show up. They are there, Nature is kind of willing to bring them to human knowledge, but one needs to find the right perspective to see them! This concept may be illustrated by a picture I made during my visit at the wonderful Iguazù Falls, at the border among Brazil, Argentina and Paraguay: the beauty of Nature is just there to be seen and appreciated by the proper sensitive eyes!