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Abstract: Quantum information is scrambled via a chaotic time evolution in many-body systems. Clarifying how and to what extent we can recover the quantum information from the scrambled state is a central issue of today's physics, since it commonly appears as an essential factor in various topics including irreversibility of quantum chaotic dynamics, fault-tolerant quantum computation, and black hole information paradox. In realistic settings, symmetry can ubiquitously exist in scrambling dynamics. Here we establish fundamental limitations on the information recovery arising from the Lie group symmetries in the scrambling dynamics. The results are applicable to many situations, including the Hayden-Preskill black hole model with symmetry where the success rate of the information recovery qualitatively changes from the original no-symmetry case. Our findings show a close relationship between information recovery, symmetry, and quantum coherence, and that symmetry hinders the quantum information recovery.

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Topics: Quantum information (67%), Quantum computer (59%), Black hole information paradox (58%) ... show more

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Abstract: We study information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the ``half-way'' point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. Our estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis.

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Topics: Quantum information (68%)

575 Citations

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Abstract: The Hayden-Preskill protocol is a quantum information theoretic model of the black hole information paradox. Based on the protocol, it was revealed that information scrambling and entanglement lead to an instant leakage of information. In this paper, we study the information paradox with symmetry in the framework of the Hayden-Preskill protocol. Symmetry is an important feature of black holes that induces yet more conceptual puzzles in the regime of quantum gravity. We especially consider an axial symmetry and clarify its consequences in the information leakage problem. Using a partial decoupling approach, we first show that symmetry induces a \emph{delay} of information leakage and an \emph{information remnant}, both of which can be macroscopically large for certain initial conditions. We then clarify the physics behind the delay and the information remnant. By introducing the concept of \emph{clipping of entanglement}, we show that the delay is characterized by thermodynamic properties of the black hole associated with the symmetry. We also show that the information remnant is closely related to the symmetry-breaking of the black hole. These relations indicate the existence of non-trivial microscopic-macroscopic correspondences in the information leakage problem.

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Topics: Black hole information paradox (60%), Quantum information (56%), Symmetry (physics) (51%) ... show more

2 Citations

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Abstract: It is known that continuous symmetries induce fundamental restrictions on the accuracy of quantum error correction (QEC). Here we systematically study the competition between continuous symmetries and QEC in a quantitative manner. We first define meaningful measures of approximate symmetries based on the degree of covariance and charge conservation violation, which induce corresponding notions of approximately covariant codes, and then derive a series of trade-off bounds between these different approximate symmetry measures and QEC accuracy by leveraging insights and techniques from approximate QEC, quantum metrology, and resource theory. From a quantum computation perspective, our results indicate general limits on the precision and density of transversal logical gates. For concrete examples, we showcase two explicit types of approximately covariant codes that nearly saturate certain bounds, respectively obtained from quantum Reed--Muller codes and thermodynamic codes. Finally, we discuss potential applications of our theory to several important topics in physics.

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Topics: Quantum error correction (61%), Quantum computer (56%), Quantum metrology (54%) ... show more

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Abstract: Quantum error correction and symmetries play central roles in quantum information science and physics. It is known that quantum error-correcting codes that obey (covariant with respect to) continuous symmetries cannot correct erasure errors perfectly (a well-known result in this regard being the Eastin-Knill theorem in the context of fault-tolerant quantum computing), in contrast to the case without symmetry constraints. Furthermore, several quantitative fundamental limits on the accuracy of such covariant codes for approximate quantum error correction are known. Here, we consider the quantum error correction capability of uniformly random covariant codes. In particular, we analytically study the most essential cases of U(1) and SU(d) symmetries, and show that for both symmetry groups the error of the covariant codes generated by Haar-random symmetric unitaries, i.e. unitaries that commute with the group actions, typically scale as O(n^(-1)) in terms of both the average- and worst-case purified distances against erasure noise, saturating the fundamental limits to leading order. We note that the results hold for symmetric variants of unitary 2-designs, and comment on the convergence problem of symmetric random circuits. Our results not only indicate (potentially efficient) randomized constructions of optimal U(1)- and SU(d)-covariant codes, but also reveal fundamental properties of random symmetric unitaries, which underlie important models of complex quantum systems in wide-ranging physical scenarios with symmetries, such as black holes and many-body spin systems. Our study and techniques may have broad relevance for both physics and quantum computing.

