@inproceedings{bib_Eras_2026, AUTHOR = {Badhani, Himanshu and GS, Dhanuja and Choudhary, Swati and Anand, Vishal and Das, Siddhartha }, TITLE = {Erasure cost of a quantum process: a thermodynamic meaning of the dynamical min-entropy}, BOOKTITLE = {Quantum Science and Technology}. YEAR = {2026}}

The erasure of information is fundamentally an irreversible logical operation, carrying profound consequences for the energetics of computation and information processing. We investigate the thermodynamic costs associated with erasing (and preparing) quantum processes. Specifically, we analyze an arbitrary bipartite unitary gate acting on logical and ancillary input-output systems, where the ancillary input is always initialized in the ground state. We focus on the adversarial erasure cost of the reduced dynamics—that is, the minimal thermodynamic work cost to erase the logical output of the gate for any logical input, assuming full access to the ancilla but no access to any purifying reference of the logical input state. We determine that this adversarial erasure cost is directly proportional to the negative min-entropy of the reduced dynamics, thereby giving the dynamical min-entropy a clear operational meaning. The dynamical min-entropy can take positive and negative values, depending on the underlying quantum dynamics. The negative value of the erasure cost implies that the extraction of thermodynamic work is possible instead of its consumption during the process. A key foundation of this result is the quantum process decoupling theorem, which quantitatively relates the decoupling ability of a process with its min-entropy. This insight bridges thermodynamics, information theory, and the fundamental limits of quantum computation.

@inproceedings{bib_Samp_2025, AUTHOR = {Chakraborty, Shantanav and Das, Siddhartha and Ghorui, Arnab and Hazra, Soumyabrata and Singh, Uttam }, TITLE = {Sample Complexity of black box work extraction}, BOOKTITLE = {Quantum Science and Technology}. YEAR = {2025}}

Extracting work from a physical system is one of the cornerstones of quantum thermodynamics. The extractable work, as quantified by ergotropy, necessitates a complete description of the quantum system. This is significantly more challenging when the state of the underlying system is unknown, as quantum tomography is extremely inefficient. In this article, we analyze the number of samples of the unknown state required to extract work. With only a single copy of an unknown state, we prove that ergotropy approaches zero in the asymptotic limit, rendering work extraction nearly impossible. In contrast, when multiple copies are available, we quantify the sample complexity required to estimate extractable work, establishing a scaling relationship that balances the desired accuracy with success probability. Our work develops a sample-efficient protocol to assess the utility of unknown states as quantum batteries and opens avenues for estimating thermodynamic quantities using near-term quantum computers.

@inproceedings{bib_Caus_2025, AUTHOR = {Buscemi, Francesco and Gangwar, Rajeev and Goswami, Kaumudibikash and Badhani, Himanshu and Pandit, Tanmoy and Mohan, Brij and Das, Siddhartha and Bera, Manabendra Nath }, TITLE = {Causal and Noncausal Revivals of Information: A New Regime of Non-Markovianity in Quantum Stochastic Processes}, BOOKTITLE = {PRX Quantum}. YEAR = {2025}}

The study of information revivals, witnessing the violation of certain data-processing inequalities, has provided an important paradigm in the study of non-Markovian quantum stochastic processes. Although often used interchangeably, we argue here that the notions of “revivals” and “backflows,” i.e., flows of information from the environment back into the system, are distinct: an information revival can occur without any backflow ever taking place. In this paper, we examine in detail the phenomenon of noncausal revivals and relate them to the theory of short Markov chains and squashed non-Markovianity. We also provide an operational condition, in terms of system-only degrees of freedom, to witness the presence of genuine backflow that cannot be explained by noncausal revivals. As a byproduct, we demonstrate that focusing on processes with genuine backflows, while excluding those with only noncausal revivals, resolves the issue of nonconvexity of Markovianity, thus enabling the construction of a convex resource theory of genuine quantum non-Markovianity

@inproceedings{bib_Squa_2025, AUTHOR = {Gangwar, Rajeev and Pandit, Tanmoy and Goswami, Kaumudibikash and Das, Siddhartha and Bera, Manabendra Nath }, TITLE = {Squashed quantum non-Markovianity: a measure of genuine quantum non-Markovianity in states}, BOOKTITLE = {Quantum Journal}. YEAR = {2025}}

We introduce a new information-theoretic measure to characterize non-Markovianity in a tripartite quantum state of genuine quantum origin. This measure reveals a novel tripartite quantum correlation and may be interpreted as entanglement between two systems when conditioned on the third (memory) system. We formulate a convex resource-theoretic framework to characterize genuine quantum non-Markovianity in states as a quantum resource and provide operational meaning to this resource in various information processing tasks.

