QIC Abstracts

 Vol.12 No.1&2, January 1, 2012

Research Articles:

Practical four-dimensional quantum key distribution without entanglement (pp0001-0008)
William T. Buttler, Steven K. Lamoreaux, and Justin R. Torgerson

We describe a four-dimensional ($\mathcal{D} = 4$) single-photon quantum cryptography protocol with up to twenty ($\mathcal{D} \times (2^2 + 1)$) possible states generated by a polarization-, phase- and time-encoding transmitter. This protocol can be experimentally realized with existing technology, drawing from time- and polarization-encoded systems. The protocol is error tolerant and has a maximum raw bit rate of two raw bits per detection, which when combined with state detection efficiency yields a qubit rate of up to one per transmission under ideal assumptions, or up to twice the raw bit rate of two-dimensional protocols such as the well-known BB84 protocol.

Bell violations through independent bases games (pp0009-0020)
Oded Regev
In a recent paper, Junge and Palazuelos presented two two-player games exhibiting interesting properties. In their first game, entangled players can perform notably better than classical players. The quantitative gap between the two cases is remarkably large, especially as a function of the number of inputs to the players. In their second game, entangled players can perform notably better than players that are restricted to using a maximally entangled state (of arbitrary dimension). This was the first game exhibiting such a behavior. The analysis of both games is heavily based on non-trivial results from Banach space theory and operator space theory. Here we provide alternative proofs of these two results. Our proofs are arguably simpler, use elementary probabilistic techniques and standard quantum information arguments, and also give better quantitative bounds.

Impossibility of succinct quantum proofs for collision-freeness (pp0021-0028)
Scott Aaronson

We show that any quantum algorithm to decide whether a function f[n] \rightarrow [n] is a permutation or far from a permutation\ must make \Omega( n^{1/3}/w) queries to f, even if the algorithm is given a w-qubit quantum witness in support of f being a permutation. This implies that there exists an oracle A such that {SZK}^{A}\not \subset {QMA}^{A}, answering an eight-year-old open question of the author. \ Indeed, we show that relative to some oracle, {SZK} is not in the counting class {A}_0{PP} defined by Vyalyi. The proof is a fairly simple extension of the quantum lower bound for the collision problem.

Black-box Hamiltonian simulation and unitary implementation (pp0029-0062)
Andrew M. Childs and Dominic W. Berry
We present general methods for simulating black-box Hamiltonians using quantum walks. These techniques have two main applications: simulating sparse Hamiltonians and implementing black-box unitary operations. In particular, we give the best known simulation of sparse Hamiltonians with constant precision. Our method has complexity linear in both the sparseness $\sparseness$ (the maximum number of nonzero elements in a column) and the evolution time $t$, whereas previous methods had complexity scaling as $\sparseness^4$ and were superlinear in $t$. We also consider the task of implementing an arbitrary unitary operation given a black-box description of its matrix elements. Whereas standard methods for performing an explicitly specified $\UN\times\UN$ unitary operation use $\tilde O(\UN^2)$ elementary gates, we show that a black-box unitary can be performed with bounded error using $O(\UN^{2/3} (\log\log \UN)^{4/3})$ queries to its matrix elements. In fact, except for pathological cases, it appears that most unitaries can be performed with only $\tilde O(\sqrt{\UN})$ queries, which is optimal.

Mixed maximally entangled states (pp0063-0073)
Z. G. Li, M. G. Zhao, S. M. Fei, H. Fan, and W. M. Liu
We find that the mixed maximally entangled states exist and prove that the form of the mixed maximally entangled states is unique in terms of the entanglement of formation. Moreover, even if the entanglement is quantified by other entanglement measures, this conclusion is still proven right. This result is a supplementary to the generally accepted fact that all maximally entangled states are pure. These states possess important properties of the pure maximally entangled states, for example, these states can be used as a resource for faithful teleportation and they can be distinguished perfectly by local operations and classical communication.

