arXiv subjects

M. Thenabadu

Publications and source records attributed to M. Thenabadu.

4 recordsLinked to original sources

arXiv2026

Physical currents for stochastic Einstein-Podolsky-Rosen quantum trajectories

Theories of the measured homodyne current generated by a stochastic Schr\"odinger equation (SSE) can be tested in a simulation of the Einstein-Podolsky-Rosen (EPR) correlations for a two-mode squeezed state. We carry out such a simulation, and determine the correct stochastic term for the measured current in the broad-band limit. Stratonovich rather than Ito stochastic noise agrees with experiment. We show that this is relevant to measurement noise and errors in quantum technologies. By analyzing the SSE trajectories as measurement settings are changed, we propose a modern version of Schrodinger's gedanken experiment, where one measures position and momenta simultaneously, ``one by direct, the other by indirect measurement''.

quant-ph↗

arXiv2023

Weak versus deterministic macroscopic realism, and Einstein-Podolsky-Rosen's elements of reality

Violation of Leggett-Garg inequalities allows proof of the incompatibility between quantum mechanics and the combined premises (called macrorealism) of macroscopic realism (MR) and non-invasive measurability (NIM). Arguments can be given that the incompatibility arises because MR fails $-$ or else, that NIM fails. In this paper, we consider a strong failure of macrorealism, involving superpositions of coherent states, where the NIM premise is replaced by Bell-locality. We follow recent work and propose validity of a subset of Einstein-Podolsky-Rosen (EPR) and Leggett-Garg premises, referred to as \emph{weak macroscopic realism} (wMR). In finding consistency with wMR, we identify that the Leggett-Garg inequalities are violated because of failure of both MR and NIM, but also that both are valid in a less restrictive sense. Weak MR is distinguished from \emph{deterministic macroscopic realism} (dMR) by recognizing that a measurement involves a reversible unitary interaction that establishes the measurement setting. Weak MR posits a predetermined value for the measurement outcome, for the system defined at the time after the interaction, when the measurement setting is experimentally specified. An extended definition of wMR considers the element of reality defined by EPR for a system A, where one can predict with certainty the outcome of a measurement on A, by measurement on a system B. Weak MR posits that the element of reality exists once the unitary interaction determining the setting at B has occurred. We show compatibility of systems violating Leggett-Garg inequalities with wMR, but point out that dMR is falsifiable. We compare with other MR models, and give an argument for wMR on the basis that wMR resolves inconsistencies pointed out by Leggett and Garg between failure of macrorealism and assumptions intrinsic to quantum measurement theory.

quant-ph↗

arXiv2023

A macroscopic quantum three-box paradox: finding consistency with weak macroscopic realism

The quantum three-box paradox considers a ball prepared in a superposition of being in one of three Boxes. Bob makes measurements by opening either Box 1 or Box 2. After performing some unitary operations (shuffling), Alice can infer with certainty that the ball was detected by Bob, regardless of which box he opened, if she detects the ball after opening Box 3. The paradox is that the ball would have been found with certainty in either box, if that box had been opened. Resolutions of the paradox include that Bob's measurement cannot be made non-invasively, or else that realism cannot be assumed at the quantum level. Here, we strengthen the case for the former argument, by constructing macroscopic versions of the paradox. Macroscopic realism implies that the ball is in one of the boxes, prior to Bob or Alice opening any boxes. We demonstrate consistency of the paradox with macroscopic realism, if carefully defined (as weak macroscopic realism, wMR) to apply to the system at the times prior to Alice or Bob opening any Boxes, but after the unitary operations associated with preparation or shuffling. By solving for the dynamics of the unitary operations, and comparing with mixed states, we demonstrate agreement between the predictions of wMR and quantum mechanics: The paradox only manifests if Alice's shuffling combines both local operations (on Box 3) and nonlocal operations, on the other Boxes. Following previous work, the macroscopic paradox is shown to correspond to a violation of a Leggett-Garg inequality, which implies non-invasive measurability, if wMR holds.

quant-ph↗

arXiv2020

Testing macroscopic local realism using cat-states and Bell inequalities in time

We show how one may test macroscopic local realism where, different from conventional Bell tests, all relevant measurements need only distinguish between two macroscopically distinct states of the system being measured. Here, measurements give macroscopically distinguishable outcomes for a system observable and do not resolve microscopic properties (of order $\hbar$). Macroscopic local realism assumes: (1) macroscopic realism (the system prior to measurement is in a state which will lead to just one of the macroscopically distinguishable outcomes) and (2) macroscopic locality (a measurement on a system at one location cannot affect the macroscopic outcome of the measurement on a system at another location, if the measurement events are spacelike separated). To obtain a quantifiable test, we define $M$-scopic local realism where the outcomes are separated by an amount $\sim M$. We first show for $N$ up to $20$ that $N$-scopic Bell violations are predicted for entangled superpositions of $N$ bosons (at each of two sites). Secondly, we show violation of $M$-scopic local realism for entangled superpositions of coherent states of amplitude $α$, for arbitrarily large $M=α$. In both cases, the systems evolve dynamically according to a local nonlinear interaction. The first uses nonlinear beam splitters realised through nonlinear Josephson interactions; the second is based on nonlinear Kerr interactions. To achieve the Bell violations, the traditional choice between two spin measurement settings is replaced by a choice between different times of evolution at each site.

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