obtained by casting a single photon on a balanced beamsplitter, where e.g. |10〉b1,b2 , indicates ... more obtained by casting a single photon on a balanced beamsplitter, where e.g. |10〉b1,b2 , indicates one photon excitation in the Fock space of exit mode b1 and the vacuum of the Fock space relative to exit mode b2, see Fig.(1). The form of such state appears to be similar to the singlet state of two level systems, which is known to maximally violate a Bell’s inequality. The two states are however intrinsically different in terms of the number of particles involved and |ψ〉b1,b2 can be thought of as a plain superposition of the photon in either of the beams.
The title of our work is a paraphrase of the title of Asher Peres' paper \textit{Unperformed ... more The title of our work is a paraphrase of the title of Asher Peres' paper \textit{Unperformed experiments have no results}. We show what are the lessons to be learned from the gedankenexperiments presented by Frauchiger and Renner (claim that quantum theory cannot consistently describe the use of itself), and Brukner (a no-go theorem for observer independent facts). One has to remember Bohr's remark "the unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of experimental arrangement", which specifically to the gedankenexperiments means: in all your quantum mechanical thinking about measurements, think in terms of the full quantum measurement theory. The theory sees measurement as composed of two stages: pre-measurement (entanglement, i.e. quantum correlation, of the measured system with the pointer variable), and next decoherence via interaction with an environment, which leaves a record of the result. T...
We demonstrate here that for a given mixed multi-qubit state if there are at least two observers ... more We demonstrate here that for a given mixed multi-qubit state if there are at least two observers for whom mutual Einstein-Podolsky-Rosen steering is possible, i.e. each observer is able to steer the other qubits into two different pure states by spontaneous collapses due to von Neumann type measurements on his/her qubit, then nonexistence of local realistic models is fully equivalent to quantum entanglement (this is not so without this condition). This result leads to an enhanced version of Gisin's theorem (originally: all pure entangled states violate local realism). Local realism is violated by all mixed states with the above steering property. The new class of states allows one e.g. to perform three party secret sharing with just pairs of entangled qubits, instead of three qubit entanglements (which are currently available with low fidelity). This significantly increases the feasibility of having high performance versions of such protocols. Finally, we discuss some possible a...
We review series of multiqubit Bell's inequalities which apply to correlation functions and p... more We review series of multiqubit Bell's inequalities which apply to correlation functions and present conditions that quantum states must satisfy to violate such inequalities.
With two independent parametric down-converters one can realize an “event-ready” Bell-EPR experim... more With two independent parametric down-converters one can realize an “event-ready” Bell-EPR experiment. Monitoring for sharply coincident idlers results in entangling and pre-selecting the independent signals, which are then fed into an interferometric setup. We give the general conditions for high visibility and particle collection efficiency for Bell-type experiments involving two independent sources. These ideas can also lead to a realizable GHZ experiment.
Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographi... more Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. Here we show, that a entangled 4-photon state can be directly created by parametric down-conversion. This state exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. We have used this state for four-particle test of local realistic theories. Therefore this state can be used for new types of quantum communication. We also report on possibilities for the experimentally realization of a 3-photon entangled state, the so called W-state, and discuss some of its properties.
We prove that for every Bell’s inequality and for a broad class of protocols, there always exists... more We prove that for every Bell’s inequality and for a broad class of protocols, there always exists a multi-party communication complexity problem, for which the protocol assisted by states which violate the inequality is more efficient than any classical protocol. Moreover, for that advantage Bell’s inequality violation is a necessary and sufficient criterion. Thus, violation of Bell’s inequalities has a significance beyond that of a non-optimal-witness of non-separability.