Entanglement distillation from quasifree fermions (pp0074-0104)
Zoltan Kadar, Michael Keyl, and Dirk Schlingemann
We develop a scheme to distill entanglement from bipartite Fermionic systems in an arbitrary quasifree state. It can be applied if either one system containing infinite one-copy entanglement is available or if an arbitrary amount of equally prepared systems can be used. We show that the efficiency of the proposed scheme is in general very good and in some cases even optimal. Furthermore we apply it to Fermions hopping on an infinite lattice and demonstrate in this context that an efficient numerical analysis is possible for more than $10^6$ lattice sites.

On learning finite-state quantum sources (pp0105-0118)
Brendan Juba

We examine the complexity of learning the distributions produced by finite-state quantum sources. We show how prior techniques for learning hidden Markov models can be adapted to the {\em quantum generator} model to find that the analogous state of affairs holds: information-theoretically, a polynomial number of samples suffice to approximately identify the distribution, but computationally, the problem is as hard as learning parities with noise, a notorious open question in computational learning theory.

Relative state measures of correlations in bipartite quantum systems (pp0119-0137)
Pierre Rudolfsson and Erik Sjoqvist

Everett's concept of relative state can be viewed as a map that contains information about correlations between measurement outcomes on two quantum systems. We demonstrate how geometric properties of the relative state map can be used to develop operationally well-defined measures of the total correlation in bipartite quantum systems of arbitrary state space dimension. These measures are invariant under local unitary transformations and non-increasing under local operations. We show that some known correlation measures have a natural interpretation in terms of relative states.

Some bounds on the minimum number of queries required for quantum channel perfect discrimination (pp0138-0148)
Cheng Lu, Jianxin Chen, and Runyao Duan
We prove a lower bound on the $q$-maximal fidelities between two quantum channels $\E_0$ and $\E_1$ and an upper bound on the $q$-maximal fidelities between a quantum channel $\E$ and an identity $\I$. Then we apply these two bounds to provide a simple sufficient and necessary condition for sequential perfect distinguishability between $\E$ and $\I$ and provide both a lower bound and an upper bound on the minimum number of queries required to sequentially perfectly discriminating $\E$ and $\I$. Interestingly, in the $2$-dimensional case, both bounds coincide. Based on the optimal perfect discrimination protocol presented in \cite{DFY09}, we can further generalize the lower bound and upper bound to the minimum number of queries to perfectly discriminating $\E$ and $I$ over all possible discrimination schemes. Finally the two lower bounds are shown remain working for perfectly discriminating general two quantum channels $\E_0$ and $\E_1$ in sequential scheme and over all possible discrimination schemes respectively.

Recovery in quantum error correction for general noise without measurement (pp0149-0158)
Chi-Kwong Li, Mikio Nakahara, Yiu-Tung Poon, Nung-Sing Sze, and Hiroyuki Tomita
It is known that one can do quantum error correction without syndrome measurement, which is often done in operator quantum error correction (OQEC). However, the physical realization could be challenging, especially when the recovery process involves high-rank projection operators and a superoperator. We use operator theory to improve OQEC so that the implementation can always be done by unitary gates followed by a partial trace operation. Examples are given to show that our error correction scheme outperforms the existing ones in various scenarios.

Nearly deterministic controlled-NOT gate with weak cross-Kerr nonlinearities (pp0159-0170)
Xiao-Ming Xiu, Li Dong, Ya-Jun Gao, and X. X. Yi
On the basis of the probe coherent state and weak cross-Kerr nonlinearities, we present a scheme of a nearly deterministic Controlled-NOT gate. In this construction, feed-forward methods, quantum nondemolition detectors and several optical elements are applied. It is a potentially practical quantum gate with certain features. First, the lack of auxiliary photons is allowable, which decreases consumption of resources. Secondly, employment of the signal photon from either of target output ports and three quantum nondemolition detectors enable the success probability to approach unit and judge whether the signal photons lose or not. Thirdly, the displacement measurement is adopted, and thus the Controlled-NOT gate works against photon loss of the probe coherent state. Finally, in order to circumvent the effect of dephasing, the monochromatic signal photons are exploited.

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