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Ujjwal Sen

Harish Chandra Research Institute, Physics, Faculty Member

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Masters in applied mathematics at the University of Calcutta. PhD in physics with Ryszard Horodecki at the University of Gdansk. Humboldt postdoctoral fellow with Maciej Lewenstein at the Leibniz University Hannover. Ramon-y-Cajal fellow with Maciej Lewenstein at ICFO-The Institute of Photonic Sciences, Barcelona. Assistant professor in Physics at the Indian Institute of Technology Delhi. Currently, professor in physics at the Harish-Chandra Research Institute, Allahabad. Researcher in quantum information technologies and its interface with other sciences.
Supervisors: Ryszard Horodecki and Maciej Lewenstein

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Amarjit Budhiraja

University of California, Riverside

Università degli Studi dell'Insubria

Walmir Miranda Filho

University of Cambridge

Aubrey Wulfsohn

University of Warwick

Thierry Huillet

Erika Hausenblas

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Papers by Ujjwal Sen

Frequentist-Approach Inspired Theory of Quantum Random Phenomena Predicts Signaling

SSRN Electronic Journal, 2021

Different ensembles of the same density matrix are indistinguishable within the modern Kolmogorov... more Different ensembles of the same density matrix are indistinguishable within the modern Kolmogorov probability measure theory of quantum random phenomena. We find that changing the framework from the Kolmogorov one to a frequentist-inspired theory of quantum random phenomena-à la von Mises-would lift the indistinguishability, and potentially cost us the no-signaling principle (i.e., leads to superluminal communication). We believe that this adds to the recent works on the search for a suitable representation of the state of a quantum system. While erstwhile arguments for potential modifications in the representation of the quantum state were restricted to possible variations in the formalism of the quantum theory, we indicate a possible fallout of altering the underlying theory of random processes.

Distinguishibility of the Bell states

More than two multipartite orthogonal states cannot always be discriminated (with certainty) if o... more More than two multipartite orthogonal states cannot always be discriminated (with certainty) if only local operations and classical communication (LOCC) are allowed. Using an existing inequality among the measures of entanglement, we show that any three Bell states cannot be discriminated by LOCC. Exploiting the inequality, we calculate the distillable entanglement of a certain class of (4\otimes 4) mixed states

Entanglemnt vs. Non-commutativity in Teleportation

Quantum thermal transistors: Operation characteristics in steady state versus transient regimes

Physical Review A, 2021

We show that a quantum thermal transistor can also cause the transistor effect-where one out of t... more We show that a quantum thermal transistor can also cause the transistor effect-where one out of three terminals can control the flow of heat current in the other two-with good amplification properties in the transient regime for certain paradigmatic initial states. We find three broad classes of transient quantum thermal transistors-the first having a smaller amplification than the steady state quantum thermal transistor, the second with better amplification but a smaller operating region in terms of temperature, and the third that gives higher amplification with a larger operating region. The last type is of particular interest as it also operates in the region where the steady state thermal transistors lose the transistor effect. We discuss in some detail certain initial states for which the cases of necessarily transient transistors arise. We analyze the time variation of the amplification factor of transient thermal transistors and estimate the preferable time and duration for which they can work efficiently. Cumulative studies of the differences in magnitudes of the amplifications of heat currents at the non-base terminals of the quantum thermal transistor, first with respect to the base-terminal temperature and next with time, are also presented.

The Separability versus Entanglement Problem

Lectures on Quantum Information

VI. Bell inequalities A. Detection of entanglement by Bell inequality VII. Classification of bipa... more

Tuning interaction strength leads to an ergodic-nonergodic transition of quantum correlations in the anisotropic Heisenberg spin model

Physical Review A, 2013

We investigate the time dynamics of quantum correlations of the anisotropic Heisenberg model in a... more We investigate the time dynamics of quantum correlations of the anisotropic Heisenberg model in a time-dependent magnetic field, in one-dimensional, ladder, and two-dimensional lattices. We find that quantum correlation measures in the entanglement-separability paradigm are ergodic in these systems irrespective of system parameters. However, information-theoretic quantum correlation measures can also be nonergodic, and exhibit a transition from nonergodic to ergodic behavior with the change of interaction strength in the direction of the magnetic field. We also observe that the transition point changes drastically as we go from one-dimensional and ladder lattices to the two-dimensional one.

