Physics is fascinating because of the intellectual excitement it provides and because of the applications it offers. In the Group of Applied Physics (GAP) at Geneva University we get our inspiration from both of these motivations. Optics, in this respect, has a privileged place. Indeed, in modern optics, experiments and theory progress hand-in-hand, and practical applications are close behind. Consequently, we can work both on conceptual issues and on applications. Moreover, it is a very good time for optics! The fascinating new insight about quantum mechanics brought about by recent quantum optics experiments on one side, and the tremendous development of optical communications on the other, illustrates our privileged position!
The American Research Council has recently declared optics as the technology of the 21st century. In contrast, a famous physicist, Michael Berry, has declared that the 21st century will be shaped by quantum physics, in a way similar to electrodynamics, which shaped the 20th century. Our position in GAP-Quantique, at the crossroads between optics and quantum physics, ensures our participation to both challenges.
A recent collaborative project between our group, the University of Cambridge and the National University of Singapore has succeeded in experimentally demonstrating the feasibility of secure bit commitment and has been published in the Physics Review Letters. This protocol allows at individual to lock away, or in other words commit to, some information and reveal it at some time later, without the possibility of changing it. This could be used for a fair voting or bidding system where no one should know anyone else intent before a specified time. Full security is guaranteed by exchanging quantum information between the communicating parties in addition to the exchange of classical information with distant agents, which gives rise to a secure commitment time thanks to the laws of special relativity. In our case the agents were placed in Geneva and Singapore allowing for a commitment time close to the maximum achievable on the surface of the earth. Further details of our work have been discussed in a Physics Focus article published by the American Physical Society.
A detailed and popular discussion of the future of communication technology, as well as specific parameters and capabilities of modern quantum systems are available on the Phys.org website. The principal advantages of using a solid-state quantum memory on rare-earth ion doped crystals and the prospects of their use for quantum devices are discussed. In an interview with Professor Nicolas Gisin talks about the recent achievements of our group in the direction of a quantum repeater within the framework of the European QUREP project.
Research into the strange phenomenon known as quantum entanglement - once described as 'spooky' by Albert Einstein - could revolutionise ICT over the coming years, enabling everything from ultra-fast computing to completely secure long-distance communications…Read more>>>
Quantum theory has historically been presented as the theory describing the microscopic world. However, the technological progress of the last decades is now allowing us to experimentally test the theory in different regimes. Many questions can be raised at this point, e.g. whether quantum mechanics applies at any scale, and how to caharacterize the “macroscopic”. Probing these questions using accessible tools like linear optics is a fascinating challenge.
In our letter published in Nature Physics, we experimentally demonstrate the presence of entanglement in a system involving “macroscopic” states of light. The number of photons (order of 500) is large enough to be easily seen by the human eye (provided that the wavelength is in the visible spectrum!) and the two macro components of the state can be efficiently distinguished using “classical detectors”, i.e. detectors that only resolve large photon number differences . This makes the state analogous to a Schrödinger cat state. Starting by generating heralded single photon entanglement between two spatially separated optical modes, we subsequently displace one of these modes towards the macroscopic domain. A final displacement back to the single photon regime allows us to measure a well-defined entanglement witness and set a lower bound on the concurrence (a measure of entanglement) .
Our results, also discussed in a News & Views article published in the same issue of Nature Physics, highlight the idea that although observing macroscopic entanglement with coarse-grained measurements is very challenging, the creation of quantum macro systems can be straightforward. This suggests that quantumness is a concept that extends into our macroscopic world and provides us with renewed motivation to look for quantum effects in Nature.
In our first paper published in the all new Physical Review X journal, we propose a novel approach to device-independent quantum key distribution (DIQKD). This family of protocols enables the generation of a secret key via the violation of a Bell inequality, but crucially does not require precise models for the devices used in the protocol. This fact makes DIQKD the ultimate solution in terms of security against device flaws and side-channel attacks. A drawback in existing DIQKD protocols is the requirement of a two party Bell test, which opens up the detection loophole as soon as a realistic distance (channel loss) is introduced between the two users. In this work, we propose a way of circumventing this with a Bell test carried out by only one party, enabling DIQKD over longer distances.
Congratulations to Dr. Christoph Clausen (pictured center of photo) for defending his doctoral thesis in February. The work involved the development of a source of entangled photons which could interact with solid state crystals acting as a quantum memory. This lead to a demonstration of entanglement transfer to the crystals followed by successful retrieval of the quantum information. This opens the door to a possible quantum repeater in the future. His thesis is available from our website.