arXiv subjects

Josey Stevens

Publications and source records attributed to Josey Stevens.

5 recordsLinked to original sources

arXiv2026

Procrustes Tomography -- reconstructing noisy quantum channels made easy

What is the most expensive part of quantum device characterization? Clearly, the answer is quantum process tomography. However, especially for noisy intermediate-scale quantum (NISQ) computers, a comprehensive understanding of the noisy quantum dynamics is essential in interpreting the computational output. In this work, we introduce an efficient method -- Procrustes tomography -- that outperforms established methods in a number of aspects. After a pedagogical and constructive introduction, we demonstrate the utility of the method for representative examples of noisy quantum channels.

quant-ph↗

arXiv2026

Energy-error tradeoff in encoding quantum error correction

While it has been widely recognized that genuine quantum advantage for practical problems might only be achieved with fault-tolerant quantum computers, it is still not entirely clear whether the required quantum error correction will be physically feasible. In the present work, we carefully analyze the required energy resources to encode the logical qubit states for repetition, perfect, and Steane codes. We find that there is a universal trade-off between the target precision and the required energetic resources. Importantly, we find that the energetic resources intimately depend on the specific physical realization of a quantum error correction code, and that the required resources scale exponentially with the targeted precision of the encoding.

quant-ph↗

arXiv2026

Variational Gibbs State Preparation on Trapped-Ion Devices

We implement a variational quantum algorithm for Gibbs state preparation of a transverse-field Ising model on IonQ's quantum computers. To this end, we train the variational parameters via classical simulation and perform state tomography on the quantum devices to evaluate the fidelity of the prepared Gibbs state. As a main result, we find that fidelity decreases (non-monotonically) as a function of the inverse temperature $\beta$ of the system. Fidelity also decreases as a function of the size of the system. Interestingly, we find that a Gibbs state prepared for a specified $\beta$ is a better representative of a Gibbs state prepared for a $\textit{lower}$ $\beta$; or in other words, thermal fluctuations in the quantum hardware lead to digital heating, that is, an increase in the temperature of the prepared Gibbs state above what was intended.

quant-ph↗

arXiv2025

Hamiltonian quantum gates -- energetic advantage from entangleability

Hamiltonian quantum gates controlled by classical electromagnetic fields form the basis of any realistic model of quantum computers. In this letter, we derive a lower bound on the field energy required to implement such gates and relate this energy to the expected gate error. We study the entangleability (ability to entangle qubits) of Hamiltonians and highlight how this feature of quantum gates can provide a means for more energetically efficient computation. Ultimately, we show that a universal quantum computer can be realized with vanishingly low energetic requirements but at the expense of arbitrarily large complexity.

quant-ph↗

arXiv2019

Quantum to classical transition in an information ratchet

Recent years have seen a flurry of research activity in the study of minimal and autonomous information ratchets. However, the existing classical and quantum models are somewhat hard to compare, and, hence, quantifying possible quantum supremacy in information ratchets has been elusive. We propose a first step towards filling this void between quantum and classical ratchets by introducing a new model with continuous variables -- a quantum particle in a box coupled to a stream of qubits. The dynamics is solved exactly, and we analyze the quantum to classical transition in terms of a natural time scale parameter for the model.

cond-mat.stat-mech↗