4,084 research outputs found
Nonclassical correlation in NMR quadrupolar systems
The existence of quantum correlation (as revealed by quantum discord), other
than entanglement and its role in quantum-information processing (QIP), is a
current subject for discussion. In particular, it has been suggested that this
nonclassical correlation may provide computational speedup for some quantum
algorithms. In this regard, bulk nuclear magnetic resonance (NMR) has been
successfully used as a test bench for many QIP implementations, although it has
also been continuously criticized for not presenting entanglement in most of
the systems used so far. In this paper, we report a theoretical and
experimental study on the dynamics of quantum and classical correlations in an
NMR quadrupolar system. We present a method for computing the correlations from
experimental NMR deviation-density matrices and show that, given the action of
the nuclear-spin environment, the relaxation produces a monotonic time decay in
the correlations. Although the experimental realizations were performed in a
specific quadrupolar system, the main results presented here can be applied to
whichever system uses a deviation-density matrix formalism.Comment: Published versio
Experimental Implementation of Logical Bell State Encoding
Liquid phase NMR is a general purpose test-bed for developing methods of
coherent control relevant to quantum information processing. Here we extend
these studies to the coherent control of logical qubits and in particular to
the unitary gates necessary to create entanglement between logical qubits. We
report an experimental implementation of a conditional logical gate between two
logical qubits that are each in decoherence free subspaces that protect the
quantum information from fully correlated dephasing.Comment: 9 Pages, 5 Figure
Bounds on the entanglability of thermal states in liquid-state nuclear magnetic resonance
The role of mixed state entanglement in liquid-state nuclear magnetic
resonance (NMR) quantum computation is not yet well-understood. In particular,
despite the success of quantum information processing with NMR, recent work has
shown that quantum states used in most of those experiments were not entangled.
This is because these states, derived by unitary transforms from the thermal
equilibrium state, were too close to the maximally mixed state. We are thus
motivated to determine whether a given NMR state is entanglable - that is, does
there exist a unitary transform that entangles the state? The boundary between
entanglable and nonentanglable thermal states is a function of the spin system
size and its temperature . We provide new bounds on the location of this
boundary using analytical and numerical methods; our tightest bound scales as
, giving a lower bound requiring at least proton
spins to realize an entanglable thermal state at typical laboratory NMR
magnetic fields. These bounds are tighter than known bounds on the
entanglability of effective pure states.Comment: REVTeX4, 15 pages, 4 figures (one large figure: 414 K
Experimental implementation of a NMR entanglement witness
Entanglement witnesses (EW) allow the detection of entanglement in a quantum
system, from the measurement of some few observables. They do not require the
complete determination of the quantum state, which is regarded as a main
advantage. On this paper it is experimentally analyzed an entanglement witness
recently proposed in the context of Nuclear Magnetic Resonance (NMR)
experiments to test it in some Bell-diagonal states. We also propose some
optimal entanglement witness for Bell-diagonal states. The efficiency of the
two types of EW's are compared to a measure of entanglement with tomographic
cost, the generalized robustness of entanglement. It is used a GRAPE algorithm
to produce an entangled state which is out of the detection region of the EW
for Bell-diagonal states. Upon relaxation, the results show that there is a
region in which both EW fails, whereas the generalized robustness still shows
entanglement, but with the entanglement witness proposed here with a better
performance
Quantum information processing by NMR using a 5-qubit system formed by dipolar coupled spins in an oriented molecule
Quantum Information processing by NMR with small number of qubits is well
established. Scaling to higher number of qubits is hindered by two major
requirements (i) mutual coupling among qubits and (ii) qubit addressability. It
has been demonstrated that mutual coupling can be increased by using residual
dipolar couplings among spins by orienting the spin system in a liquid
crystalline matrix. In such a case, the heteronuclear spins are weakly coupled
but the homonuclear spins become strongly coupled. In such circumstances, the
strongly coupled spins can no longer be treated as qubits. However, it has been
demonstrated elsewhere, that the energy levels of a strongly coupled N
spin-1/2 system can be treated as an N-qubit system. For this purpose the
various transitions have to be identified to well defined energy levels. This
paper consists of two parts. In the first part, the energy level diagram of a
heteronuclear 5-spin system is obtained by using a newly developed
heteronuclear z-cosy (HET-Z-COSY) experiment. In the second part,
implementation of logic gates, preparation of pseudopure states, creation of
entanglement and entanglement transfer is demonstrated, validating the use of
such systems for quantum information processing.Comment: 23 pages, 8 figure
Complete quantum teleportation using nuclear magnetic resonance
Quantum mechanics provides spectacular new information processing abilities
(Bennett 1995, Preskill 1998). One of the most unexpected is a procedure called
quantum teleportation (Bennett et al 1993) that allows the quantum state of a
system to be transported from one location to another, without moving through
the intervening space. Partial implementations of teleportation (Bouwmeester et
al 1997, Boschi et al 1998) over macroscopic distances have been achieved using
optical systems, but omit the final stage of the teleportation procedure. Here
we report an experimental implementation of the full quantum teleportation
operation over inter-atomic distances using liquid state nuclear magnetic
resonance (NMR). The inclusion of the final stage enables for the first time a
teleportation implementation which may be used as a subroutine in larger
quantum computations, or for quantum communication. Our experiment also
demonstrates the use of quantum process tomography, a procedure to completely
characterize the dynamics of a quantum system. Finally, we demonstrate a
controlled exploitation of decoherence as a tool to assist in the performance
of an experiment.Comment: 15 pages, 2 figures. Minor differences between this and the published
versio
Experimental realization of the Yang-Baxter Equation via NMR interferometry
The Yang-Baxter equation is an important tool in theoretical physics, with
many applications in different domains that span from condensed matter to
string theory. Recently, the interest on the equation has increased due to its
connection to quantum information processing. It has been shown that the
Yang-Baxter equation is closely related to quantum entanglement and quantum
computation. Therefore, owing to the broad relevance of this equation, besides
theoretical studies, it also became significant to pursue its experimental
implementation. Here, we show an experimental realization of the Yang-Baxter
equation and verify its validity through a Nuclear Magnetic Resonance (NMR)
interferometric setup. Our experiment was performed on a liquid state
Iodotrifluoroethylene sample which contains molecules with three qubits. We use
Controlled-transfer gates that allow us to build a pseudo-pure state from which
we are able to apply a quantum information protocol that implements the
Yang-Baxter equation.Comment: 10 pages and 6 figure
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