669 research outputs found

### Gapped quantum liquids and topological order, stochastic local transformations and emergence of unitarity

In this work we present some new understanding of topological order,
including three main aspects: (1) It was believed that classifying topological
orders corresponds to classifying gapped quantum states. We show that such a
statement is not precise. We introduce the concept of \emph{gapped quantum
liquid} as a special kind of gapped quantum states that can "dissolve" any
product states on additional sites. Topologically ordered states actually
correspond to gapped quantum liquids with stable ground-state degeneracy.
Symmetry-breaking states for on-site symmetry are also gapped quantum liquids,
but with unstable ground-state degeneracy. (2) We point out that the
universality classes of generalized local unitary (gLU) transformations
(without any symmetry) contain both topologically ordered states and
symmetry-breaking states. This allows us to use a gLU invariant -- topological
entanglement entropy -- to probe the symmetry-breaking properties hidden in the
exact ground state of a finite system, which does not break any symmetry. This
method can probe symmetry- breaking orders even without knowing the symmetry
and the associated order parameters. (3) The universality classes of
topological orders and symmetry-breaking orders can be distinguished by
\emph{stochastic local (SL) transformations} (i.e.\ \emph{local invertible
transformations}): small SL transformations can convert the symmetry-breaking
classes to the trivial class of product states with finite probability of
success, while the topological-order classes are stable against any small SL
transformations, demonstrating a phenomenon of emergence of unitarity. This
allows us to give a new definition of long-range entanglement based on SL
transformations, under which only topologically ordered states are long-range
entangled.Comment: Revised version. Figures and references adde

### Universal Entanglers for Bosonic and Fermionic Systems

A universal entangler (UE) is a unitary operation which maps all pure product
states to entangled states. It is known that for a bipartite system of
particles $1,2$ with a Hilbert space $\mathbb{C}^{d_1}\otimes\mathbb{C}^{d_2}$,
a UE exists when $\min{(d_1,d_2)}\geq 3$ and $(d_1,d_2)\neq (3,3)$. It is also
known that whenever a UE exists, almost all unitaries are UEs; however to
verify whether a given unitary is a UE is very difficult since solving a
quadratic system of equations is NP-hard in general. This work examines the
existence and construction of UEs of bipartite bosonic/fermionic systems whose
wave functions sit in the symmetric/antisymmetric subspace of
$\mathbb{C}^{d}\otimes\mathbb{C}^{d}$. The development of a theory of UEs for
these types of systems needs considerably different approaches from that used
for UEs of distinguishable systems. This is because the general entanglement of
identical particle systems cannot be discussed in the usual way due to the
effect of (anti)-symmetrization which introduces "pseudo entanglement" that is
inaccessible in practice. We show that, unlike the distinguishable particle
case, UEs exist for bosonic/fermionic systems with Hilbert spaces which are
symmetric (resp. antisymmetric) subspaces of
$\mathbb{C}^{d}\otimes\mathbb{C}^{d}$ if and only if $d\geq 3$ (resp. $d\geq
8$). To prove this we employ algebraic geometry to reason about the different
algebraic structures of the bosonic/fermionic systems. Additionally, due to the
relatively simple coherent state form of unentangled bosonic states, we are
able to give the explicit constructions of two bosonic UEs. Our investigation
provides insight into the entanglement properties of systems of
indisitinguishable particles, and in particular underscores the difference
between the entanglement structures of bosonic, fermionic and distinguishable
particle systems.Comment: 15 pages, comments welcome, TQC2013 Accepted Tal

### Unextendible Product Basis for Fermionic Systems

We discuss the concept of unextendible product basis (UPB) and generalized
UPB for fermionic systems, using Slater determinants as an analogue of product
states, in the antisymmetric subspace \wedge^ N \bC^M. We construct an
explicit example of generalized fermionic unextendible product basis (FUPB) of
minimum cardinality $N(M-N)+1$ for any $N\ge2,M\ge4$. We also show that any
bipartite antisymmetric space \wedge^ 2 \bC^M of codimension two is spanned
by Slater determinants, and the spaces of higher codimension may not be spanned
by Slater determinants. Furthermore, we construct an example of complex FUPB of
$N=2,M=4$ with minimum cardinality $5$. In contrast, we show that a real FUPB
does not exist for $N=2,M=4$ . Finally we provide a systematic construction for
FUPBs of higher dimensions using FUPBs and UPBs of lower dimensions.Comment: 17 pages, no figure. Comments are welcom

