Abstract We establish the comprehensive theoretical framework for an exact description of the open system dynamics of permutationally invariant (PI) states in arbitrary N -qudit systems when this dynamics preserves the PI symmetry over time. Thanks to the powerful Schur–Weyl duality formalism, we unveil the internal links between the canonical time-local Lindblad-like master equation and the Markovian or non-Markovian dynamics of each PI degree of freedom (Schur subspaces). Our approach does not require one to compute the Schur transform as it operates directly within the restricted PI operator subspace of the Liouville space, whose dimension only scales polynomially with the number of qudits. We introduce the concept of 3 ν -symbol matrix, where ν here denotes an integer partition, that proves to be very useful in this context.
Abstract We establish the comprehensive theoretical framework for an exact description of the open system dynamics of permutationally invariant (PI) states in arbitrary N -qudit systems when this dynamics preserves the PI symmetry over time. Thanks to the powerful Schur–Weyl duality formalism, we unveil the internal links between the canonical time-local Lindblad-like master equation and the Markovian or non-Markovian dynamics of each PI degree of freedom (Schur subspaces). Our approach does not require one to compute the Schur transform as it operates directly within the restricted PI operator subspace of the Liouville space, whose dimension only scales polynomially with the number of qudits. We introduce the concept of 3 ν -symbol matrix, where ν here denotes an integer partition, that proves to be very useful in this context.
We propose a generic and systematic decoherence-free scheme to encode quantum information into an open quantum system based focusing on symmetry. Under a given symmetry, the Liouville space is decomposed into invariant subspaces characterized by a tensor-product structure. A decoherence-free subsystem is then identified as a factor of the tensor product. Unlike decoherence-free subspaces, which typically require strong symmetries, decoherence-free systems are permitted under less restrictive weak symmetries. Specifically, we primarily concern the permutation symmetry in conjunction with the unitary symmetry and utilize the Schur-Weyl duality, which facilitates numerous efficient and systematic calculations based on the well-established group representation theory. Employing the isomorphism between the Liouville space and the fictitious Hilbert space, we construct a super-Schur basis, which block-diagonalizes the super-operators that describe the noisy quantum channels, both in the Kraus representation and in terms of the quantum master equation. Each block reveals the tensor-product structure and facilitates the identification of physically relevant decoherence-free subsystems under the specified weak symmetry.
We describe the implications of permutation symmetry for the state space and dynamics of quantum mechanical systems of matrices of general size N. We solve the general 11-parameter permutation invariant quantum matrix harmonic oscillator Hamiltonian and calculate the canonical partition function. The permutation invariant sector of the Hilbert space, for general Hamiltonians, can be described using partition algebra diagrams forming the bases of a tower of partition algebras Pk(N). The integer k is interpreted as the degree of matrix oscillator polynomials in the quantum mechanics. Families of interacting Hamiltonians are described which are diagonalized by a representation theoretic basis for the permutation invariant subspace which we construct for $N\ensuremath{\ge}2k$. These include Hamiltonians for which the low-energy states are permutation invariant and can give rise to large ground state degeneracies related to the dimensions of partition algebras. A symmetry-based mechanism for quantum many body scars discussed in the literature can be realized in these matrix systems with permutation symmetry. A mapping of the matrix index values to lattice sites allows a realization of the mechanism in the context of modified Bose-Hubbard models. Extremal correlators analogous to those studied in AdS/CFT are shown to obey selection rules based on Clebsch-Gordan multiplicites (Kronecker coefficients) of symmetric groups.
We investigate Permutation-Invariant (PI) quantum error-correcting codes encoding a logical qudit of dimension dL in PI states using physical qudits of dimension dP. We extend the Knill--Laflamme (KL) conditions for d−1 deletion errors from qubits to qudits and investigate numerically both qubit (dL=dP=2) and qudit (dL>2 or dP>2) PI codes. We analyze the scaling of the block length n in terms of the code distance d, and compare to existing families of PI codes due to Ouyang, Aydin--Alekseyev--Barg (AAB) and Pollatsek--Ruskai (PR). Our three main findings are: (i) We conjecture that qubit PI codes correcting up to d−1 deletion errors have block length n(d)≥(3d2+1)/4, which implies an upper bound d≤12n−3/3 on their code distance, and that PR codes can saturate this bound. (ii) For qudit PI codes encoding a single qudit we numerically observe that increasing dP results in n monotonically decreasing and approaching the quantum Singleton bound n(d)≥2d−1. (iii) We propose a semi-analytic extension of the qubit AAB construction to qudits that finds explicit solutions by solving a linear program. Our results therefore provide key insights into lower bounds on the block length scaling of both qubit and qudit PI codes, and demonstrate the benefit of increased physical local dimension in the context of PI codes.
We show that all nonrelativistic quantum processes, whether open or closed, are either unitary or probabilistic unitary, i.e., probabilistic combinations of unitary evolutions. This means that, for open quantum systems, their continuous dynamics can always be described by the Lindblad master equation with all jump operators being unitary. We call this formalism the probabilistic unitary formulation of open quantum system dynamics. This formalism is shown to be exact under all cases, and does not rely on any assumptions other than the continuity and differentiability of the density matrix. Moreover, it requires as few as $d\ensuremath{-}1$ jump operators, instead of ${d}^{2}\ensuremath{-}1$, to describe the open dynamics in the most general case, where d is the dimension of Hilbert space of the system. Importantly, different from the conventional Lindblad master equation, this formalism is state dependent, meaning that the Hamiltonian, jump operators, and rates in general all depend on the current state of the density matrix. Hence one needs to know the explicit expression of the density matrix in order to write down the probabilistic unitary master equation explicitly. Experimentally, the formalism provides a scheme to control a quantum state to evolve along designed nonunitary quantum trajectories, and can be potentially useful in quantum computing and quantum control scenes since only unitary resources are needed for implementation.
The dynamics of finite dimension open quantum systems is studied with the help of the simplest possible form of projection operators, namely the ones which project only onto one-dimensional subspaces. The simplicity of the action of the projection operators always leads to an analytical solution of the dynamical master equation, even in the non-Markovian case, in any perturbative order. The analytical solution correctly reproduces the short-time dynamics, and can be used to recursively recover the dynamics for an arbitrary time interval with arbitrary precision. The necessary number of relevant degrees of freedom to completely characterise an open quantum system is ( n − 1 ) ( n + 2 ) / 2 , where n is the dimension of the Hilbert space of the open system. The method is illustrated by two examples, the relaxation of a qubit in a thermal bath and the dynamics of two interacting qubits in a common environment.
We derive an explicit formula for the intrinsic MacWilliams transform for permutation-invariant qudit codes. Such codes naturally live in symmetric power representations, where the relevant error sectors are determined by the irreducible decomposition of the conjugation action on the associated operator space. Using the multiplicity-free structure of this decomposition and the corresponding intertwiner algebra, we identify the intrinsic MacWilliams matrix with a finite Racah transform. The entries are given by a terminating hypergeometric series, and the rows of the matrix are Racah orthogonal polynomials with parameters determined explicitly by the block length and local dimension. Computing the spectrum of the degree-one twirl reveals that this spectrum lies on an affine quadratic lattice. Then we derive a tridiagonal multiplication rule from the representation theory of the adjoint sector. As consequences, we obtain closed-form orthogonality, detailed-balance, and involutivity identities for the transform. The resulting formula supplies an explicit MacWilliams matrix for computing linear programming bounds on permutation-invariant qudit codes.