Journal of Physics B: Atomic and Molecular Physics
The problem of the choice of the electromagnetic gauge is investigated in the framework of collis... more The problem of the choice of the electromagnetic gauge is investigated in the framework of collision theory in the presence of strong radiation fields. Charged-particle potential scattering is considered as an example. The analysis is based on the dipole approximation for the radiation field, and on the velocity and length gauges. Distinguishing features of field-assisted particle scattering are, among others,
obtained by casting a single photon on a balanced beamsplitter, where e.g. |10〉b1,b2 , indicates ... more obtained by casting a single photon on a balanced beamsplitter, where e.g. |10〉b1,b2 , indicates one photon excitation in the Fock space of exit mode b1 and the vacuum of the Fock space relative to exit mode b2, see Fig.(1). The form of such state appears to be similar to the singlet state of two level systems, which is known to maximally violate a Bell’s inequality. The two states are however intrinsically different in terms of the number of particles involved and |ψ〉b1,b2 can be thought of as a plain superposition of the photon in either of the beams.
The title of our work is a paraphrase of the title of Asher Peres' paper \textit{Unperformed ... more The title of our work is a paraphrase of the title of Asher Peres' paper \textit{Unperformed experiments have no results}. We show what are the lessons to be learned from the gedankenexperiments presented by Frauchiger and Renner (claim that quantum theory cannot consistently describe the use of itself), and Brukner (a no-go theorem for observer independent facts). One has to remember Bohr's remark "the unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of experimental arrangement", which specifically to the gedankenexperiments means: in all your quantum mechanical thinking about measurements, think in terms of the full quantum measurement theory. The theory sees measurement as composed of two stages: pre-measurement (entanglement, i.e. quantum correlation, of the measured system with the pointer variable), and next decoherence via interaction with an environment, which leaves a record of the result. T...
We demonstrate here that for a given mixed multi-qubit state if there are at least two observers ... more We demonstrate here that for a given mixed multi-qubit state if there are at least two observers for whom mutual Einstein-Podolsky-Rosen steering is possible, i.e. each observer is able to steer the other qubits into two different pure states by spontaneous collapses due to von Neumann type measurements on his/her qubit, then nonexistence of local realistic models is fully equivalent to quantum entanglement (this is not so without this condition). This result leads to an enhanced version of Gisin's theorem (originally: all pure entangled states violate local realism). Local realism is violated by all mixed states with the above steering property. The new class of states allows one e.g. to perform three party secret sharing with just pairs of entangled qubits, instead of three qubit entanglements (which are currently available with low fidelity). This significantly increases the feasibility of having high performance versions of such protocols. Finally, we discuss some possible a...
We review series of multiqubit Bell's inequalities which apply to correlation functions and p... more We review series of multiqubit Bell's inequalities which apply to correlation functions and present conditions that quantum states must satisfy to violate such inequalities.
With two independent parametric down-converters one can realize an “event-ready” Bell-EPR experim... more With two independent parametric down-converters one can realize an “event-ready” Bell-EPR experiment. Monitoring for sharply coincident idlers results in entangling and pre-selecting the independent signals, which are then fed into an interferometric setup. We give the general conditions for high visibility and particle collection efficiency for Bell-type experiments involving two independent sources. These ideas can also lead to a realizable GHZ experiment.
Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographi... more Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. Here we show, that a entangled 4-photon state can be directly created by parametric down-conversion. This state exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. We have used this state for four-particle test of local realistic theories. Therefore this state can be used for new types of quantum communication. We also report on possibilities for the experimentally realization of a 3-photon entangled state, the so called W-state, and discuss some of its properties.
We prove that for every Bell’s inequality and for a broad class of protocols, there always exists... more We prove that for every Bell’s inequality and for a broad class of protocols, there always exists a multi-party communication complexity problem, for which the protocol assisted by states which violate the inequality is more efficient than any classical protocol. Moreover, for that advantage Bell’s inequality violation is a necessary and sufficient criterion. Thus, violation of Bell’s inequalities has a significance beyond that of a non-optimal-witness of non-separability.
Journal of Physics B: Atomic and Molecular Physics
The problem of the choice of the electromagnetic gauge is investigated in the framework of collis... more The problem of the choice of the electromagnetic gauge is investigated in the framework of collision theory in the presence of strong radiation fields. Charged-particle potential scattering is considered as an example. The analysis is based on the dipole approximation for the radiation field, and on the velocity and length gauges. Distinguishing features of field-assisted particle scattering are, among others,
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Papers by Marek Zukowski