Patterns of genuine multipartite entanglement in frustrated quantum spin systems

Physical Review A, 2014

We investigate the behavior of genuine multipartite entanglement of paradigmatic frustrated quant... more We investigate the behavior of genuine multipartite entanglement of paradigmatic frustrated quantum spin systems. We consider six different spin models, whose frustration ranges from being very high to very low. We find that the highly frustrated quantum spin systems are near-maximally genuine multipartite entangled, while those with low frustration do not possess a similar definite behavior with regard to their genuine multipartite entanglement.

Shared purity of multipartite quantum states

Physical Review A, 2014

Fidelity plays an important role in measuring distances between pairs of quantum states, of singl... more Fidelity plays an important role in measuring distances between pairs of quantum states, of single as well as multiparty systems. Based on the concept of fidelity, we introduce a physical quantity, shared purity, for arbitrary pure or mixed quantum states of shared systems of an arbitrary number of parties in arbitrary dimensions. We find that it is different from quantum correlations. However, we prove that a maximal shared purity between two parties excludes any shared purity of these parties with a third party, thus ensuring its quantum nature. Moreover, we show that all generalized GHZ states are monogamous, while all generalized W states are non-monogamous with respect to this measure. We apply the quantity to investigate the quantum XY spin models, and observe that it can faithfully detect the quantum phase transition present in these models. We perform a finite-size scaling analysis and find the scaling exponent for this quantity.

Quantum discord surge heralds entanglement revival in an infinite spin chain

EPL (Europhysics Letters), 2012

A measure of quantum correlation defined from an information-theoretic perspective, namely, quant... more A measure of quantum correlation defined from an information-theoretic perspective, namely, quantum discord, is applied to study the time-evolved nonequilibrium state of the infinite anisotropic quantum XY spin chain in a transverse time-dependent field. In particular, we probe whether the collapse and revival of nearest-neighbor entanglement of the state seen with a varying initial applied field strength, at a fixed evolution time, may be predicted from the behavior of the quantum correlation measure. For this quantum many-body system, realizable with currently available technology, we find that the revival of entanglement of the evolved state happens if there is an increase in quantum discord in the vicinity of entanglement collapse.

Cumulative quantum work-deficit versus entanglement in the dynamics of an infinite spin chain

Physics Letters A, 2014

We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of t... more We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of timeevolved states of the anisotropic infinite quantum XY spin chain, in a transverse time-dependent magnetic field, can be quantitatively characterized by the dynamics of an information-theoretic quantum correlation measure, namely, quantum work-deficit (QWD). We show that only those nonequilibrium states exhibit entanglement resurrection after death, on changing the field parameter during the DPT, for which the cumulative bipartite QWD is above a threshold. The results point to an interesting interrelation between two quantum correlation measures that are conceptualized from different perspectives.

Detection loophole in measurement-device-independent entanglement witnesses

Physical Review A

There always exists an entanglement witness for every entangled quantum state. Negativity of the ... more There always exists an entanglement witness for every entangled quantum state. Negativity of the expectation value of an entanglement witness operator guarantees entanglement of the corresponding state, given that the measurement devices involved are perfect, i.e., the performed measurements actually constitute the witness operator for the state under consideration. In a realistic situation, there are two possible ways of measurements to drive the process away from the ideal one. First, wrong measurements may be performed, and, second, while the measurement operators are implemented correctly, the detection process may be noisy. Entanglement witnesses are prone to both of these imperfections. The concept of measurement-device-independent entanglement witnesses was introduced to remove the first problem. We analyze the "detection loophole" in the context of measurement-device-independent entanglement witnesses, which deal with the second problem of imprecise measurements. We obtain an upper bound, a threshold value, on the entanglement witness function in the measurement-device-independent entanglement witness scenario, below which entanglement is guaranteed for given nonideal detector efficiencies, that can involve both lost events and dark counts. We also present a threshold value on the function in the "worst-case scenario" where the threshold of the witness operator for guaranteeing entanglement assumes its maximal negative value.

