9 research outputs found
The mother of all protocols: Restructuring quantum information's family tree
We give a simple, direct proof of the "mother" protocol of quantum
information theory. In this new formulation, it is easy to see that the mother,
or rather her generalization to the fully quantum Slepian-Wolf protocol,
simultaneously accomplishes two goals: quantum communication-assisted
entanglement distillation, and state transfer from the sender to the receiver.
As a result, in addition to her other "children," the mother protocol generates
the state merging primitive of Horodecki, Oppenheim and Winter, a fully quantum
reverse Shannon theorem, and a new class of distributed compression protocols
for correlated quantum sources which are optimal for sources described by
separable density operators. Moreover, the mother protocol described here is
easily transformed into the so-called "father" protocol whose children provide
the quantum capacity and the entanglement-assisted capacity of a quantum
channel, demonstrating that the division of single-sender/single-receiver
protocols into two families was unnecessary: all protocols in the family are
children of the mother.Comment: 25 pages, 6 figure
Optimal superdense coding of entangled states
We present a one-shot method for preparing pure entangled states between a
sender and a receiver at a minimal cost of entanglement and quantum
communication. In the case of preparing unentangled states, an earlier paper
showed that a 2n-qubit quantum state could be communicated to a receiver by
physically transmitting only n+o(n) qubits in addition to consuming n ebits of
entanglement and some shared randomness. When the states to be prepared are
entangled, we find that there is a reduction in the number of qubits that need
to be transmitted, interpolating between no communication at all for maximally
entangled states and the earlier two-for-one result of the unentangled case,
all without the use of any shared randomness. We also present two applications
of our result: a direct proof of the achievability of the optimal superdense
coding protocol for entangled states produced by a memoryless source, and a
demonstration that the quantum identification capacity of an ebit is two
qubits.Comment: Final Version. Several technical issues clarifie
Speed-up via Quantum Sampling
The Markov Chain Monte Carlo method is at the heart of efficient
approximation schemes for a wide range of problems in combinatorial enumeration
and statistical physics. It is therefore very natural and important to
determine whether quantum computers can speed-up classical mixing processes
based on Markov chains. To this end, we present a new quantum algorithm, making
it possible to prepare a quantum sample, i.e., a coherent version of the
stationary distribution of a reversible Markov chain. Our algorithm has a
significantly better running time than that of a previous algorithm based on
adiabatic state generation. We also show that our methods provide a speed-up
over a recently proposed method for obtaining ground states of (classical)
Hamiltonians.Comment: 8 pages, fixed some minor typo
Quantum algorithm for approximating partition functions
We present a quantum algorithm based on classical fully polynomial randomized approximation schemes (FPRASs) for estimating partition functions that combine simulated annealing with the Monte Carlo Markov chain method and use nonadaptive cooling schedules. We achieve a twofold polynomial improvement in time complexity: a quadratic reduction with respect to the spectral gap of the underlying Markov chains and a quadratic reduction with respect to the parameter characterizing the desired accuracy of the estimate output by the FPRAS. Both reductions are intimately related and cannot be achieved separately. First, we use Grover\u27s fixed-point search, quantum walks, and phase estimation to efficiently prepare approximate coherent encodings of stationary distributions of the Markov chains. The speed up we obtain in this way is due to the quadratic relation between the spectral and phase gaps of classical and quantum walks. The second speed up with respect to accuracy comes from generalized quantum counting used instead of classical sampling to estimate expected values of quantum observables
Quantum Speed-up for Approximating Partition Functions
We achieve a quantum speed-up of fully polynomial randomized approximation
schemes (FPRAS) for estimating partition functions that combine simulated
annealing with the Monte-Carlo Markov Chain method and use non-adaptive cooling
schedules. The improvement in time complexity is twofold: a quadratic reduction
with respect to the spectral gap of the underlying Markov chains and a
quadratic reduction with respect to the parameter characterizing the desired
accuracy of the estimate output by the FPRAS. Both reductions are intimately
related and cannot be achieved separately.