### Codes for Simultaneous Transmission of Quantum and Classical Information

We consider the characterization as well as the construction of quantum codes
that allow to transmit both quantum and classical information, which we refer
to as `hybrid codes'. We construct hybrid codes $[\![n,k{: }m,d]\!]_q$ with
length $n$ and distance $d$, that simultaneously transmit $k$ qudits and $m$
symbols from a classical alphabet of size $q$. Many good codes such as
$[\![7,1{: }1,3]\!]_2$, $[\![9,2{: }2,3]\!]_2$, $[\![10,3{: }2,3]\!]_2$,
$[\![11,4{: }2,3]\!]_2$, $[\![11,1{: }2,4]\!]_2$, $[\![13,1{: }4,4]\!]_2$,
$[\![13,1{: }1,5]\!]_2$, $[\![14,1{: }2,5]\!]_2$, $[\![15,1{: }3,5]\!]_2$,
$[\![19,9{: }1,4]\!]_2$, $[\![20,9{: }2,4]\!]_2$, $[\![21,9{: }3,4]\!]_2$,
$[\![22,9{: }4,4]\!]_2$ have been found. All these codes have better parameters
than hybrid codes obtained from the best known stabilizer quantum codes.Comment: 6 page

### Codeword Stabilized Quantum Codes for Asymmetric Channels

We discuss a method to adapt the codeword stabilized (CWS) quantum code
framework to the problem of finding asymmetric quantum codes. We focus on the
corresponding Pauli error models for amplitude damping noise and phase damping
noise. In particular, we look at codes for Pauli error models that correct one
or two amplitude damping errors. Applying local Clifford operations on graph
states, we are able to exhaustively search for all possible codes up to length
$9$. With a similar method, we also look at codes for the Pauli error model
that detect a single amplitude error and detect multiple phase damping errors.
Many new codes with good parameters are found, including nonadditive codes and
degenerate codes.Comment: 5 page

### Concatenated Codes for Amplitude Damping

We discuss a method to construct quantum codes correcting amplitude damping
errors via code concatenation. The inner codes are chosen as asymmetric
Calderbank-Shor-Steane (CSS) codes. By concatenating with outer codes
correcting symmetric errors, many new codes with good parameters are found,
which are better than the amplitude damping codes obtained by any previously
known construction.Comment: 5 page

### On Corporate Hedging and Firm Focus and on Bank Board Structure

This dissertation consists of two essays: one looks at the relation between firm focus and hedging in the REIT industry, and the other compares bank board structures in China and the US. The first essay presented in Chapter 2 examines the relation between corporate hedging and firm focus in the REIT industry by using a sample of REITs in 2005 and in 2007. We find 46.41% utilization rate in 2005 and 43.41% in 2007. Consistent with our hypothesis, we find that, relative to diversified firms, focused firms are more likely to engage in hedging. Focused firms also tend to be involved in greater amount of hedging. We also document a negative relation between hedging and transparency, although the evidence is not overwhelming. Consistent with previous literature, there is a strong firm size effect. The second essay presented in Chapter 3 examines the relation between bank performance and board structure by using a sample of 74 US banks and 53 Chinese banks for the period 2002 to 2006. Indeed, the empirical relation between board structure and performance is virtually non-existing in China. In particular, for the US sample, the board size is found to be significantly and negatively correlated with ROA, but a larger board also tends to be associated with lower costs. For Chinese banks, the evidence indicates that governance variables are not significantly correlated with performances with the exception of block ownership: there is strong evidence that the relation between block ownership and bank performance is negative. Additionally, we find substantial differences in board structure between the two countries; in particular the average board size and the proportion of outside directors for US banks are almost twice of those in China

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