Quantum no-go theorems in causality respecting systems in the presence of closed timelike curves: Tweaking the Deutsch condition

EPL (Europhysics Letters)

We consider causality respecting (CR) quantum systems interacting with closed timelike curves (CT... more We consider causality respecting (CR) quantum systems interacting with closed timelike curves (CTCs), within the Deutsch model. We introduce the concepts of popping-up and elimination of quantum information and use them to show that no-cloning and no-deleting, which are true in CR quantum systems, are no more valid in the same that are interacting with CTCs. We also find limits on the possibility of creation of entanglement between a CR system and a CTC, and the same between two CR systems in the presence of a CTC. We prove that teleportation of quantum information, even in its approximate version, from a CR region to a CTC is disallowed. Interestingly, we find that tweaking the Deutsch model, by allowing the input and output not to be the same, leads to a nontrivial approximate teleportation beyond the classical limit.

Canonical distillation of entanglement

Physics Letters A

Distilling highly entangled quantum states from weaker ones is a process that is crucial for effi... more Distilling highly entangled quantum states from weaker ones is a process that is crucial for efficient and long-distance quantum communication, and has implications for several other quantum information protocols. We introduce the notion of distillation under limited resources, and specifically focus on the energy constraint. The corresponding protocol, which we call the canonical distillation of entanglement, naturally leads to the set of canonically distillable states. We show that for non-interacting Hamiltonians, almost no states are canonically distillable, while the situation can be drastically different for interacting ones. Several paradigmatic Hamiltonians are considered for bipartite as well as multipartite canonical distillability. The results have potential applications for practical quantum communication devices.

Adiabatic freezing of long-range quantum correlations in spin chains

EPL (Europhysics Letters), 2016

We consider a process to create quasi long-range quantum discord between the non-interacting end ... more We consider a process to create quasi long-range quantum discord between the non-interacting end spins of a quantum spin chain, with the end spins weakly coupled to the bulk of the chain. The process is not only capable of creating long-range quantum correlation but the latter remains frozen, when certain weak end-couplings are adiabatically varied below certain thresholds. We term this phenomenon as adiabatic freezing of quantum correlation. We observe that the freezing is robust to moderate thermal fluctuations and is intrinsically related to the cooperative properties of the quantum spin chain. In particular, we find that the energy gap of the system remains frozen for these adiabatic variations, and moreover, considering the end spins as probes, we show that the interval of freezing can detect the anisotropy transition in quantum XY spin chains. Importantly, the adiabatic freezing of long-range quantum correlations can be simulated with contemporary experimental techniques.

Constructive interference between disordered couplings enhances multiparty entanglement in quantum Heisenberg spin glass models

New Journal of Physics, 2016

Disordered systems form one of the centrestages of research in many body sciences and lead to a p... more Disordered systems form one of the centrestages of research in many body sciences and lead to a plethora of interesting phenomena and applications. A paradigmatic disordered system consists of an one-dimensional array of quantum spin-1/2 particles, governed by the Heisenberg spin glass Hamiltonian with natural or engineered quenched disordered couplings in an external magnetic field. These systems allow disorder-induced enhancement for bipartite and multipartite observables. Here we show that simultaneous application of independent quenched disorders results in disorder-induced enhancement, while the same is absent with individual application of the same disorders. We term the phenomenon as constructive interference and the corresponding parameter stretches as the Venus regions. Interestingly, it has only been observed for multiparty entanglement and is absent for the single-and two-party physical quantities.

