Welcome to the Quantum Optics Theory Group

Our research is focused on quantum optics, i.e. the application of quantum theory to phenomena involving light and its interaction with atomic and mechanical systems. The tools of quantum optics, combined with those of quantum information sciences, are providing new and unexpected possibilities that we use for both applied physics and fundamental research. Most of our studies are done in close collaboration with leading experimental groups.

Research Events

06 May 2019

Please join us in congratulating Alexey Melnikov for winning the Cozzarelli Prize in the physical and mathematical sciences category. This award acknowledges Melnikov et al. "Active learning machine learns to create new quantum experiments", a paper published in PNAS, in which reinforcement learning is used to explore a space of possible quantum experiments in order to find one (or more) design fulfilling a specific aim. This price was delivered personally to Alexey and his coauthors during a ceremony in Washington D.C.. Alexey Melnikov appeared recently on national russian television news, in which he talks (Russian) about his prize.

21 Dec 2018

Bell tests provide certification techniques that are device-independent, that is, they do not rely on a detailed description of the hardware. In a recent article published in Physical Review Letter, we showed how to certify the quality of specific type of joint measurements, namely Bell-state measurements, device independently. Our certificate is noise-tolerant, hence opening a way for an experimental realisation. These results might play an important role for certifying the building blocks of future quantum networks.

05 Nov 2018

Please join us in congratulating Roman Schmied, Jean-Daniel Bancal, Baptiste Allard, Matteo Fadel, Valerio Scarani, Philipp Treutlein and Nicolas Sangouard for winning the Paul Ehrenfest Best Paper Award for Quantum Foundations 2017. The committee awarded the prize to Schmied et al. “for the certification of Bell nonlocality in a many-body quantum system composed of hundreds of atoms, thereby confirming the presence of quantum effects at the mesoscopic scale”. Some of the authors were present to receive the award in Vienna, as shown in the photograph. The prize-winning work can be found here.

02 Nov 2018

The field of quantum computing has made great strides in the past decade or so. As the field continues to proceed towards performing quantum-enabled computation at increasing scale and precision, certification of the constituent ingredients of a quantum computer becomes increasingly crucial. In this work we provide a method based on Bell’s theorem to certify coherent operations for the storage, processing and transfer of quantum information. This completes the set of tools needed to certify all the building blocks of a quantum computer. These results have been published in Physical Review Letters, and have been highlighted by the SNSF and also in the Department website.

12 Jul 2018

There have been intense efforts in recent years in the field of cavity optomechanics, resulting in impressive levels of control of mechanical systems with light. For example, optomechanical systems in the weak coupling and resolved sideband regime can be driven so that they produce correlated mechanical and photonic excitations. As a first step towards certifying the use of these systems for quantum networks, we propose a witness for optomechanical entanglement suited for these states. While we require a detailed description of the measurements and settings, we do not require prior assumptions on the state description, and find that our proposed witness is robust to inefficiencies. This witness provides a path towards a future device-independent characterisation of optomechanical entanglement. These results have been published in Physical Review Letters.

5 Jun 2018

When light interacts with a crystal or a molecule, it can transfer energy to and from microscopic vibrations of their constituent atoms. The total energy is conserved, so that the difference in energy between the incoming and outgoing light equals the vibrational energy deposited in or recovered from the material. Prof. Galland from Lausanne and Prof. Sangouard have developed a new technique to measure in real time the creation and destruction of individual quanta of vibrational energy (phonons) using ultra-short laser pulses and single photon detectors. They recorded the birth and death of single phonons in diamond and their technique can be applied to many different Raman active vibrational modes opening the way for detecting exotic vibrational quantum states such as entangled states where energy is delocalised over several vibrational modes. These results have been published in Physical Review Letters and selected as an Editor’s suggestion.

31 May 2018

Quantum theory is by far our best description of the microscopic world. Surprisingly, there is nothing in the theory preventing macroscopic systems to be in quantum states. In principle, a cat could be dead and alive at the same time. Dr. Sekatski, Prof. Sangouard, and collaborators from Innsbruck and Geneva, have written a review article presenting recent results on the difficulties and prospects of creating, maintaining, and detecting macroscopic quantum states. This review, which outlines the role of macroscopic quantum states in foundational questions as well as practical applications, has been published in the prestigious journal Review Modern Physics and has been chosen for the cover of the next issue.

27 Oct 2017

Quantum theory predicts that two or more spins can be entangled so that they loose their individual properties and are described as a whole only. It is however challenging to detect this quantum property when it is shared between many spins. In a recent project driven in collaboration with the Treutlein group in Basel, we have shown in particular that a large number of experimental runs are required to convincingly conclude that quantum correlations are present in a large number of spins. These results have been published in Physical Review Letters.

25 Nov 2016

Quantum networks are essential in creating quantum correlations across widely separated nodes, and there has thus been intense activity across many groups to realise their widespread implementation. We have recently investigated the ability to produce tunable single photons -- a crucial step to interface various systems and to design hybrid architectures aiming to make the best use of these systems. We reported on our collaboration with ICFO in Nature Communications, where we have demonstrated a tunable source of single photons from a cold atomic ensemble, with a tunability of the photon duration across more than 3 orders of magnitude up to 10 microseconds.
This work has been featured in Opli, and also in the Department website.

