1,615 research outputs found
A most compendious and facile quantum de Finetti theorem
In its most basic form, the finite quantum de Finetti theorem states that the reduced k-partite density operator of an n-partite symmetric state can be approximated by a convex combination of k-fold product states. Variations of this result include Renner's “exponential” approximation by “almost-product” states, a theorem which deals with certain triples of representations of the unitary group, and the result of D'Cruz et al. [e-print quant-ph/0606139;Phys. Rev. Lett. 98, 160406 (2007)] for infinite-dimensional systems. We show how these theorems follow from a single, general de Finetti theorem for representations of symmetry groups, each instance corresponding to a particular choice of symmetry group and representation of that group. This gives some insight into the nature of the set of approximating states and leads to some new results, including an exponential theorem for infinite-dimensional systems
On exchangeable continuous variable systems
We investigate permutation-invariant continuous variable quantum states and their covariance matrices. We provide a complete characterization of the latter with respect to permutation invariance and exchangeability and representing convex combinations of tensor power states. On the level of the respective density operators this leads to necessary criteria for all these properties which become necessary and sufficient for Gaussian states. For these we use the derived results to provide de Finetti-type theorems for various distance measures
Classification of topologically protected gates for local stabilizer codes
Given a quantum error correcting code, an important task is to find encoded
operations that can be implemented efficiently and fault-tolerantly. In this
Letter we focus on topological stabilizer codes and encoded unitary gates that
can be implemented by a constant-depth quantum circuit. Such gates have a
certain degree of protection since propagation of errors in a constant-depth
circuit is limited by a constant size light cone. For the 2D geometry we show
that constant-depth circuits can only implement a finite group of encoded gates
known as the Clifford group. This implies that topological protection must be
"turned off" for at least some steps in the computation in order to achieve
universality. For the 3D geometry we show that an encoded gate U is
implementable by a constant-depth circuit only if the image of any Pauli
operator under conjugation by U belongs to the Clifford group. This class of
gates includes some non-Clifford gates such as the \pi/8 rotation. Our
classification applies to any stabilizer code with geometrically local
stabilizers and sufficiently large code distance.Comment: 6 pages, 2 figure
A de Finetti representation for finite symmetric quantum states
Consider a symmetric quantum state on an n-fold product space, that is, the
state is invariant under permutations of the n subsystems. We show that,
conditioned on the outcomes of an informationally complete measurement applied
to a number of subsystems, the state in the remaining subsystems is close to
having product form. This immediately generalizes the so-called de Finetti
representation to the case of finite symmetric quantum states.Comment: 22 pages, LaTe
A Reference Model to Support Risk Identification in Cloud Networks
The rising adoption of cloud computing and increasing interconnections among its actors lead to the emergence of network-like structures and new associated risks. A major obstacle for addressing these risks is the lack of transparency concerning the underlying network structure and the dissemination of risks therein. Existing research does not consider the risk perspective in a cloud network’s context. We address this research gap with the construction of a reference model that can display such networks and therefore supports risk identification. We evaluate the reference model through real-world examples and interviews with industry experts and demonstrate its applicability. The model provides a better understanding of cloud networks and causalities between related risks. These insights can be used to develop appropriate risk management strategies in cloud networks. The reference model sets a basis for future risk quantification approaches as well as for the design of (IT) tools for risk analysis
Limitations of local update recovery in stabilizer-GKP codes: a quantum optimal transport approach
Local update recovery seeks to maintain quantum information by applying local
correction maps alternating with and compensating for the action of noise.
Motivated by recent constructions based on quantum LDPC codes in the
finite-dimensional setting, we establish an analytic upper bound on the
fault-tolerance threshold for concatenated GKP-stabilizer codes with local
update recovery. Our bound applies to noise channels that are tensor products
of one-mode beamsplitters with arbitrary environment states, capturing, in
particular, photon loss occurring independently in each mode. It shows that for
loss rates above a threshold given explicitly as a function of the locality of
the recovery maps, encoded information is lost at an exponential rate. This
extends an early result by Razborov from discrete to continuous variable (CV)
quantum systems.
