Monitored quantum systems undergo Measurement-induced Phase Transitions (MiPTs) stemming from the interplay between measurements and unitary dynamics. When the detector readout is post- selected to match a given value, the dynamics is generated by a Non-Hermitian Hamiltonian with MiPTs characterized by different universal features. Here, we derive a partial post-selected stochastic Schrodinger equation based on a microscopic description of continuous weak measurement. This formalism connects the monitored and post-selected dynamics to a broader family of stochastic evolution. We apply the formalism to a chain of free fermions subject to partial post-selected monitoring of local fermion parities. Within a 2-replica approach, we obtained an effective bosonized Hamiltonian in the strong post-selected limit. Using a renormalization group analysis, we find that the universality of the non-Hermitian MiPT is stable against a finite (weak) amount of stochasticity. We further show that the passage to the monitored universality occurs abruptly at finite partial post-selection, which we confirm from the numerical finite size scaling of the MiPT. Our approach establishes a way to study MiPTs for arbitrary subsets of quantum trajectories and provides a potential route to tackle the experimental post-selected problem.
LAST-MODIFIED:20241010T150215Z SEQUENCE:14690645 X-ACCESS:1 X-HITS:724 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20240620T140000 DTEND;TZID=Europe/Paris:20240620T150000 UID:9D0DB262-98CC-498A-98DE-BA8398214E6B SUMMARY:Séminaire TQM: Nicolas Bergeal (ESPCI) CREATED:20240604T140256Z DTSTAMP:20240604T140256Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/nicolas-bergeal-espci DESCRIPTION:Superconducting oxides interfaces\NThe achievement of high-quality epitaxial interfaces involving transition metal oxides offersa unique opportunity to design artificial materials that host novel electronic phases. The discoveryof a high mobility two-dimensional electron gas (2-DEG) confined in a quantum well at theinterface between two insulating oxides LaAlO3 and SrTiO3 is perhaps one of the most prominentexamples in the field[1]. Unlike more conventional semiconductor based quantum wells,conducting electrons at LaAlO3/SrTiO3 fill 3d-bands, which gives a favourable ground for theemergence of complex electronic phases. In particular, 2D superconductivity [2,3] and strongRashba spin orbit coupling [4] have been reported in such interfaces. More recently, the discoveryof a superconducting 2-DEG in (111)-oriented KTaO3-based heterostructures injected newmomentum into the realm of oxide interfaces [5,6]. In this system, the superconductingTc can exceed 2K, nearly an order of magnitude higher than that observed in SrTiO3-basedinterfaces. Additionally, the increased mass of Ta compared to Ti leads to significantly enhancedspin-orbit effects, as recently demonstrated [7]. Consequently, KTaO3-based 2-DEGs have thepotential to enable the realization of topological superconducting phases—a concept originallyproposed for SrTiO3-based 2-DEGs but hitherto unattainable due to the limitations of the relevantenergy scales.\NA key feature of these electronic systems lies in the possibility to control their carrier densityby electric field effect, which results in gate-tunability of both superconductivity and Rashba spin-orbit coupling. In this talk, I will review complementary dc and microwave transport measurementsconducted on SrTiO3 and KTaO3-based interfaces employing both back-gate and top-gateconfigurations. I will discuss, in particular, gate-induced multigap superconductivity [8,9] and therole of phase fluctuations within the Berezinskii-Kosterlitz-Thouless model [10]. I will also presentthe realization of field effect devices whose physical properties, including superconductivity andRashba spin-orbit coupling, can be tuned over a wide range of electrostatic doping, and discuss thepotential of oxides interfaces for the realization of mesoscopic devices [11].\N[1] A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004).[2] A. Caviglia et al., Nature 456, 624–627 (2008).[3] J. Biscaras et al., Nature Communications 1, 89 (2010).[4] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010).[5] C. Liu, et al. Science 371, 716–721(2021).[6] Chen, Z. et al. Science 372, 721–724 (2021).[7] Vicente-Arche, L. M. et al. Adv. Mater. 2102102 (2021).[7] S. Varotto, et al. Nature Commun. 13, 6165 (2022).[8] G. Singh et al., Nature Mat. 18, 948–954 (2019).[9] G. Singh et al., Phys. Rev. B 105, 064512 (2022).[10] Mallik et al. Nature Commun. 13, 4625 (2022).[11] A.Jouan et al. Nature Elec. 3, 201–206 (2020). X-ALT-DESC;FMTTYPE=text/html:The achievement of high-quality epitaxial interfaces involving transition metal oxides offers