Distribution of Bell-inequality violation versus multiparty-quantum-correlation measures

Physical Review A, 2016

Violation of a Bell inequality guarantees the existence of quantum correlations in a quantum stat... more Violation of a Bell inequality guarantees the existence of quantum correlations in a quantum state. A pure bipartite quantum state, having nonvanishing quantum correlation, always violates a Bell inequality. Such correspondence is absent for multipartite pure quantum states. For a shared multipartite quantum state, we establish a connection between the monogamy of Bell inequality violation and genuine multi-site entanglement as well as monogamy-based multiparty quantum correlation measures. We find that generalized Greenberger-Horne-Zeilinger states and another singleparameter family states which we refer to as the "special Greenberger-Horne-Zeilinger" states have the status of extremal states in such relations.

Capacities of noiseless quantum channels for massive indistinguishable particles: Bosons versus fermions

Phys Rev a, 2007

We consider information transmission through a noiseless quantum channel, where the information i... more We consider information transmission through a noiseless quantum channel, where the information is encoded into massive indistinguishable particles: bosons or fermions. We study the situation in which the particles are noninteracting. The encoding input states obey a set of physically motivated constraints on the mean values of the energy and particle number. In such a case, the determination of both classical and quantum capacity reduces to a constrained maximization of entropy. In the case of noninteracting bosons, signatures of Bose Einstein condensation can be observed in the behavior of the capacity. A major motivation for these considerations is to compare the information carrying capacities of channels that carry bosons with those that carry fermions. We show analytically that fermions generally provide higher channel capacity, i.e., they are better suited for transferring bits as well as qubits, in comparison to bosons. This holds for a large range of power law potentials, and for moderate to high temperatures. Numerical simulations seem to indicate that the result holds for all temperatures. Also, we consider the low temperature behavior for the three-dimensional box and harmonic trap, and again we show that the fermionic capacity is higher than the bosonic one for sufficiently low temperatures.

Distinguishability of Bell States

Phys Rev Lett, 2001

Entanglement versus noncommutativity in teleportation

Physical Review a, Feb 13, 2002

Role of an information-theoretic measure of quantum correlation in a dynamical phase transition of entanglement

The nearest-neighbor entanglement of the evolved state of an asymmetric quantum XY spin chain, in... more

Frequentist-Approach Inspired Theory of Quantum Random Phenomena Predicts Signaling

SSRN Electronic Journal, 2021

Different ensembles of the same density matrix are indistinguishable within the modern Kolmogorov... more Different ensembles of the same density matrix are indistinguishable within the modern Kolmogorov probability measure theory of quantum random phenomena. We find that changing the framework from the Kolmogorov one to a frequentist-inspired theory of quantum random phenomena-à la von Mises-would lift the indistinguishability, and potentially cost us the no-signaling principle (i.e., leads to superluminal communication). We believe that this adds to the recent works on the search for a suitable representation of the state of a quantum system. While erstwhile arguments for potential modifications in the representation of the quantum state were restricted to possible variations in the formalism of the quantum theory, we indicate a possible fallout of altering the underlying theory of random processes.

Distinguishibility of the Bell states

More than two multipartite orthogonal states cannot always be discriminated (with certainty) if o... more More than two multipartite orthogonal states cannot always be discriminated (with certainty) if only local operations and classical communication (LOCC) are allowed. Using an existing inequality among the measures of entanglement, we show that any three Bell states cannot be discriminated by LOCC. Exploiting the inequality, we calculate the distillable entanglement of a certain class of (4\otimes 4) mixed states

Entanglemnt vs. Non-commutativity in Teleportation

Quantum thermal transistors: Operation characteristics in steady state versus transient regimes

