Collisional quantum thermometry

Stella Seah, Stefan Nimmrichter, Daniel Grimmer, Jader P. Santos, Valerio Scarini, Gabriel T. Landi

We introduce a general framework for thermometry based on collisional models, where ancillas probe thetemperature of the environment through an intermediary system. This allows for the generation of correlatedancillas even if they are initially independent. Using tools from parameter estimation theory, we show through aminimal qubit model that individual ancillas can already outperform the thermal Cramer-Rao bound. In addition,when probed collectively, these ancillas may exhibit superlinear scalings of the Fisher information, especiallyfor weak system-ancilla interactions. Our approach sets forth the notion of metrology in a sequential interactionssetting, and may inspire further advances in quantum thermometry.

Almost thermal operations: inhomogeneous reservoirs

Angeline Shu, Yu Cai, Stella Seah, Stefan Nimmrichter, Valerio Scarini

The resource theory of thermal operations explains the state transformations that are possible ina very specific thermodynamic setting: there is only one thermal bath, auxiliary systems can onlybe in the corresponding thermal state (free states), and the interaction must commute with the freeHamiltonian (free operation). In this paper we study the mildest deviation: the reservoir particlesare subject to inhomogeneities, either in the local temperature (introducing resource states) or inthe local Hamiltonian (generating a resource operation). For small inhomogeneities, the two modelsgenerate the same channel and thus the same state transformations. However, their thermodynamicsis significantly different when it comes to work generation or to the interpretation of the “secondlaws of thermal operations”.

Error suppression in adiabatic quantum computing with qubit ensembles

Naeimeh Mohseni, Marek Narozniak, Alexey N Pyrkov, Valentin Ivannikov, Jonathan P Dowling

In the standard approach to adiabatic quantum computing (AQC), quantum information storedon qubits are adiabatically evolved to find the ground state of a problem Hamiltonian. Here weinvestigate a variation of AQC where qubit ensembles are used in place of qubits. We identify twodistinct regimes for a given problem Hamiltonian under thismapping as a function of the ensemblesizeN. At a critical ensemble sizeNc, the nature of the first excited state changes from beingmacroscopically distinct spin configuration to a single particle perturbation of the ground state.AboveNcthe minimum gap for large ensembles is well predicted by mean-field theory and theAQC performance improves withN, realizing error-suppression due to duplication of the quantuminformation. While belowNcthe performance is mixed, and can increase withN. For randomlychosen problem instances Nc tends to be smaller than realistic ensemble sizes, hence we expect theensemble version of AQC to work well in a great majority of cases. Our approach shows it is possibleto perform AQC without the necessity of controlling individual qubits, allowing an alternative routetowards implementing AQC.