11 May 2016

So far, quantum mechanics has been able to predict the behaviour of microscopic systems with unprecedented accuracy. Intensive efforts are devoted to the study of quantum phenomena at the macroscopic scale. In a recent publication in Physical Review Letters, we report on the storage of photonic entangled states with increasing size in an ensemble of atoms trapped in a solid, clearly demonstrating micro-macro light-matter entanglement.

3rd May 2016

Tremendous progress has been realized in quantum optics for engineering and detecting the quantum properties of light. Today, photon pairs are routinely created in entangled states. Entanglement is revealed using single-photon detectors in which a single photon triggers an avalanche current. The resulting signal is then processed and stored in a computer. We have proposed an approach to get rid of all the electronic devices between the photons and the experimentalist i.e. to use the experimentalist’s eye to detect entanglement.
These results have just been published in the latest issue of Optica, and have been highlighted by several media and scientific blogs already, including by the MIT Technology Review or Phys.org. The work has also been covered by the University News, and also in the Department website.

22nd Apr 2016

The microscopic world is governed by the rules of quantum mechanics, where the properties of a particle can be completely undetermined and yet strongly correlated with those of other particles. Physicists from the University of Basel have observed these so-called Bell correlations for the first time between hundreds of atoms. Their findings are published in the scientific journal Science.
This work has also been covered in the media in Phys.org, the University News, and also in the Department website. See our publications page for more links.

13th Apr 2016

The John Templeton Foundation is a US based entity that serves as a philanthropic catalyst for discoveries in a large number of fields, including results leading to a deeper understanding of the nature of reality within the realm of quantum theory, cosmology, astronomy, or chemistry. It provides research grants for projects that are unlikely to be supported by conventional funding sources.
The Foundation has recently formalized a generous grant for a project led by Professor Nicolas Sangouard, entitled Is the human eye able to see entanglement?.

18th Feb 2016

While small systems like single atoms and photons have been widely used as tests of quantum theory, recent experiments have been implemented to test quantum predictions with more massive systems. This includes the detection of electromechanical entanglement or non-classical correlations in optomechanical systems. These results have been obtained under strong assumptions on the functioning of the measurement devices and/or on the mechanical systems. In a recent paper published in Physical Review Letters, we have shown how to perform a Bell test in optomechanical systems which allows one to certify the quantum behavior of optomechanical systems device-inpendently. Given recent experimental results where photon counting techniques are used in optomechanical systems, our proposal might be implemented sooner than expected!

28th Sep 2015

What are the quantum states analogous to the Schrödinger cat states? A superposition of coherent states with opposite phases seems a likely candidate, since a coherent state is the quantum state of light closest to a classical state. By extending several macroscopicity measures to multimode continuous variable states, we have shown that the size of two-mode squeezed states is comparable to superpositions of coherent states while being much simpler to create in practice. Furthermore, we have found strong requirements on the precision of the measurements to reveal the quantum features of these states.
Our paper, Two-mode squeezed states as Schrödinger cat-like states has been published in the Journal of the Optical Society of America B and has been featured in a Spotlight by the OSA.

1st Jul 2015

In the July/August issue of the French media « La Recherche » focusing on quantum revolutions, Professor Nicolas Gisin and Nicolas Sangouard have been commissioned to write an article on quantum theory at macroscopic scales. They discuss various definitions of the notion of « macroscopic », present experiments aiming to create Schrödinger’s cat like states, and describe the difficulty to reveal the quantum nature of macroscopic states. The text, which is written in French, is made to be accessible to a lay audience.

4th May 2015

How can one detect entanglement between multiple optical paths sharing a single photon? We have addressed this question by proposing a scalable protocol, which only uses local measurements, does not require postselection, nor assumptions about the photon number in each path. Furthermore, it guarantees that entanglement lies in a subspace with at most one photon per optical path and reveals genuinely multipartite entanglement. In collaboration with the group of Prof. Zbinden and Dr. Thew, we have demonstrated its scalability and resistance to loss by performing various experiments with two and three optical paths. These results have been published in Physical Review Letters. We anticipate applications of our results for quantum network certification.
Our paper, Revealing Genuine Optical-Path Entanglement has been published in Physical Review Letters.

22nd Oct 2014

Harnessing nonlinearities strong enough to allow single photons to interact with one another is central to numerous advanced applications in quantum information science. We took the first step along this line by exploiting a parametric process in a nonlinear crystal. By benefiting from their inherently large bandwidth to work with pulsed systems at high repetition rates, we have shown in collaboration with the group of Prof. Zbinden and Dr. Thew at Geneva, the first non-linear interaction between two true independent single photons (Fock states).
Our paper, Nonlinear Interaction between Single Photons has been published in Physical Review Letters and selected for a Viewpoint in Physics. The paper was also chosen for an Editor's Suggestion.

22nd Sept 2014

Can a cat be simultaneously dead AND alive? Surprisingly, quantum theory says yes. This naturally raises the question: Why don't we observe such cats in our everyday life? In collaboration with Dr. Sekatski from Innsbruck (Austria), and Prof. Gisin from Geneva, we have shown quantitatively that macroscopic quantum states are basically indistinguishable from classical states if they interact with their environment, even very weakly or if they are observed with detectors with coarse-graining. These results give us insights into the reasons that make the observation of quantum behavior hard in macroscopic systems and might find important applications e.g. in quantum metrology.
Our paper, How Difficult Is It to Prove the Quantumness of Macroscopic States? has been published in Physical Review Letters, and showcased in the department website.