@inproceedings{bib_Prac_2023, AUTHOR = {Sadhu, Abhishek and Somayajula, Meghana Ayyala and Horodecki, Karol and Das, Siddhartha }, TITLE = {Practical limitations on robustness and scalability of quantum Internet}, BOOKTITLE = {Technical Report}. YEAR = {2023}}

As quantum theory allows for information processing and computing tasks that otherwise are not possible with classical systems, there is a need and use of quantum Internet beyond existing network systems. At the same time, the realization of a desirably functional quantum Internet is hindered by fundamental and practical challenges such as high loss during transmission of quantum systems, decoherence due to interaction with the environment, fragility of quantum states, etc. We study the implications of these constraints by analyzing the limitations on the scaling and robustness of quantum Internet. Considering quantum networks, we present practical bottlenecks for secure communication, delegated computing, and resource distribution among end nodes. Motivated by the power of abstraction in graph theory (in association with quantum information theory), we consider graph-theoretic quantifiers to assess network robustness and provide critical values of communication lines for viable communication over quantum Internet. In particular, we begin by discussing limitations on usefulness of isotropic states as device-independent quantum key repeaters which otherwise could be useful for device-independent quantum key distribution. We consider some quantum networks of practical interest, ranging from satellite-based networks connecting far-off spatial locations to currently available quantum processor architectures within computers, and analyze their robustness to perform quantum information processing tasks. Some of these tasks form primitives for delegated quantum computing, e.g., entanglement distribution and quantum teleportation.

@inproceedings{bib_Spee_2023, AUTHOR = {Pandey, Vivek and Shrimali, Divyansh and Mohan, Brij and Das, Siddhartha and Pati, Arun Kumar }, TITLE = {Speed limits on correlations in bipartite quantum systems}, BOOKTITLE = {Physical Review A}. YEAR = {2023}}

Quantum speed limit is bound on the minimum time a quantum system requires to evolve from an initial state to final state under a given dynamical process. It sheds light on how fast a desired state transformation can take place which is pertinent for design and control of quantum technologies. In this paper, we derive speed limits on correlations such as entanglement, Bell-CHSH correlation, and quantum mutual information of quantum systems evolving under dynamical processes. Our main result is speed limit on an entanglement monotone called negativity which holds for arbitrary dimensional bipartite quantum systems and processes. Another entanglement monotone which we consider is the concurrence. To illustrate efficacy of our speed limits, we analytically and numerically compute the speed limits on the negativity, concurrence, and Bell-CHSH correlation for various quantum processes of practical interest. We are able to show that for practical examples we have considered, some of the speed limits we derived are actually attainable and hence these bounds can be considered to be tight.

@inproceedings{bib_Test_2023, AUTHOR = {Sadhu, Abhishek and Das, Siddhartha }, TITLE = {Testing of quantum nonlocal correlations under constrained free will and imperfect detectors}, BOOKTITLE = {Physical Review A}. YEAR = {2023}}

In this work, we deal with the relaxation of two central assumptions in standard locally realistic hidden variable (LRHV) inequalities: free will in choosing measurement settings, and the presence of perfect detectors at the measurement devices. Quantum correlations violating LRHV inequalities are called quantum nonlocal correlations. In principle, in an adversarial situation, there could be a hidden variable introducing bias in the selection of measurement settings, but observers with no access to that hidden variable could be unaware of the bias. In practice, however, detectors do not have perfect efficiency. A main focus of this paper is the introduction of the framework in which given a quantum state with nonlocal behavior under constrained free will, we can determine the threshold values of detector parameters (detector inefficiency and dark counts) such that the detectors are robust enough to certify nonlocality. We also introduce a class of LRHV inequalities with constrained free will, and we discuss their implications in the testing of quantum nonlocal correlation

@inproceedings{bib_Uppe_2022, AUTHOR = {Kaur, Eneet and , Karol Horodecki and Das, Siddhartha }, TITLE = {Upper Bounds on Device-Independent Quantum Key Distribution Rates in Static and Dynamic Scenarios}, BOOKTITLE = {Physical Review Applied}. YEAR = {2022}}

In this work, we develop upper bounds on key rates for device-independent quantum key distribution (DI-QKD) protocols and devices. We study the reduced cc-squashed entanglement and show that it is a convex functional. As a result, we show that the convex hull of the currently known bounds is a tighter upper bound on the device-independent key rates of the standard Clauser-Horne-Shimony-Holt (CHSH)- based protocol. We further provide tighter bounds for DI-QKD key rates achievable by any protocol applied to the CHSH-based device. This bound is based on reduced relative entropy of entanglement optimized over decompositions into local and nonlocal parts. In the dynamical scenario of quantum channels, we obtain upper bounds for device-independent private capacity for the CHSH-based protocols. We show that the device-independent private capacity for the CHSH-based protocols on depolarizing and erasure channels is limited by the secret key capacity of dephasing channels.