First, we use Grover's fixed point search, quantum walks and phase estimation
to efficiently prepare approximate coherent encodings of stationary
distributions of the Markov chains. The speed-up we obtain in this way is due
to the quadratic relation between the spectral and phase gaps of classical and
quantum walks. Second, we generalize the method of quantum counting, showing
how to estimate expected values of quantum observables. Using this method
instead of classical sampling, we obtain the speed-up with respect to accuracy.Comment: 17 pages; v3: corrected typos, added a reference about efficient
implementations of quantum walk
Generalized remote state preparation: Trading cbits, qubits and ebits in quantum communication
We consider the problem of communicating quantum states by simultaneously
making use of a noiseless classical channel, a noiseless quantum channel and
shared entanglement. We specifically study the version of the problem in which
the sender is given knowledge of the state to be communicated. In this setting,
a trade-off arises between the three resources, some portions of which have
been investigated previously in the contexts of the quantum-classical trade-off
in data compression, remote state preparation and superdense coding of quantum
states, each of which amounts to allowing just two out of these three
resources. We present a formula for the triple resource trade-off that reduces
its calculation to evaluating the data compression trade-off formula. In the
process, we also construct protocols achieving all the optimal points. These
turn out to be achievable by trade-off coding and suitable time-sharing between
optimal protocols for cases involving two resources out of the three mentioned
above.Comment: 15 pages, 2 figures, 1 tabl
Unification of Quantum Information Theory
We present the unification of many previously disparate results in noisy quantum Shannon theory and the unification of all of noiseless quantum Shannon theory. More specifically we deal here with bipartite, unidirectional, and memoryless quantum Shannon theory. We find all the optimal protocols and quantify the relationship between the resources used, both for the one-shot and for the ensemble case, for what is arguably the most fundamental task in quantum information theory: sharing entangled states between a sender and a receiver. We find that all of these protocols are derived from our one-shot superdense coding protocol and relate nicely to each other. We then move on to noisy quantum information theory and give a simple, direct proof of the "mother" protocol, or rather her generalization to the Fully Quantum Slepian-Wolf protocol(FQSW). FQSW simultaneously accomplishes two goals: quantum communication-assisted entanglement distillation, and state transfer from the sender to the receiver. As a result, in addition to her other "children," the mother protocol generates the state merging primitive of Horodecki, Oppenheim, and Winter as well as a new class of distributed compression protocols for correlated quantum sources, which are optimal for sources described by separable density operators. Moreover, the mother protocol described here is easily transformed into the so-called "father" protocol, demonstrating that the division of single-sender/single-receiver protocols into two families was unnecessary: all protocols in the family are children of the mother
Quantifying Quantum Nonlocality
Quantum mechanics is nonlocal, meaning it cannot be described by any classical local hidden variable model. In this thesis we study two aspects of quantum nonlocality.
Part I addresses the question of what classical resources are required to simulate nonlocal quantum correlations. We start by constructing new local models for noisy entangled quantum states. These constructions exploit the connection between nonlocality and Grothendieck's inequality, first noticed by Tsirelson. Next, we consider local models augmented by a limited amount of classical communication. After generalizing Bell inequalities to this setting, we show that (i) one bit of communication is sufficient to simulate the correlations of projective measurements on a maximally entangled state of two qubits, and (ii) five bits of communication are sufficient to simulate the joint correlation of two-outcome measurements on any bipartite quantum state. The latter result can be interpreted as a stronger (constrained) version of Grothendieck's inequality.
In part II, we investigate the monogamy of nonlocal correlations. In a setting where three parties, A, B, and C, share an entangled quantum state of arbitrary dimension, we: (i) bound the trade-off between AB's and AC's violation of the CHSH inequality, obtaining an intriguing generalization of Tsirelson's bound, and (ii) demonstrate that forcing B and C to be classically correlated prevents A and B from violating certain Bell inequalities. We study not only correlations that arise within quantum theory, but also arbitrary correlations that do not allow signaling between separate groups of parties. These results are based on new techniques for obtaining Tsirelson bounds, or bounds on the quantum value of a Bell inequality, and have applications to interactive proof systems and cryptography.</p