a unique opportunity to design artificial materials that host novel electronic phases. The discovery
of a high mobility two-dimensional electron gas (2-DEG) confined in a quantum well at the
interface between two insulating oxides LaAlO3 and SrTiO3 is perhaps one of the most prominent
examples in the field[1]. Unlike more conventional semiconductor based quantum wells,
conducting electrons at LaAlO3/SrTiO3 fill 3d-bands, which gives a favourable ground for the
emergence of complex electronic phases. In particular, 2D superconductivity [2,3] and strong
Rashba spin orbit coupling [4] have been reported in such interfaces. More recently, the discovery
of a superconducting 2-DEG in (111)-oriented KTaO3-based heterostructures injected new
momentum into the realm of oxide interfaces [5,6]. In this system, the superconducting
Tc can exceed 2K, nearly an order of magnitude higher than that observed in SrTiO3-based
interfaces. Additionally, the increased mass of Ta compared to Ti leads to significantly enhanced
spin-orbit effects, as recently demonstrated [7]. Consequently, KTaO3-based 2-DEGs have the
potential to enable the realization of topological superconducting phases—a concept originally
proposed for SrTiO3-based 2-DEGs but hitherto unattainable due to the limitations of the relevant
energy scales.
A key feature of these electronic systems lies in the possibility to control their carrier density
by electric field effect, which results in gate-tunability of both superconductivity and Rashba spin-
orbit coupling. In this talk, I will review complementary dc and microwave transport measurements
conducted on SrTiO3 and KTaO3-based interfaces employing both back-gate and top-gate
configurations. I will discuss, in particular, gate-induced multigap superconductivity [8,9] and the
role of phase fluctuations within the Berezinskii-Kosterlitz-Thouless model [10]. I will also present
the realization of field effect devices whose physical properties, including superconductivity and
Rashba spin-orbit coupling, can be tuned over a wide range of electrostatic doping, and discuss the
potential of oxides interfaces for the realization of mesoscopic devices [11].
[1] A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004).
[2] A. Caviglia et al., Nature 456, 624–627 (2008).
[3] J. Biscaras et al., Nature Communications 1, 89 (2010).
[4] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010).
[5] C. Liu, et al. Science 371, 716–721(2021).
[6] Chen, Z. et al. Science 372, 721–724 (2021).
[7] Vicente-Arche, L. M. et al. Adv. Mater. 2102102 (2021).
[7] S. Varotto, et al. Nature Commun. 13, 6165 (2022).
[8] G. Singh et al., Nature Mat. 18, 948–954 (2019).
[9] G. Singh et al., Phys. Rev. B 105, 064512 (2022).
[10] Mallik et al. Nature Commun. 13, 4625 (2022).
[11] A.Jouan et al. Nature Elec. 3, 201–206 (2020).
Monopole Quantum Numbers and Projective Representations in Stable and Fragile Topological Crystalline Insulators
Over the past 15 years, a dizzying array of noninteracting topological insulator (TI) and topological crystalline insulator (TCI) phases have been theoretically predicted and identified in real materials. While the TI states are well understood, the TCI states – which comprise the majority of topological materials in nature – exhibit more complicated classification groups and boundary states and carry more ambiguous response signatures. For earlier variants of interacting symmetry-protected topological states (SPTs), both the classification and response were clarified through the many-body quantum numbers of the 0D collective excitations bound to crystal and electromagnetic defects, such as magnetic fluxes and monopoles. In particular, when 0D defects exhibit fractionalized quantum numbers, or more generally projective representations of the local many-body symmetry group, this can indicate the presence of quantized responses in the bulk that are governed by long-wavelength topological field theories that are stable to symmetric interactions. In this talk, I will introduce numerical methods for computing defect quantum numbers in stable and fragile TCI states via the reduced density matrix, revealing a deep connection between defect quantum numbers and the entanglement spectrum. Surprisingly, we find that when crystal symmetries are included in the local symmetry group, defects can appear to transform projectively even in Wannierizable (fragile) insulators, casting doubt on the suitability of magnetic monopoles for characterizing the TCI states present in real 3D materials. Our results represent a crucial step towards describing TCIs beyond tight-binding models and frameworks like “higher-order topology,” and facilitate more direct connections between free-fermion TCIs and interacting SPTs.