Physical Review A, 2021

We show that a quantum thermal transistor can also cause the transistor effect-where one out of t... more We show that a quantum thermal transistor can also cause the transistor effect-where one out of three terminals can control the flow of heat current in the other two-with good amplification properties in the transient regime for certain paradigmatic initial states. We find three broad classes of transient quantum thermal transistors-the first having a smaller amplification than the steady state quantum thermal transistor, the second with better amplification but a smaller operating region in terms of temperature, and the third that gives higher amplification with a larger operating region. The last type is of particular interest as it also operates in the region where the steady state thermal transistors lose the transistor effect. We discuss in some detail certain initial states for which the cases of necessarily transient transistors arise. We analyze the time variation of the amplification factor of transient thermal transistors and estimate the preferable time and duration for which they can work efficiently. Cumulative studies of the differences in magnitudes of the amplifications of heat currents at the non-base terminals of the quantum thermal transistor, first with respect to the base-terminal temperature and next with time, are also presented.

The Separability versus Entanglement Problem

Lectures on Quantum Information

VI. Bell inequalities A. Detection of entanglement by Bell inequality VII. Classification of bipa... more

Tuning interaction strength leads to an ergodic-nonergodic transition of quantum correlations in the anisotropic Heisenberg spin model

Physical Review A, 2013

We investigate the time dynamics of quantum correlations of the anisotropic Heisenberg model in a... more We investigate the time dynamics of quantum correlations of the anisotropic Heisenberg model in a time-dependent magnetic field, in one-dimensional, ladder, and two-dimensional lattices. We find that quantum correlation measures in the entanglement-separability paradigm are ergodic in these systems irrespective of system parameters. However, information-theoretic quantum correlation measures can also be nonergodic, and exhibit a transition from nonergodic to ergodic behavior with the change of interaction strength in the direction of the magnetic field. We also observe that the transition point changes drastically as we go from one-dimensional and ladder lattices to the two-dimensional one.

Patterns of genuine multipartite entanglement in frustrated quantum spin systems

Physical Review A, 2014

We investigate the behavior of genuine multipartite entanglement of paradigmatic frustrated quant... more We investigate the behavior of genuine multipartite entanglement of paradigmatic frustrated quantum spin systems. We consider six different spin models, whose frustration ranges from being very high to very low. We find that the highly frustrated quantum spin systems are near-maximally genuine multipartite entangled, while those with low frustration do not possess a similar definite behavior with regard to their genuine multipartite entanglement.

Shared purity of multipartite quantum states

Physical Review A, 2014

Fidelity plays an important role in measuring distances between pairs of quantum states, of singl... more Fidelity plays an important role in measuring distances between pairs of quantum states, of single as well as multiparty systems. Based on the concept of fidelity, we introduce a physical quantity, shared purity, for arbitrary pure or mixed quantum states of shared systems of an arbitrary number of parties in arbitrary dimensions. We find that it is different from quantum correlations. However, we prove that a maximal shared purity between two parties excludes any shared purity of these parties with a third party, thus ensuring its quantum nature. Moreover, we show that all generalized GHZ states are monogamous, while all generalized W states are non-monogamous with respect to this measure. We apply the quantity to investigate the quantum XY spin models, and observe that it can faithfully detect the quantum phase transition present in these models. We perform a finite-size scaling analysis and find the scaling exponent for this quantity.

Quantum discord surge heralds entanglement revival in an infinite spin chain

EPL (Europhysics Letters), 2012

A measure of quantum correlation defined from an information-theoretic perspective, namely, quant... more A measure of quantum correlation defined from an information-theoretic perspective, namely, quantum discord, is applied to study the time-evolved nonequilibrium state of the infinite anisotropic quantum XY spin chain in a transverse time-dependent field. In particular, we probe whether the collapse and revival of nearest-neighbor entanglement of the state seen with a varying initial applied field strength, at a fixed evolution time, may be predicted from the behavior of the quantum correlation measure. For this quantum many-body system, realizable with currently available technology, we find that the revival of entanglement of the evolved state happens if there is an increase in quantum discord in the vicinity of entanglement collapse.