@inproceedings{bib_Quan_2022, AUTHOR = {Mohan, Brij and Das, Siddhartha and Pati, Arun K }, TITLE = {Quantum speed limits for information and coherence}, BOOKTITLE = {New Journal of Physics}. YEAR = {2022}}

The quantum speed limit indicates the maximal evolution speed of the quantum system. In this work, we determine speed limits on the informational measures, namely the von Neumann entropy, maximal information, and coherence of quantum systems evolving under dynamical processes. These speed limits ascertain the fundamental limitations on the evolution time required by the quantum systems for the changes in their informational measures. Erasing of quantum information to reset the memory for future use is crucial for quantum computing devices. We use the speed limit on the maximal information to obtain the minimum time required to erase the information of quantum systems via some quantum processes of interest.

@inproceedings{bib_Fund_2022, AUTHOR = {Horodecki, Karol and Winczewski, Marek and Das, Siddhartha }, TITLE = {Fundamental limitations on the device-independent quantum conference key agreement}, BOOKTITLE = {Physical Review A}. YEAR = {2022}}

Building a quantum secure internet is one of the most important challenges in the field of quantum technologies [1,2]. It would ensure worldwide information-theoretically secure communication. The idea of quantum repeaters [3–5] gives hope that this dream will come true. However, the level of quantum security proposed originally in a seminal article by Bennett and Brassard [6] seems to be insufficient due to the fact that on the way between an honest manufacturer and an honest user, an active hacker can change with the inner workings of a quantum device, making it totally insecure [7]. Indeed, the hardware Trojan-horse attacks on random number generators are known [8], and the active hacking on quantum devices became a standard testing approach since the seminal attack by Makarov [9]. The idea of device-independent (DI) security overcomes this obstacle [7,10] (see also [11] and references therein). Although difficult to be done in practice, it has been demonstrated quite recently in several recent experiments [12–14]. In parallel, the study of the limitations of this approach in terms of upper bounds on the distillable key has been put forward [15–18]. However, these approaches focus on pointto-point quantum device-independent secure communication. In this paper we introduce the upper bounds on the performance of the device-independent conference key agreement (DI-CKA) [19,20]. The task of the conference agreement is to distribute to N > 2 honest parties the same secure key for one-time-pad encryption. A protocol achieving this task in a device-independent manner has been shown in Ref. [20]. We set an upper bound on the performance of such protocols in a network setting. We focus on physical behaviors with N users (for arbitrary N > 2), where each user is both the sender and receiver of the behavior treated as a black box. This situation is a special case of a network describable with a multiplex quantum channel where inputs and outputs are classical with quantum phenomena going inside the physical behavior [21]. All N trusted parties have the role of both the sender to and receiver from the channel and their goal is to obtain a secret key in a device-independent way against a quantum adversary. Aiming at upper bounds on the device-independent key, we narrow the consideration to the independent and identically distributed case. In this scenario, the honest parties share n identical devices. All the N parties set (classical) inputs x = (x1,..., xN ) to each of the n shared devices P(a|x) and receive (classical) outputs a = (a1,..., aN ) from each of them. We restrict our consideration to quantum devices. Such devices are realized by certain measurements M ≡ ⊗N i=1Mxi ai on quantum states ρA1,...,AN ≡ ρN(A). We define these devices (ρN(A),M) = Tr[ρN(A) ⊗N i=1 Mxi ai ]. In this work we provide upper bounds on the deviceindependent conference key distillation rates for arbitrary multipartite states. As the first main result, we introduce a multipartite generalization of the cc-squashed entanglement provided in Ref. [22] and developed in Ref. [18]. With a little abuse of notation with respect to that used in Refs. [16,18], for the sake of the reader, we will omit the fact that the measure is multipartite as well as reduce the abbreviation cc in its name and here call it just reduced c-squashed entanglement,