LAST-MODIFIED:20250113T150401Z SEQUENCE:2423844 X-ACCESS:1 X-HITS:560 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20250206T140000 DTEND;TZID=Europe/Paris:20250206T150000 UID:F310CF16-9671-4B9D-8012-F6702A983651 SUMMARY:[Séminaire TQM] Luca de Medici (LPEM, ESPCI) CREATED:20241217T134102Z DTSTAMP:20241217T134102Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/seminaire-tqm-luca-de-medici-lpem-espci DESCRIPTION:Luca de’ Medici (ESPCI, Paris)\NInsight into Hund metals and interplay with Mott physics\NHund metals are paramagnetic phases in which high-spin local configurations dominate. This paradigm is now a useful guidance to interpret the physics of many transition-metal compounds, like Ruthenates and Iron-based superconductors. I will show how this physics is extremized by moving towards a half-filled Mott insulator, and that it gives rise to charge instabilities and heavy fermionic behavior along the way.\NA. Georges and G. Kotliar, Physics Today 77, 46 (2024)\NM. Chatzieleftheriou et al. Phys. Rev. Lett. 130, 066401 (2023)\NM. Crispino et al. ArXiv:2312.06511 (2023) X-ALT-DESC;FMTTYPE=text/html:Luca de’ Medici (ESPCI, Paris)
Insight into Hund metals and interplay with Mott physics
Hund metals are paramagnetic phases in which high-spin local configurations dominate. This paradigm is now a useful guidance to interpret the physics of many transition-metal compounds, like Ruthenates and Iron-based superconductors. I will show how this physics is extremized by moving towards a half-filled Mott insulator, and that it gives rise to charge instabilities and heavy fermionic behavior along the way.
A. Georges and G. Kotliar, Physics Today 77, 46 (2024)
M. Chatzieleftheriou et al. Phys. Rev. Lett. 130, 066401 (2023)
M. Crispino et al. ArXiv:2312.06511 (2023)
LAST-MODIFIED:20250204T082218Z SEQUENCE:4214476 X-ACCESS:1 X-HITS:660 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20250213T140000 DTEND;TZID=Europe/Paris:20250213T150000 UID:CE29AE90-36C9-417D-99BC-FD5A65695061 SUMMARY:Nicolas Regnault (LPENS, Paris) CREATED:20241217T134153Z DTSTAMP:20241217T134153Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/nicolas-regnault-lpens-paris DESCRIPTION:Nicolas Regnault (Flatiron institute, NY et LPENS Paris)\NEngineering quantum phases of matter through moire materials: The case of Fractional Chern insulators\NProgress in stacking two dimensional materials, such as graphene or transition metal dichalcogenides (TMDs), has paved the way to engineer new structures relying on moire patterns. These patterns induced for example by slightly twisting one layer compared to the other, could lead to strongly correlated quantum phases such as superconductivity or the quantum anomalous Hall effects. In the realm of condensed matter physics, the fractional quantum Hall effect stands as a singular experimental manifestation of topological order, characterized by the presence of anyons—quasiparticles that bear fractional charge and exhibit exchange statistics diverging from conventional fermions and bosons. This phenomenon, observed over four decades ago, was still missing the direct observation of similar topological orders arising purely from band structure—without the application of strong magnetic fields. In 2023 within the span of a few months, several pioneering experiments have illuminated this once theoretical domain. Studies on twisted homobilayer MoTe2 and pentalayer rhombohedral graphene placed on hBN have finally unveiled the existence of fractional Chern insulators (FCIs), the zero-magnetic field analog of fractional quantum Hall states.\NThe journey to this point, preceded by over a decade of theoretical frameworks and predictions surrounding FCIs, yet the experimental revelations have proved to be richer and more surprising than expected. In this talk, we will present how the combination of ab-initio and quantum many-body calculations can help us capture the different features observed in experiments. We will discuss the potential future for this exciting booming field, including the possible observation of fractional topological insulators, a yet-never observed topological ordered phase preserving the time reversal symmetry. X-ALT-DESC;FMTTYPE=text/html:Nicolas Regnault (Flatiron institute, NY et LPENS Paris)