Cumulative quantum work-deficit versus entanglement in the dynamics of an infinite spin chain

Physics Letters A, 2014

We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of t... more We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of timeevolved states of the anisotropic infinite quantum XY spin chain, in a transverse time-dependent magnetic field, can be quantitatively characterized by the dynamics of an information-theoretic quantum correlation measure, namely, quantum work-deficit (QWD). We show that only those nonequilibrium states exhibit entanglement resurrection after death, on changing the field parameter during the DPT, for which the cumulative bipartite QWD is above a threshold. The results point to an interesting interrelation between two quantum correlation measures that are conceptualized from different perspectives.

Detection loophole in measurement-device-independent entanglement witnesses

Physical Review A

There always exists an entanglement witness for every entangled quantum state. Negativity of the ... more There always exists an entanglement witness for every entangled quantum state. Negativity of the expectation value of an entanglement witness operator guarantees entanglement of the corresponding state, given that the measurement devices involved are perfect, i.e., the performed measurements actually constitute the witness operator for the state under consideration. In a realistic situation, there are two possible ways of measurements to drive the process away from the ideal one. First, wrong measurements may be performed, and, second, while the measurement operators are implemented correctly, the detection process may be noisy. Entanglement witnesses are prone to both of these imperfections. The concept of measurement-device-independent entanglement witnesses was introduced to remove the first problem. We analyze the "detection loophole" in the context of measurement-device-independent entanglement witnesses, which deal with the second problem of imprecise measurements. We obtain an upper bound, a threshold value, on the entanglement witness function in the measurement-device-independent entanglement witness scenario, below which entanglement is guaranteed for given nonideal detector efficiencies, that can involve both lost events and dark counts. We also present a threshold value on the function in the "worst-case scenario" where the threshold of the witness operator for guaranteeing entanglement assumes its maximal negative value.

Quantum no-go theorems in causality respecting systems in the presence of closed timelike curves: Tweaking the Deutsch condition

EPL (Europhysics Letters)

We consider causality respecting (CR) quantum systems interacting with closed timelike curves (CT... more We consider causality respecting (CR) quantum systems interacting with closed timelike curves (CTCs), within the Deutsch model. We introduce the concepts of popping-up and elimination of quantum information and use them to show that no-cloning and no-deleting, which are true in CR quantum systems, are no more valid in the same that are interacting with CTCs. We also find limits on the possibility of creation of entanglement between a CR system and a CTC, and the same between two CR systems in the presence of a CTC. We prove that teleportation of quantum information, even in its approximate version, from a CR region to a CTC is disallowed. Interestingly, we find that tweaking the Deutsch model, by allowing the input and output not to be the same, leads to a nontrivial approximate teleportation beyond the classical limit.

Canonical distillation of entanglement

Physics Letters A

Distilling highly entangled quantum states from weaker ones is a process that is crucial for effi... more Distilling highly entangled quantum states from weaker ones is a process that is crucial for efficient and long-distance quantum communication, and has implications for several other quantum information protocols. We introduce the notion of distillation under limited resources, and specifically focus on the energy constraint. The corresponding protocol, which we call the canonical distillation of entanglement, naturally leads to the set of canonically distillable states. We show that for non-interacting Hamiltonians, almost no states are canonically distillable, while the situation can be drastically different for interacting ones. Several paradigmatic Hamiltonians are considered for bipartite as well as multipartite canonical distillability. The results have potential applications for practical quantum communication devices.