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Topics: Quantum error correction (66%), Quantum information science (61%), Quantum computer (60%) ... show more

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Abstract: We study the problem of quantum hacking, which is the procedure of quantum-information extraction from and installation on a quantum network given only partial access. This problem generalizes a central topic in contemporary physics -- information recovery from systems undergoing scrambling dynamics, such as the Hayden--Preskill protocol in black-hole studies. We show that a properly prepared partially entangled probe state can generally outperform a maximally entangled one in quantum hacking. Moreover, we prove that finding an optimal decoder for this stronger task is equivalent to that for Hayden--Preskill-type protocols, and supply analytical formulas for the optimal hacking fidelity of large networks. In the two-user scenario where Bob attempts to hack Alice's data, we find that the optimal fidelity increases with Bob's hacking space relative to Alice's user space. However, if a third neutral party, Charlie, is accessing the computer concurrently, the optimal hacking fidelity against Alice drops with Charlie's user-space dimension, rendering targeted quantum hacking futile in high-dimensional multi-user scenarios without classical attacks. When applied to the black-hole information problem, the limited hacking fidelity implies a reflectivity decay of a black hole as an information mirror.

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Topics: Quantum network (56%)

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43 results found

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Abstract: In the classical theory black holes can only absorb and not emit particles. However it is shown that quantum mechanical effects cause black holes to create and emit particles as if they were hot bodies with temperature\(\frac{{h\kappa }}{{2\pi k}} \approx 10^{ - 6} \left( {\frac{{M_ \odot }}{M}} \right){}^ \circ K\) where κ is the surface gravity of the black hole. This thermal emission leads to a slow decrease in the mass of the black hole and to its eventual disappearance: any primordial black hole of mass less than about 1015 g would have evaporated by now. Although these quantum effects violate the classical law that the area of the event horizon of a black hole cannot decrease, there remains a Generalized Second Law:S+1/4A never decreases whereS is the entropy of matter outside black holes andA is the sum of the surface areas of the event horizons. This shows that gravitational collapse converts the baryons and leptons in the collapsing body into entropy. It is tempting to speculate that this might be the reason why the Universe contains so much entropy per baryon.

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Topics: Black hole (62%), Hawking radiation (61%), Extremal black hole (60%) ... show more

10,022 Citations

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01 Jan 1969-

Abstract: A review. Quantum detection theory is a reformulation, in quantum-mechanical terms, of statistical decision theory as applied to the detection of signals in random noise. Density operators take the place of the probability density functions of conventional statistics. The optimum procedure for choosing between two hypotheses, and an approximate procedure valid at small signal-to-noise ratios and called threshold detection, are presented. Quantum estimation theory seeks best estimators of parameters of a density operator. A quantum counterpart of the Cramer-Rao inequality of conventional statistics sets a lower bound to the mean-square errors of such estimates. Applications at present are primarily to the detection and estimation of signals of optical frequencies in the presence of thermal radiation.

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Topics: Quantum phase estimation algorithm (63%), Quantum algorithm (62%), Estimator (59%) ... show more

3,781 Citations

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Abstract: Foreword to 2nd English edition.- Foreword to 2nd Russian edition.- Preface.- Chapters: I. Statistical Models.- II. Mathematics of Quantum Theory.- III. Symmetry Groups in Quantum Mechanics.- IV. Covariant Measurements and Optimality.- V. Gaussian States.- VI Unbiased Measurements.- Supplement - Statistical Structure of Quantum Theory and Hidden Variables.- References.

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Topics: Quantum statistical mechanics (77%), Quantum probability (77%), Probabilistic logic (54%)

2,121 Citations

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Abstract: If a quantum system of Hilbert space dimension mn is in a random pure state, the average entropy of a subsystem of dimension m\ensuremath{\le}n is conjectured to be ${\mathit{S}}_{\mathit{m},\mathit{n}}$= ${\mathit{S}}_{\mathit{k}=\mathit{n}+1}^{\mathit{mn}}$ 1/k-m-1/2n and is shown to be \ensuremath{\simeq}lnm-m/2n for 1\ensuremath{\ll}m\ensuremath{\le}n. Thus there is less than one-half unit of information, on average, in the smaller subsystem of a total system in a random pure state.

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1,168 Citations

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Abstract: The following theorem is established. Among all static, asymptotically flat vacuum space-times with closed simply connected equipotential surfaces ${g}_{00}=\mathrm{constant}$, the Schwarzschild solution is the only one which has a nonsingular infinite-red-shift surface ${g}_{00}=0$. Thus there exists no static asymmetric perturbation of the Schwarzschild manifold due to internal sources (e.g., a quadrupole moment) which will preserve a regular event horizon. Possible implications of this result for asymmetric gravitational collapse are briefly discussed.

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Topics: Deriving the Schwarzschild solution (59%), Schwarzschild metric (59%), Kerr metric (58%) ... show more

1,031 Citations