Engineering quantum phases of matter through moire materials: The case of Fractional Chern insulators
Progress in stacking two dimensional materials, such as graphene or transition metal dichalcogenides (TMDs), has paved the way to engineer new structures relying on moire patterns. These patterns induced for example by slightly twisting one layer compared to the other, could lead to strongly correlated quantum phases such as superconductivity or the quantum anomalous Hall effects. In the realm of condensed matter physics, the fractional quantum Hall effect stands as a singular experimental manifestation of topological order, characterized by the presence of anyons—quasiparticles that bear fractional charge and exhibit exchange statistics diverging from conventional fermions and bosons. This phenomenon, observed over four decades ago, was still missing the direct observation of similar topological orders arising purely from band structure—without the application of strong magnetic fields. In 2023 within the span of a few months, several pioneering experiments have illuminated this once theoretical domain. Studies on twisted homobilayer MoTe2 and pentalayer rhombohedral graphene placed on hBN have finally unveiled the existence of fractional Chern insulators (FCIs), the zero-magnetic field analog of fractional quantum Hall states.
The journey to this point, preceded by over a decade of theoretical frameworks and predictions surrounding FCIs, yet the experimental revelations have proved to be richer and more surprising than expected. In this talk, we will present how the combination of ab-initio and quantum many-body calculations can help us capture the different features observed in experiments. We will discuss the potential future for this exciting booming field, including the possible observation of fractional topological insulators, a yet-never observed topological ordered phase preserving the time reversal symmetry.
LAST-MODIFIED:20250204T082139Z SEQUENCE:4214386 X-ACCESS:1 X-HITS:720 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20250327T140000 DTEND;TZID=Europe/Paris:20250327T150000 UID:3E40A94B-D7F1-4650-8F44-37438A445642 SUMMARY:[Séminaire TQM] Tarik Yefsah (LKB Paris) CREATED:20241217T134255Z DTSTAMP:20241217T134255Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/seminaire-tqm-tarik-yefsah-lkb-paris LAST-MODIFIED:20241217T134255Z SEQUENCE:0 X-ACCESS:1 X-HITS:529 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20260319T140000 DTEND;TZID=Europe/Paris:20260319T150000 UID:59E7F989-D968-470D-A5C2-D499B4EAB90A SUMMARY:[Séminaire TQM] Hugues Pothier (CEA Saclay) CREATED:20260310T165346Z DTSTAMP:20260310T165346Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/seminaire-tqm-hugues-pothier-cea-saclay DESCRIPTION:The Nobel Prize in Physics 2025: Quantum physics with electrical circuits\NThe Nobel Prize in Physics 2025 was awarded to John Clarke, Michel Devoret and John Martinis “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”. I will describe their experiments, which gave birth to the now flourishing domain of quantum electronics.\N X-ALT-DESC;FMTTYPE=text/html:The Nobel Prize in Physics 2025 was awarded to John Clarke, Michel Devoret and John Martinis “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”. I will describe their experiments, which gave birth to the now flourishing domain of quantum electronics.
