Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:quant-ph/0506047 · Communications in Theoretical Physics

No-relationship Between Impossibility of Faster-Than-Light Quantum Communication and Distinction of Ensembles with the Same Density Matrix

Chuan Xi Wang, Gui‐Lu Long, Yang Sun

Published Oct 1, 2005• 7 citations
arXivHTMLPDF

Abstract

It has been claimed in the literature that impossibility of faster-than-light quantum communication has an origin of indistinguishability of ensembles with the same density matrix. We show that the two concepts are not related. We argue that even with an ideal single-atom-precision measurement, it is generally impossible to produce two ensembles with exactly the same density matrix; or to produce ensembles with the same density matrix, classical communication is necessary. Hence the impossibility of faster-than-light communication does not imply the indistinguishability of ensembles with the same density matrix.

DOI
10.1088/6102/44/4/622

Related papers

Relating quantum incoherence, entanglement and superluminal signalling

Stanislav Filatov, Marcis P. Auzinsh

Hereby we inspect two-partite entanglement using thought experiment that relates properties of incoherently mixed states to the impossibility of faster-than-light (FTL) signalling. We show that if there appears a way to distinguish ensembles of particles that are described by the same density matrix, but are generated using different pure states - properties of entanglement (namely, non-classical correlations) could be employed to create an FTL signalling device. We do not claim FTL signalling is possible, rather, we establish the logical connection between the aforementioned properties of current physical theory which has not so far been evident.

arXiv:2001.08867•Jan 24, 2020•0 citations

Faster-Than-Light Communication using Quantum Entanglement is Possible

Florentín Smarandache

In this little note we refute the statement that Faster-Than-Light Communication using Quantum Entanglement is Impossible, by proving that the alleged proof of the mainstream is irrelevant. Also, the Faster-Than-Light Communication does NOT mean sending information into the past. We recall the many paradoxes of the theory of relativity that make it unrealistic, while the Absolute Theory of Relativity [1] is the realistic one.

Information Sciences with Applications•Dec 19, 2025•0 citations

Limit on Nonlocality in Any World in Which Communication Complexity Is Not Trivial

Gilles Brassard A, Harry Buhrman, Noah Linden +3 more

Bell proved that quantum entanglement enables two spacelike separated parties to exhibit classically impossible correlations. Even though these correlations are stronger than anything classically achievable, they cannot be harnessed to make instantaneous (faster than light) communication possible. Yet, Popescu and Rohrlich have shown that even stronger correlations can be defined, under which instantaneous communication remains impossible. This raises the question: Why are the correlations achievable by quantum mechanics not maximal among those that preserve causality? We give a partial answer to this question by showing that slightly stronger correlations would result in a world in which communication complexity becomes trivial.

arXiv:quant-ph/0508042•Physical Review Letters•Jun 27, 2006•404 citations

Quantum Interpretation of Faster Than Light Communication Via Entanglement Property of Two Particles

Mark Timothy Sheldrick

It is possible to use two particles (A and B) with entangled properties to transmit information at faster than light speeds. This can be done, not by trying to modulate the results of how particle A is measured, but by modulating whether particle A is measured or not measured. The effect of this modulating method is to place particle B in either a single value (but arbitrary) state or leave it in a superposition of states. It is then possible for the reciever of particle B to distinguish between these two states by the use of an appropriately designed interferometer. Such a device can be designed to produce an interference pattern only when particle B is in a superposition of values and a straight simple image when particle B has a single (arbitrary) defined value. Under the Copenhagen interpretation and existing experimental results, this method will produce a way of signalling fater than light. It will require the use of multiple entangled pairs of particles to effectively transmit a single bit of information. Under the Multi-Worlds Interpretation, it will not be so easy to communicate faster than light by this mechanism, as detector B will be detecting photon B as a superposition of values in both cases.

•Aug 1, 2014•0 citations

No-faster-than-light-signaling implies linear evolution. A re-derivation

Angelo Bassi, Kasra Hejazi

Abstract There is a growing interest, both from the theoretical as well as experimental side, to test the validity of the quantum superposition principle, and of theories which explicitly violate it by adding nonlinear terms to the Schrödinger equation. We review the original argument elaborated by Gisin (1989 Helv. Phys. Acta 62 363), which shows that the non-superluminal-signaling condition implies that the dynamics of the density matrix must be linear. This places very strong constraints on the permissible modifications of the Schrödinger equation, since they have to give rise, at the statistical level, to a linear evolution for the density matrix. The derivation is done in a heuristic way here and is appropriate for the students familiar with the textbook quantum mechanics and the language of density matrices.

