arXiv subjects

Ron Rubin

Publications and source records attributed to Ron Rubin.

6 recordsLinked to original sources

arXiv2026

Entangled measurements are necessary for optimal tomography of mixed fermionic Gaussian states and of bosonic Gaussian states near the vacuum

We prove that learning an unknown mixed fermionic Gaussian state on $m$ modes to trace distance $ε$ requires $Ω(m^3/ε^2)$ copies when measurements act on one copy at a time, even with arbitrary POVMs, fresh ancillas and classical adaptivity, but without quantum memory between copies. The bound holds for $0<ε\le1/3600$ and separates this model from the known collective rate $Θ(m^2/ε^2)$. It follows from a uniform single-copy Fisher-information budget and a dimension-independent comparison between trace distance and Gaussian parameters. On covariance matrices of operator norm at most $1-c$, we prove a Frobenius-to-trace-norm continuity bound with constant $[2c(2-c)]^{-1/2}$; matchgate shadows then attain $O(m^3/(cε^2))$ copies. This also gives explicit error certificates for thermal free-fermion states. For a class of mixed passive bosonic Gaussian states with total mean photon number at most one, a reduction to bounded-block qudit tomography gives a single-copy lower bound $Ω(m^3/(ε^2\sqrt{\log(m/ε)}))$, versus collective complexity $Θ(m^2/ε^2)$. These separations answer the mixed-state measurement-resource question posed by Chen et al. A two-mode example has optimal two-copy Fisher trace $5$, versus the separable ceiling $4$. An IBM replication gave a pre-registered witness lower bound $4.12$ at one-sided $95\%$ shot-noise confidence; its interpretation assumes the prescribed preparations and a common measurement channel and does not bound preparation systematics.

quant-ph↗

arXiv2026

Localized charges, reset noise, and boundary memory in matrix-product-conserving quantum chains

We construct boundary observables that retain memory for a quantified time window in quantum chains whose basis configurations carry a conserved ordered product of matrix labels. Under strong-irreducibility and proximality hypotheses on the label matrices, projective contraction of random matrix products localizes a conserved charge in the root-mean-square over uniform basis configurations. A finite-time inequality then converts readout overlap and reset leakage into a correlation bound: for a conserved reference observable $H$, a readout $F$, accumulated squared leakage $\mathcal B$, and the normalized Hilbert-Schmidt inner product, $\langle F,Φ(F)\rangle\ge 2\langle H,F\rangle^2/(\|H\|_2^2+\mathcal B)-\|F\|_2^2$. Here $Φ$ is a sequence of conserving channels and partial resets. The bound holds for each such circuit, without averaging its gates. It yields an exponential memory window in the distance from the noisy region, with quantitative corrections for spatially distributed noise and imperfect conservation. For a thirteen-state elementary-matrix model, exact polynomial certificates give root-mean-square localization error $e^{-r/1400}$ between the charge and its restriction to the last $r$ sites, and limiting variance at least $1/19$ for a rational charge. A related normalized-Gram charge gives an eight-site certificate retaining more than half the specified readout correlation through two boundary-reset rounds. Uniform depolarization imposes an inverse-noise-rate lifetime ceiling. A two-qubit IBM experiment illustrates the general reset inequality; it does not realize the matrix-product chain. We provide proofs, exact certificate programs, and an independent recount of the archived experimental outcomes.

quant-ph↗

arXiv1998

Still Baking

We present here a simple proof of the non-existence of a non-periodic invariant point for the quantum baker's map propagator presented in Rubin and Salwen (Annals of Physics, 1998), for Planck's constant h=1/N and N a positive integer.

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arXiv1998

Quantum Mechanics on a Torus

We present here a canonical description for quantizing classical maps on a torus. We prove theorems analagous to classical theorems on mixing and ergodicity in terms of a quantum Koopman space $ L^2 (A_\hbar},τ_\hbar) $ obtained as the completion of the algebra of observables $ A_\hbar $ in the norm induced by the following inner product $(A,B) =τ_{\hbar}(A^{\dagger}B) $, where $τ_{\hbar}$ is a linear functional on the algebra analogous to the classical ``integral over phase space.'' We also derive explicit formulas connecting this formulation to the $θ$-torus decomposition of Bargmann space introduced in ref. \QCITE{cite}{}{KLMR}.

quant-ph↗

arXiv1998

A Canonical Quantization of the Baker's Map

We present here a canonical quantization for the baker's map. The method we use is quite different from that used in Balazs and Voros (ref. \QCITE{cite}{}{BV}) and Saraceno (ref. \QCITE{cite}{}{S}). We first construct a natural ``baker covering map'' on the plane $\QTO{mathbb}{\mathbb{R}}^{2}$. We then use as the quantum algebra of observables the subalgebra of operators on $L^{2}(\QTO{mathbb}{\mathbb{R}}) $ generated by $\left\{\exp (2πi\hat{x}) ,\exp (2πi\hat{p}) \right\} $ . We construct a unitary propagator such that as $\hbar \to 0$ the classical dynamics is returned. For Planck's constant $h=1/N$, we show that the dynamics can be reduced to the dynamics on an $N$-dimensional Hilbert space, and the unitary $N\times N$ matrix propagator is the same as given in ref. \QCITE{cite}{}{BV} except for a small correction of order $h$. This correction is shown to preserve the classical symmetry $x\to 1-x$ and $p\to 1-p$ in the quantum dynamics for periodic boundary conditions.

quant-ph↗

arXiv1998

A Parity-Conserving Canonical Quantization for the Baker's Map

We present here a complete description of the quantization of the baker's map. The method we use is quite different from that used in Balazs and Voros [BV] and Saraceno [S]. We use as the quantum algebra of observables the operators generated by {exp(2 Pi ix),exp (2 Pi ip)} and construct a unitary propagator such that as Planck's constant tends to zero,the classical dynamics is returned. For Planck's constant satisfying the integrality condition 1/N with N even, and for periodic boundary conditions for the wave functions on the torus, we show that the dynamics can be reduced to the dynamics on an N-dimensional Hilbert space, and the unitary N by N matrix propagator is the same as given in [BV] except for a small correction of order Planck's constant. This correction is is shown to preserve the symmetry x->1-x and p->1-p of the classical map for periodic boundary conditions.

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