142 research outputs found
Quantum Key Distribution with Classical Bob
Secure key distribution among two remote parties is impossible when both are
classical, unless some unproven (and arguably unrealistic)
computation-complexity assumptions are made, such as the difficulty of
factorizing large numbers. On the other hand, a secure key distribution is
possible when both parties are quantum.
What is possible when only one party (Alice) is quantum, yet the other (Bob)
has only classical capabilities? We present a protocol with this constraint,
and prove its robustness against attacks: we prove that any attempt of an
adversary to obtain information (and even a tiny amount of information)
necessarily induces some errors that the legitimate users could notice.Comment: 4 and a bit pages, 1 figure, RevTe
Signatures of non-classicality in mixed-state quantum computation
We investigate signatures of non-classicality in quantum states, in
particular, those involved in the DQC1 model of mixed-state quantum computation
[Phys. Rev. Lett. 81, 5672 (1998)]. To do so, we consider two known
non-classicality criteria. The first quantifies disturbance of a quantum state
under locally noneffective unitary operations (LNU), which are local unitaries
acting invariantly on a subsystem. The second quantifies measurement induced
disturbance (MID) in the eigenbasis of the reduced density matrices. We study
the role of both figures of non-classicality in the exponential speedup of the
DQC1 model and compare them vis-a-vis the interpretation provided in terms of
quantum discord. In particular, we prove that a non-zero quantum discord
implies a non-zero shift under LNUs. We also use the MID measure to study the
locking of classical correlations [Phys. Rev. Lett. 92, 067902 (2004)] using
two mutually unbiased bases (MUB). We find the MID measure to exactly
correspond to the number of locked bits of correlation. For three or more MUBs,
it predicts the possibility of superior locking effects.Comment: Published version, containing additional discussion on the role of
non-classicality in the locking of classical correlation
Quantum discord and the power of one qubit
We use quantum discord to characterize the correlations present in the
quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev.
Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the
completely mixed state coupled to a single control qubit that has nonzero
purity. The initial state, operations, and measurements in the model all point
to a natural bipartite split between the control qubit and the mixed ones.
Although there is no entanglement between these two parts, we show that the
quantum discord across this split is nonzero for typical instances of the DQC1
ciruit. Nonzero values of discord indicate the presence of nonclassical
correlations. We propose quantum discord as figure of merit for characterizing
the resources present in this computational model.Comment: 4 Pages, 1 Figur
Entanglement Is Not Necessary for Perfect Discrimination between Unitary Operations
We show that a unitary operation (quantum circuit) secretely chosen from a
finite set of unitary operations can be determined with certainty by
sequentially applying only a finite amount of runs of the unknown circuit. No
entanglement or joint quantum operations is required in our scheme. We further
show that our scheme is optimal in the sense that the number of the runs is
minimal when discriminating only two unitary operations.Comment: 4 pages(in Revtex4), 2 figures (eps). Corrected some typos and two
equations. Main results unchanged. Essentially the journal versio
"Quantumness" versus "classicality" of quantum states and quantum protocols
Entanglement is one of the pillars of quantum mechanics and quantum information processing, and as a result, the quantumness of nonentangled states has typically been overlooked and unrecognized until the last decade. We give a robust definition for the classicality versus quantumness of a single multipartite quantum state, a set of states, and a protocol using quantum states. We show a variety of nonentangled (separable) states that exhibit interesting quantum properties, and we explore the "zoo" of separable states; several interesting subclasses are defined based on the diagonalizing bases of the states, and their nonclassical behavior is investigated.The work of BG was funded by EPSRC and Sidney Sussex College, Cambridge. T.M was funded by the Wolfson Foundation and the Israeli MOD Research and Technology Unit. AB and TM were partly supported The Gerald Schwartz & Heather Reis- man Foundation
Evolution of elastic and mechanical properties during fault shear. The roles of clay content, fabric development, and porosity
Phyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite-quartz fault gouges (10–100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite-poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300–1,800 m/s, Vs ~ 900–1,100 m/s, K ~ 1–4 GPa, and G ~ 1–2 GPa, and low smectite gouges have values of Vp ~ 2,300–2,500 m/s, Vs ~ 1,200–1,300 m/s, K ~ 5–8 GPa, and G ~ 2.5–3 GPa. We find that, even in smectite-poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine-grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces
Private quantum decoupling and secure disposal of information
Given a bipartite system, correlations between its subsystems can be
understood as information that each one carries about the other. In order to
give a model-independent description of secure information disposal, we propose
the paradigm of private quantum decoupling, corresponding to locally reducing
correlations in a given bipartite quantum state without transferring them to
the environment. In this framework, the concept of private local randomness
naturally arises as a resource, and total correlations get divided into
eliminable and ineliminable ones. We prove upper and lower bounds on the amount
of ineliminable correlations present in an arbitrary bipartite state, and show
that, in tripartite pure states, ineliminable correlations satisfy a monogamy
constraint, making apparent their quantum nature. A relation with entanglement
theory is provided by showing that ineliminable correlations constitute an
entanglement parameter. In the limit of infinitely many copies of the initial
state provided, we compute the regularized ineliminable correlations to be
measured by the coherent information, which is thus equipped with a new
operational interpretation. In particular, our results imply that two
subsystems can be privately decoupled if their joint state is separable.Comment: Child of 0807.3594 v2: minor changes v3: presentation improved, one
figure added v4: extended version with a lot of discussions and examples v5:
published versio
Experimental investigation of classical and quantum correlations under decoherence
It is well known that many operations in quantum information processing
depend largely on a special kind of quantum correlation, that is, entanglement.
However, there are also quantum tasks that display the quantum advantage
without entanglement. Distinguishing classical and quantum correlations in
quantum systems is therefore of both fundamental and practical importance. In
consideration of the unavoidable interaction between correlated systems and the
environment, understanding the dynamics of correlations would stimulate great
interest. In this study, we investigate the dynamics of different kinds of
bipartite correlations in an all-optical experimental setup. The sudden change
in behaviour in the decay rates of correlations and their immunity against
certain decoherences are shown. Moreover, quantum correlation is observed to be
larger than classical correlation, which disproves the early conjecture that
classical correlation is always greater than quantum correlation. Our
observations may be important for quantum information processing.Comment: 7 pages, 4 figures, to appear in Nature Communication
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