10,196 research outputs found
Simulating quantum operations with mixed environments
We study the physical resources required to implement general quantum
operations, and provide new bounds on the minimum possible size which an
environment must be in order to perform certain quantum operations. We prove
that contrary to a previous conjecture, not all quantum operations on a
single-qubit can be implemented with a single-qubit environment, even if that
environment is initially prepared in a mixed state. We show that a mixed
single-qutrit environment is sufficient to implement a special class of
operations, the generalized depolarizing channels.Comment: 4 pages Revtex + 1 fig, pictures at
http://stout.physics.ucla.edu/~smolin/tetrahedron .Several small correction
Sudden death of effective entanglement
Sudden death of entanglement is a well-known effect resulting from the finite
volume of separable states. We study the case when the observer has a limited
measurement capability and analyse the effective entanglement, i.e.
entanglement minimized over the output data. We show that in the well defined
system of two quantum dots monitored by single electron transistors, one may
observe a sudden death of effective entanglement when real, physical
entanglement is still alive. For certain measurement setups, this occurs even
for initial states for which sudden death of physical entanglement is not
possible at all. The principles of the analysis may be applied to other
analogous scenarios, such as etimation of the parameters arising from quantum
process tomography.Comment: final version, 5 pages, 3 figure
Direct Characterization of Quantum Dynamics: General Theory
The characterization of the dynamics of quantum systems is a task of both
fundamental and practical importance. A general class of methods which have
been developed in quantum information theory to accomplish this task is known
as quantum process tomography (QPT). In an earlier paper [M. Mohseni and D. A.
Lidar, Phys. Rev. Lett. 97, 170501 (2006)] we presented a new algorithm for
Direct Characterization of Quantum Dynamics (DCQD) of two-level quantum
systems. Here we provide a generalization by developing a theory for direct and
complete characterization of the dynamics of arbitrary quantum systems. In
contrast to other QPT schemes, DCQD relies on quantum error-detection
techniques and does not require any quantum state tomography. We demonstrate
that for the full characterization of the dynamics of n d-level quantum systems
(with d a power of a prime), the minimal number of required experimental
configurations is reduced quadratically from d^{4n} in separable QPT schemes to
d^{2n} in DCQD.Comment: 17 pages, 6 figures, minor modifications are mad
Realization of logically labeled effective pure states for bulk quantum computation
We report the first use of "logical labeling" to perform a quantum
computation with a room-temperature bulk system. This method entails the
selection of a subsystem which behaves as if it were at zero temperature -
except for a decrease in signal strength - conditioned upon the state of the
remaining system. No averaging over differently prepared molecules is required.
In order to test this concept, we execute a quantum search algorithm in a
subspace of two nuclear spins, labeled by a third spin, using solution nuclear
magnetic resonance (NMR), and employing a novel choice of reference frame to
uncouple nuclei.Comment: PRL 83, 3085 (1999). Small changes made to improve readability and
remove ambiguitie
Graph Concatenation for Quantum Codes
Graphs are closely related to quantum error-correcting codes: every
stabilizer code is locally equivalent to a graph code, and every codeword
stabilized code can be described by a graph and a classical code. For the
construction of good quantum codes of relatively large block length,
concatenated quantum codes and their generalizations play an important role. We
develop a systematic method for constructing concatenated quantum codes based
on "graph concatenation", where graphs representing the inner and outer codes
are concatenated via a simple graph operation called "generalized local
complementation." Our method applies to both binary and non-binary concatenated
quantum codes as well as their generalizations.Comment: 26 pages, 12 figures. Figures of concatenated [[5,1,3]] and [[7,1,3]]
are added. Submitted to JM
Process reconstruction from incomplete and/or inconsistent data
We analyze how an action of a qubit channel (map) can be estimated from the
measured data that are incomplete or even inconsistent. That is, we consider
situations when measurement statistics is insufficient to determine consistent
probability distributions. As a consequence either the estimation
(reconstruction) of the channel completely fails or it results in an unphysical
channel (i.e., the corresponding map is not completely positive). We present a
regularization procedure that allows us to derive physically reasonable
estimates (approximations) of quantum channels. We illustrate our procedure on
specific examples and we show that the procedure can be also used for a
derivation of optimal approximations of operations that are forbidden by the
laws of quantum mechanics (e.g., the universal NOT gate).Comment: 9pages, 5 figure
Full characterization of a three-photon GHZ state using quantum state tomography
We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.Comment: 3 pages, 3 figure
Implementing universal multi-qubit quantum logic gates in three and four-spin systems at room temperature
In this paper, we present the experimental realization of multi-qubit gates
in macroscopic ensemble of three-qubit and four-qubit
molecules. Instead of depending heavily on the two-bit universal gate, which
served as the basic quantum operation in quantum computing, we use pulses of
well-defined frequency and length that simultaneously apply to all qubits in a
quantum register. It appears that this method is experimentally convenient when
this procedure is extended to more qubits on some quantum computation, and it
can also be used in other physical systems.Comment: 5 Pages, 2 Figure
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