To prove our result, we study a metric on bosonic states akin to the
Wasserstein distance between two CV density functions, which we call the
bosonic Wasserstein distance. It can be thought of as a CV extension of a
quantum Wasserstein distance of order 1 recently introduced by De Palma et al.
in the context of qudit systems, in the sense that it captures the notion of
locality in a CV setting. We establish several basic properties, including a
relation to the trace distance and diameter bounds for states with finite
average photon number. We then study its contraction properties under quantum
channels, including tensorization, locality and strict contraction under
beamsplitter-type noise channels. Due to the simplicity of its formulation, and
the established wide applicability of its finite-dimensional counterpart, we
believe that the bosonic Wasserstein distance will become a versatile tool in
the study of CV quantum systems.Comment: 30 pages, 2 figure
Magnetic Domain Structure of La0.7Sr0.3MnO3 thin-films probed at variable temperature with Scanning Electron Microscopy with Polarization Analysis
The domain configuration of 50 nm thick La0.7SrMnO3 films has been directly
investigated using scanning electron microscopy with polarization analysis
(SEMPA), with magnetic contrast obtained without the requirement for prior
surface preparation. The large scale domain structure reflects a primarily
four-fold anisotropy, with a small uniaxial component, consistent with
magneto-optic Kerr effect measurements. We also determine the domain transition
profile and find it to be in agreement with previous estimates of the domain
wall width in this material. The temperature dependence of the image contrast
is investigated and compared to superconducting-quantum interference device
magnetometry data. A faster decrease in the SEMPA contrast is revealed, which
can be explained by the technique's extreme surface sensitivity, allowing us to
selectively probe the surface spin polarization which due to the double
exchange mechanism exhibits a distinctly different temperature dependence than
the bulk magnetization
Investigation on constitutive IKK activity in the axon initial segment
Poster presentation: The transcription factor NF-kappaB plays a central role in the development and maintenance of the central nervous system and its constitutive activation in neurons has been repeatedly reported. Previous work from our laboratories (poster presentation: Compartimentalized NF-kappaB activity in the axon initial segment) had revealed an intriguing clustering of activated IKKalpha/beta and other downstream elements of an activated NF-kappaB cascade (phospho-IkappaBalpha, phospho-p65(Ser536)) in the axon initial segment (AIS). Accumulation of certain voltage-gated sodium channels (Na(v)1.2), M-type potassium channels (KCNQ2) as well as cytoskeletal anchoring proteins (AnkyrinG) characterise the AIS. However, it is not yet clear how AIS-localized IKK gets activated and whether this can be connected to the constitutive activation of NF-kappaB. Long-term blockade of sodium channels with tetrodotoxin, potassium-channels with linopirdine or NMDA-receptors with MK-801 did not elicit any change upon the constitutive activation of the pathway. Strikingly, the occurrence of phosphorylated IkappaBalpha was even unaltered by 24 h of incubation with protein synthesis inhibitors. Others have reported that impairment of NF-kappaB inhibits neuritogenesis. In this line we observed that the early initiation of IkappaBalpha phosphorylation was susceptible to inhibition of IKK in DIV1–2 neurons. We therefore aim to identify the interaction partners of the activated IKK complex in the AIS. Proteomic methods such as co-immunoprecipitation analyses and mass-spectrometry will help us to identify the key players in the initiation of constitutive IKK phosphorylation and activation in neurons
A strong converse for classical channel coding using entangled inputs
A fully general strong converse for channel coding states that when the rate
of sending classical information exceeds the capacity of a quantum channel, the
probability of correctly decoding goes to zero exponentially in the number of
channel uses, even when we allow code states which are entangled across several
uses of the channel. Such a statement was previously only known for classical
channels and the quantum identity channel. By relating the problem to the
additivity of minimum output entropies, we show that a strong converse holds
for a large class of channels, including all unital qubit channels, the
d-dimensional depolarizing channel and the Werner-Holevo channel. This further
justifies the interpretation of the classical capacity as a sharp threshold for
information-transmission.Comment: 9 pages, revte
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