Adiabatic freezing of long-range quantum correlations in spin chains

EPL (Europhysics Letters), 2016

We consider a process to create quasi long-range quantum discord between the non-interacting end ... more We consider a process to create quasi long-range quantum discord between the non-interacting end spins of a quantum spin chain, with the end spins weakly coupled to the bulk of the chain. The process is not only capable of creating long-range quantum correlation but the latter remains frozen, when certain weak end-couplings are adiabatically varied below certain thresholds. We term this phenomenon as adiabatic freezing of quantum correlation. We observe that the freezing is robust to moderate thermal fluctuations and is intrinsically related to the cooperative properties of the quantum spin chain. In particular, we find that the energy gap of the system remains frozen for these adiabatic variations, and moreover, considering the end spins as probes, we show that the interval of freezing can detect the anisotropy transition in quantum XY spin chains. Importantly, the adiabatic freezing of long-range quantum correlations can be simulated with contemporary experimental techniques.

Constructive interference between disordered couplings enhances multiparty entanglement in quantum Heisenberg spin glass models

New Journal of Physics, 2016

Disordered systems form one of the centrestages of research in many body sciences and lead to a p... more Disordered systems form one of the centrestages of research in many body sciences and lead to a plethora of interesting phenomena and applications. A paradigmatic disordered system consists of an one-dimensional array of quantum spin-1/2 particles, governed by the Heisenberg spin glass Hamiltonian with natural or engineered quenched disordered couplings in an external magnetic field. These systems allow disorder-induced enhancement for bipartite and multipartite observables. Here we show that simultaneous application of independent quenched disorders results in disorder-induced enhancement, while the same is absent with individual application of the same disorders. We term the phenomenon as constructive interference and the corresponding parameter stretches as the Venus regions. Interestingly, it has only been observed for multiparty entanglement and is absent for the single-and two-party physical quantities.

Distribution of Bell-inequality violation versus multiparty-quantum-correlation measures

Physical Review A, 2016

Violation of a Bell inequality guarantees the existence of quantum correlations in a quantum stat... more Violation of a Bell inequality guarantees the existence of quantum correlations in a quantum state. A pure bipartite quantum state, having nonvanishing quantum correlation, always violates a Bell inequality. Such correspondence is absent for multipartite pure quantum states. For a shared multipartite quantum state, we establish a connection between the monogamy of Bell inequality violation and genuine multi-site entanglement as well as monogamy-based multiparty quantum correlation measures. We find that generalized Greenberger-Horne-Zeilinger states and another singleparameter family states which we refer to as the "special Greenberger-Horne-Zeilinger" states have the status of extremal states in such relations.

Capacities of noiseless quantum channels for massive indistinguishable particles: Bosons versus fermions

Phys Rev a, 2007

We consider information transmission through a noiseless quantum channel, where the information i... more We consider information transmission through a noiseless quantum channel, where the information is encoded into massive indistinguishable particles: bosons or fermions. We study the situation in which the particles are noninteracting. The encoding input states obey a set of physically motivated constraints on the mean values of the energy and particle number. In such a case, the determination of both classical and quantum capacity reduces to a constrained maximization of entropy. In the case of noninteracting bosons, signatures of Bose Einstein condensation can be observed in the behavior of the capacity. A major motivation for these considerations is to compare the information carrying capacities of channels that carry bosons with those that carry fermions. We show analytically that fermions generally provide higher channel capacity, i.e., they are better suited for transferring bits as well as qubits, in comparison to bosons. This holds for a large range of power law potentials, and for moderate to high temperatures. Numerical simulations seem to indicate that the result holds for all temperatures. Also, we consider the low temperature behavior for the three-dimensional box and harmonic trap, and again we show that the fermionic capacity is higher than the bosonic one for sufficiently low temperatures.

Distinguishability of Bell States

Phys Rev Lett, 2001

Entanglement versus noncommutativity in teleportation

Physical Review a, Feb 13, 2002

Role of an information-theoretic measure of quantum correlation in a dynamical phase transition of entanglement

The nearest-neighbor entanglement of the evolved state of an asymmetric quantum XY spin chain, in... more