Quantum computation could one day make it possible to perform certain tasks that are impossible for a classical computer. One emblematic example is the factorization of very large integers, which plays a central role in cryptography. One of the reasons why today's quantum processors are not yet capable of this is that qubits (quantum analogues of bits in classical computing) and operations on these qubits still have error rates that are too high. Qubits are not perfectly isolated, and interactions with their environment are responsible for the phenomenon of decoherence, which tends to destroy the very fragile quantum superpositions that are essential to the functioning of quantum algorithms.
In a work published in January 2024 [1] researchers from IBM Quantum and IPhT have shown that, for a quantum processor based on superconducting qubit technology, it is possible to accurately model these phenomena. The study combined the results of experiments carried out on an IBM quantum processor accessible via the cloud with numerical simulations. These simulations take into account various noise terms, but also parasitic interactions between the qubits, as well as interactions with certain electrostatic charges surrounding them. To incorporate the effects of correlations between qubits, the study used specific states called graph states, which exhibit quantum entanglement. The resulting model accurately and quantitatively describes the dynamics and loss of quantum coherence of the qubits over time, and provides detailed information on both the action of the environment and unwanted interactions between the qubits. This work opens the way to better control of these systems.
The Laser Interferometer Space Antenna (LISA) [2], expected in 2035, will be a largescale space mission designed to detect one of astronomy's most elusive phenomena: gravitational waves, which are oscillations in spacetime propagating at the speed of light and which, according to the theory of general relativity, can be generated by, for example, a system of two merging black holes or two neutron stars. LISA will be the first spacebased gravitational wave observatory, and will consist of three satellites separated by 2.5 million km in a triangular formation, following the Earth in its orbit around the Sun. 
The Institut de Physique Théorique (IPhT) joined the LISA mission in 2018, when some of its researchers became associate members of the LISA Consortium, which is a vast international collaboration that combines the resources and expertise of scientists from many countries around the world. Together with ESA, its member states and NASA, the LISA consortium is working to bring the LISA mission to fruition.
As a theoretical physics laboratory, the IPhT has a diverse range of expertise, from mathematical physics and string theory to theoretical particle physics and cosmology. In recent years, gravitational waves have become a common denominator for many of these disciplines. Recent activities on gravitational waves at the IPhT, which play a major role in the international scientific community, include : the study of the twobody problem in the "postMikowskian" expansion using modern scattering amplitude techniques; the study of the twobody problem in extensions of general relativity using effective theory techniques; the derivation of constraints on dark energy and modified gravity models from gravitational wave propagation; the study of quantum effects at the black hole horizon.
All these activities involve exploring specific theoretical aspects relevant to the LISA mission, with the aim of gaining a deeper understanding of the twobody problem and, consequently, of waveform predictions from general relativity, and/or exploring new fundamental physics. In this context, some IPhT researchers are active in the LISA consortium's "Fundamental Physics" and "Cosmology" working groups, and have contributed to recent papers that define the future direction of their research [3], [4].
[1] ESA announcement
[2] LISA Consortium: lisamission.org
[3] Prospects for fundamental physics with LISA (2020)
[4] New horizons for fundamental physics with LISA(2022)
CNES announcement
Other announcements: CNRS ; IRFU


Figure a: The novel doubled axion insulator (TDAXI) phase of matter uncovered in this work can be constructed as an axion insulator (AXI) per electron "spin." Its surface exhibits an exotic quantum spin Hall response (C^{s}) consisting of a fractional quantum Hall state (C^{+} or C^{}) per spin.  Figure b: The quasi1D insulator bismuth bromide was discovered by the team as a material realization of a TDAXI 
Following pioneering experiments on elemental bismuth performed by the Meso Group at the LPSOrsay here at Saclay (https://www.nature.com/articles/s4156701802247), robustly metallic 1D hinge states have been observed in a number of candidate TCIs. However, recent theoretical advances and further experiments have called into question whether hingestate conduction truly represents a "smoking gun" signature of a TCI, or merely an interesting  but extrinsic  effect.
In a new study coled by Benjamin Wieder (IPhT), an international team has discovered new 3D bulk and 2D surface signatures of TCI states to bring clarity to this problem. By focusing on the spin degree of freedom, the team found via extensive numerical calculations that the insulating 2D surfaces of TCIs are not featureless as previously assumed, but rather contain a novel  and unremovable  pattern of electronic motion consisting of counterpropagating circular orbits that vary based on the "spin" of each electron. The team in particular found that this pattern is distinct from the behavior of electrons in all other known 2D insulators with weak electronelectron interactions, and can be measured via optical experiments. Lastly, the team identified the readily accessible material bismuth bromide as a TCI hosting this novel 2D surface phase, providing hope that the theoretical findings of this study will be experimentally validated within the near future.
This paper is published in Nature Communications.
To know more:
https://www.eurekalert.org/newsreleases/1031456
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