LAST-MODIFIED:20260310T165433Z SEQUENCE:47 LOCATION: GEO:0.00000000;0.00000000 X-LOCATION-DISPLAYNAME:Salle 523, couloir 12-13, 5è étage X-ACCESS:1 X-HITS:276 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT BEGIN:VEVENT DTSTART;TZID=Europe/Paris:20260416T140000 DTEND;TZID=Europe/Paris:20260416T150000 UID:70379E0A-C65D-46F9-B8E5-08FF134CABF1 SUMMARY:[Séminaire TQM] Freek Massee (LPS Orsay) CREATED:20250612T072754Z DTSTAMP:20250612T072754Z URL:https://www.lptmc.jussieu.fr/vie-scientifique/listeseminaires/tqm-7 DESCRIPTION:Majorana or not? A closer look at Fe(Se,Te)\NThe search for Majorana fermions in condensed matter systems has resulted in a number of putative claims of their discovery. If true, these exotic particles that are their own anti-particle could be exploited for error-free quantum computing, turning a fundamental curiosity into a billion dollar business. Unambiguous proof, however, is thus far lacking and challenging to provide. A recently proposed method to distinguish Majorana bound states from more conventional Andreev-, and Yu-Shiba-Rusinov states is to measure their shot noise [1]. Using our MHz enabled scanning tunnelling microscope [2], we set out to investigate three possible Majorana sightings in Fe(Se,Te): zero energy bound states at single Fe impurities [3] and other native impurities, linear sub-gap density of states at 1D defects [4] and vortex cores. In this talk I will discuss our findings.\N[1] Phys. Rev. B 104, L121406 (2021)[2] Rev. Sci. Instrum. 89, 093708 (2018)[3] Nature Communications 15, 8526 (2024)[4] Nature Communications 15, 3774 (2024) X-ALT-DESC;FMTTYPE=text/html:
The search for Majorana fermions in condensed matter systems has
resulted in a number of putative claims of their discovery. If true,
these exotic particles that are their own anti-particle could be
exploited for error-free quantum computing, turning a fundamental
curiosity into a billion dollar business. Unambiguous proof, however,
is thus far lacking and challenging to provide. A recently proposed
method to distinguish Majorana bound states from more conventional
Andreev-, and Yu-Shiba-Rusinov states is to measure their shot noise
[1]. Using our MHz enabled scanning tunnelling microscope [2], we set
out to investigate three possible Majorana sightings in Fe(Se,Te):
zero energy bound states at single Fe impurities [3] and other native
impurities, linear sub-gap density of states at 1D defects [4] and
vortex cores. In this talk I will discuss our findings.
[1] Phys. Rev. B 104, L121406 (2021)
[2] Rev. Sci. Instrum. 89, 093708 (2018)
[3] Nature Communications 15, 8526 (2024)
[4] Nature Communications 15, 3774 (2024)
SrTiO₃ is a “quantum paraelectric” in which dipolar fluctuations grow upon cooling, yet long-range ferroelectric order never develops. In this seminar, I will discuss the evolution of these dipolar fluctuations, as measured by inelastic neutron scattering, as the system is tuned toward superconducting and ferroelectric phases.
First, I will show that the superconducting dome of SrTiO₃ is driven by the competition between the increase in the density of states and the inevitable collapse of the quantum paraelectric phase under electron doping. Second, I will demonstrate that these dipolar fluctuations couple to a transverse acoustic mode (elastic constant c₄₄), with this coupling being most pronounced at small q-vectors. I will further show that SrTiO₃ lies near a modulated phase, as evidenced by significant softening of its transverse acoustic branch.
Both the amplitude of the coupling and the modulation vector are strongly influenced by the enhancement of the ferroelectric and antiferrodistortive (AFD) phase transitions. These findings suggest that SrTiO₃ is not only an incipient ferroelectric but also an incipient modulated material, with the modulated phase cooperating, rather than competing, with ferroelectricity and the AFD transition.
If time permits, I will also present the electric-field dependence of the thermal conductivity—another probe of acoustic phonons—in SrTiO₃. This will provide further evidence of TO–TA hybridization in SrTiO₃.
LAST-MODIFIED:20260506T060640Z SEQUENCE:0 LOCATION: GEO:0.00000000;0.00000000 X-LOCATION-DISPLAYNAME:Salle 523, couloir 12-13, 5è étage X-ACCESS:1 X-HITS:13 X-COLOR:1f7a00 X-SHOW-END-TIME:1 END:VEVENT END:VCALENDAR