arXiv:1411.1768•European Journal of Physics•Aug 4, 2015•40 citations

Quantum squeezing cannot beat the standard quantum limit

Liam Paul McGuinness

Quantum entanglement between particles is expected to allow one to perform tasks that would otherwise be impossible. In quantum sensing and metrology, entanglement is often claimed to enable a measurement precision that cannot be attained with the same number of particles and time, forgoing entanglement. Two distinct approaches exist: creation of entangled states that either i) respond quicker to the signal, or ii) are associated with lower noise and uncertainty. The second class of states are generally called squeezed states. Here we show that if our definition of success is a precision that is impossible to achieve using the same resources but without entanglement then squeezed states cannot succeed. In doing so we show that a single non-separable squeezed state provides fundamentally no better precision, per unit time, than a single particle.

arXiv:2306.14666•Jun 26, 2023•0 citations
It has been claimed in the literature that impossibility of faster-than-light quantum communication has an origin of indistinguishability of ensembles with the same density matrix. We show that the two concepts are not related. We argue that even with an ideal single-atom-precision measurement, it is generally impossible to produce two ensembles with exactly the same density matrix; or to produce ensembles with the same density matrix, classical communication is necessary. Hence the impossibility of faster-than-light communication does not imply the indistinguishability of ensembles with the same density matrix.
Hereby we inspect two-partite entanglement using thought experiment that relates properties of incoherently mixed states to the impossibility of faster-than-light (FTL) signalling. We show that if there appears a way to distinguish ensembles of particles that are described by the same density matrix, but are generated using different pure states - properties of entanglement (namely, non-classical correlations) could be employed to create an FTL signalling device. We do not claim FTL signalling is possible, rather, we establish the logical connection between the aforementioned properties of current physical theory which has not so far been evident.
Bell proved that quantum entanglement enables two spacelike separated parties to exhibit classically impossible correlations. Even though these correlations are stronger than anything classically achievable, they cannot be harnessed to make instantaneous (faster than light) communication possible. Yet, Popescu and Rohrlich have shown that even stronger correlations can be defined, under which instantaneous communication remains impossible. This raises the question: Why are the correlations achievable by quantum mechanics not maximal among those that preserve causality? We give a partial answer to this question by showing that slightly stronger correlations would result in a world in which communication complexity becomes trivial.
It is possible to use two particles (A and B) with entangled properties to transmit information at faster than light speeds. This can be done, not by trying to modulate the results of how particle A is measured, but by modulating whether particle A is measured or not measured. The effect of this modulating method is to place particle B in either a single value (but arbitrary) state or leave it in a superposition of states. It is then possible for the reciever of particle B to distinguish between these two states by the use of an appropriately designed interferometer. Such a device can be designed to produce an interference pattern only when particle B is in a superposition of values and a straight simple image when particle B has a single (arbitrary) defined value. Under the Copenhagen interpretation and existing experimental results, this method will produce a way of signalling fater than light. It will require the use of multiple entangled pairs of particles to effectively transmit a single bit of information. Under the Multi-Worlds Interpretation, it will not be so easy to communicate faster than light by this mechanism, as detector B will be detecting photon B as a superposition of values in both cases.
Abstract There is a growing interest, both from the theoretical as well as experimental side, to test the validity of the quantum superposition principle, and of theories which explicitly violate it by adding nonlinear terms to the Schrödinger equation. We review the original argument elaborated by Gisin (1989 Helv. Phys. Acta 62 363), which shows that the non-superluminal-signaling condition implies that the dynamics of the density matrix must be linear. This places very strong constraints on the permissible modifications of the Schrödinger equation, since they have to give rise, at the statistical level, to a linear evolution for the density matrix. The derivation is done in a heuristic way here and is appropriate for the students familiar with the textbook quantum mechanics and the language of density matrices.
Quantum entanglement between particles is expected to allow one to perform tasks that would otherwise be impossible. In quantum sensing and metrology, entanglement is often claimed to enable a measurement precision that cannot be attained with the same number of particles and time, forgoing entanglement. Two distinct approaches exist: creation of entangled states that either i) respond quicker to the signal, or ii) are associated with lower noise and uncertainty. The second class of states are generally called squeezed states. Here we show that if our definition of success is a precision that is impossible to achieve using the same resources but without entanglement then squeezed states cannot succeed. In doing so we show that a single non-separable squeezed state provides fundamentally no better precision, per unit time, than a single particle.