Research Topic
The hybrid approach of quantum information processing consists in the elaboration of new quantum protocols by mixing techniques and quantum states of two traditionally separated domains. Similar to what is found in the classical world, with digital and analog computing, the so-called “discrete” way of encoding information, plays on the corpuscular aspect of light, and the “continuous-encoding”, is rather based on the wave nature of photons.
Hybrid Quantum Information Processing
Hybrid Quantum Information Processing aims at overcoming the technical difficulties inherent to each of these domains, in order to achieve more efficient and less experimentally costly protocols, towards the realization of a quantum network. A hybrid quantum network could, for example, get some of its protocols running in discrete variables, where very high fidelities can be obtained, while having other protocols encoded in continuous variables, where the possibility of performing operations in a deterministic, i.e. “push-button”, way, would be a revolution and would allow for large-scale realizations.
During my Master internship, we therefore worked on a way to bridge these different domains by realizing the first “hybrid” entangled state, between continuous and discrete variables. We showed the possibility of performing this entanglement operation at a distance, through a channel with losses equivalent to 75 km of optical fiber. The experiment was based on two optical parametric oscillators, one generating squeezed vacuum states, the other generating photon pairs. A part of each of the output beams of these oscillators was taped of taken, and interfered, in an indistinguishable fashion. A single photon detection on the resulting interference signal triggered the generation of this state. These results were highlighted by the journal Nature Photonics, which used the paper for its cover.
My PhD on remote generation of quantum states
During the PhD that followed these first results, we worked on applications of this new type of entanglement, such as the remote generation of quantum states. Indeed, in a heterogeneous network framework, where different nodes operate in different ways, allowing a node to control another one, working in a different encoding, is a crucial step.
After first theoretical studies on the realization of an experiment of remote quantum state preparation and quantum teleportation, we moved to experimental practice. To do so, we had to improve the experiment: first, to make it faster, by making the detection of single photons more efficient. For that I started and maintained a collaboration with the group of Pr. Sae Woo Nam at NIST, for the creation and the optimization of a superconducting photon detector.
We demonstrated quantum detection efficiencies close to unity, at our wavelength, 1064nm, where they were previously limited to 20-40%. This allowed us to demonstrate the incredible quality of the single photon source we had.
By also working on the stability of our optical paths, in particular by working on simple, inexpensive, and robust servo techniques, we also demonstrated the generation of large squeezed optical Schrödinger cat states with very high fidelities and generation rates nearly 100 times higher than previous realizations. This work was published in Physical Review Letters.
In parallel to their very interesting modal structure, the generated states are also more robust to losses. Indeed, we showed that the decoherence of quantum states can be minimized using Gaussian operations (displacement, squeezing…), and in particular that this protocol works particularly well to protect cats from optical losses.
Quantum teleportation
Finally, we performed two experiments, the first one being the preparation of continuous quantum bits at a distance. Similar to quantum teleportation, this protocol differed in the fact that we knew in advance which state one of the nodes was wishing to generate at a distance. By performing measurements on the discrete part of the hybrid entanglement previously realized, via a homodyne detection, it was possible to generate continuous quantum bits at the other end of the optical table. We also showed perfect control over the generated qubit.
The second experiment was more fundamental and concerned the nature of the previously realized entanglement. We showed that this entanglement phenomenon violated an inequality called “Steering“, also called “Einstein Podolsky Rosen Paradox”. We thus showed for the first time that our hybrid state exhibits what Einstein called “spooky action at a distance”. This result, difficult to achieve experimentally as it requires very little optical losses and very high detection efficiencies, was a first necessary step to the implementation of quantum protocols independent of their measurement devices.
The results of these two experiments were published in Optica and Physical Review Letters.
At the end of my thesis, I also proposed a protocol to perform a DC-DC converter using a hybrid quantum teleportation protocol. For this purpose, I proposed a new type of “hybrid” Bell analyzer combining single photon detector and homodyne detection, to perform photon-number-resolved detection in an efficient way. This teleportation protocol has been complexified and adapted to perform Hybrid Entanglement Swapping between distinct and distant nodes of a quantum network. This work has been recently experimentally realized and accepted in the journal Science Advances.
Complex hybrid entangled states
Finally, we initiated the realization of new and more complex hybrid entangled states, by increasing the dimensionality and thus the possibilities of this entanglement. We multiplied the number of photon detections necessary to generate the entangled state.
I also studied how this new form of entanglement resonated with the famous Schrödinger Cat gedankenexperiment. In this experiment an atom, when de-excited, emits a poison that kills a cat. Now the atom can be in a quantum superposition state: both excited and in its fundamental state. Schrödinger thus pointed out the strangeness of this new physics model in which what is acceptable and demonstrated at the microscopic state could basically lead to ubiquitous situations where a cat would find itself being both dead and alive.
Our hybrid entanglement can also be seen as an entanglement between microscopic degrees (single photons) and macroscopic states (lasers, having different phases). By playing on the degree of squeezing of our continuous variables, we have increased in a quantum way the number of photons present in our continuous variables and studied the evolution of the generated state. We studied various criteria for measuring “macroscopicity” and showed that states of the same type as ours are good candidates for the generation of a “micro-macro” entangled state.
Associated Publications
