In briefTunable inter-core coupling of photon-number-resolved quantum light in a telecom multicore fiber
SummaryThe researchers used a telecom 39-core fiber operated away from its design wavelength to measure heralded one- and two-photon states, resolving photon number together with spatial mode or core index. The two intra-core modes couple to neighboring cores at different rates, so the choice of launch alone tunes the passive fiber's inter-core coupling from 3.9% to 13.1%. The crosstalk and mode distributions of |1> and |2> states agree closely, providing a linear-optics baseline for certifying multiphoton interference.
Chiran Wijesundara, Yasmin Sarhan, Henry Strong et al.
In briefA faster sampling method for measuring fermionic non-Gaussianity in large quantum systems
SummaryThe paper proposes a perfect-sampling method for computing the second-order magic Rényi entropy (MRE), a resource measure of fermionic non-Gaussianity, directly from matrix product states. Each sample is generated in time polynomial in the number of modes and bond dimension. Benchmarks on XXZ chains with up to 128 sites show that the MRE captures higher-order correlations at large scales that covariance-based measures cannot detect.
In briefGeneralization of Transformer-based quantum state models through in-context learning
SummaryThis paper develops a theoretical framework for analyzing how Transformer-based neural quantum states generalize under in-context learning. It establishes an inference-time mean squared error bound showing that pointwise prediction error decreases with both the number of in-context examples and the Transformer depth. The required depth scales only linearly with system size, and the analysis is extended to full rank-one density operators, with numerical simulations supporting the theory.
In briefGeometric Aspects of Entanglement: a geometric measure of multi-qubit entanglement
SummaryThis paper studies quantum entanglement geometrically, using the Fubini–Study metric on the projective Hilbert space of multi-qubit pure states. It introduces the entanglement distance (ED), which measures entanglement as an obstruction to locally minimizing the sum of squared Fubini–Study distances under local operations. For pure two-qubit states, ED reduces to an exact monotone function of the concurrence and also to a monotone function of the entropy of entanglement. These links give a geometric reading of bipartite measures, while the authors note their limits beyond two qubits.
Lucio De Simone, Lorenzo Capra, Arthur Vesperini et al.
In briefAn exact semidefinite test for when permutation-invariant Bell operators are stoquastic
SummaryThe paper studies permutationally invariant Bell operators, which detect Bell correlations in atomic ensembles. It shows that, in the thermodynamic limit, the cone of coefficients giving stoquastic operators has an exact semidefinite characterization at any operator order. Because each off-diagonal matrix element is a value of one univariate polynomial, the condition reduces to a single linear matrix inequality.
Jan Li, Owidiusz Makuta, Evert van Nieuwenburg et al.
In briefParity-time symmetry of plasma waves: a symmetry principle linking kinetic plasma waves and instabilities
SummaryThe paper shows that the susceptibility tensor of a homogeneous collisionless magnetized plasma is parity-time (PT) symmetric, and that the linearized Vlasov-Maxwell Hamiltonian is PT-symmetric. Many kinetic instabilities are identified as spontaneous PT-symmetry breaking via Krein collisions. Three examples are discussed: thermal Bernstein modes, the Dory-Guest-Harris distribution with an analytical instability threshold and growth rate, and the mode-pole resonance instability relevant to fusion energy.
In briefAnalyzing quantum circuit fault tolerance with tensor networks and symplectic geometry
SummaryThe paper presents an operator-algebraic framework for analyzing fault tolerance in quantum circuits, giving necessary and sufficient conditions for exact recovery of the ideal computation under a given noise model. For non-adaptive stabilizer circuits, it uses symplectic geometry to derive closed-form detection and logical-effect matrices that characterize circuit distance. Applied to a finite Hastings–Haah honeycomb Floquet-code circuit, these matrices certify two logical qubits with distance four, and linear programming yields upper bounds on circuit distance.
WhyIntroduces an operator-algebraic framework with SDP and LP distance bounds, validated on Floquet and Hamming codes, offering notable novelty.
