160 research outputs found
Telecom photon interface of solid-state quantum nodes
Solid-state spins such as nitrogen-vacancy (NV) center are promising
platforms for large-scale quantum networks. Despite the optical interface of NV
center system, however, the significant attenuation of its zero-phonon-line
photon in optical fiber prevents the network extended to long distances.
Therefore a telecom-wavelength photon interface would be essential to reduce
the photon loss in transporting quantum information. Here we propose an
efficient scheme for coupling telecom photon to NV center ensembles mediated by
rare-earth doped crystal. Specifically, we proposed protocols for high fidelity
quantum state transfer and entanglement generation with parameters within reach
of current technologies. Such an interface would bring new insights into future
implementations of long-range quantum network with NV centers in diamond acting
as quantum nodes.Comment: 10 pages, 5 figure
Mixed-state quantum transport in correlated spin networks
Quantum spin networks can be used to transport information between separated
registers in a quantum information processor. To find a practical
implementation, the strict requirements of ideal models for perfect state
transfer need to be relaxed, allowing for complex coupling topologies and
general initial states. Here we analyze transport in complex quantum spin
networks in the maximally mixed state and derive explicit conditions that
should be satisfied by propagators for perfect state transport. Using a
description of the transport process as a quantum walk over the network, we
show that it is necessary to phase correlate the transport processes occurring
along all the possible paths in the network. We provide a Hamiltonian that
achieves this correlation, and use it in a constructive method to derive
engineered couplings for perfect transport in complicated network topologies
Experimentally efficient methods for estimating the performance of quantum measurements
Efficient methods for characterizing the performance of quantum measurements
are important in the experimental quantum sciences. Ideally, one requires both
a physically relevant distinguishability measure between measurement operations
and a well-defined experimental procedure for estimating the distinguishability
measure. Here, we propose the average measurement fidelity and error between
quantum measurements as distinguishability measures. We present protocols for
obtaining bounds on these quantities that are both estimable using
experimentally accessible quantities and scalable in the size of the quantum
system. We explain why the bounds should be valid in large generality and
illustrate the method via numerical examples.Comment: 20 pages, 1 figure. Expanded details and typos corrected. Accepted
versio
Exact dimension estimation of interacting qubit systems assisted by a single quantum probe
Estimating the dimension of an Hilbert space is an important component of
quantum system identification. In quantum technologies, the dimension of a
quantum system (or its corresponding accessible Hilbert space) is an important
resource, as larger dimensions determine e.g. the performance of quantum
computation protocols or the sensitivity of quantum sensors. Despite being a
critical task in quantum system identification, estimating the Hilbert space
dimension is experimentally challenging. While there have been proposals for
various dimension witnesses capable of putting a lower bound on the dimension
from measuring collective observables that encode correlations, in many
practical scenarios, especially for multiqubit systems, the experimental
control might not be able to engineer the required initialization, dynamics and
observables.
Here we propose a more practical strategy, that relies not on directly
measuring an unknown multiqubit target system, but on the indirect interaction
with a local quantum probe under the experimenter's control. Assuming only that
the interaction model is given and the evolution correlates all the qubits with
the probe, we combine a graph-theoretical approach and realization theory to
demonstrate that the dimension of the Hilbert space can be exactly estimated
from the model order of the system. We further analyze the robustness in the
presence of background noise of the proposed estimation method based on
realization theory, finding that despite stringent constrains on the allowed
noise level, exact dimension estimation can still be achieved.Comment: v3: accepted version. We would like to offer our gratitudes to the
editors and referees for their helpful and insightful opinions and feedback
Spatial noise filtering through error correction for quantum sensing
Quantum systems can be used to measure various quantities in their
environment with high precision. Often, however, their sensitivity is limited
by the decohering effects of this same environment. Dynamical decoupling
schemes are widely used to filter environmental noise from signals, but their
performance is limited by the spectral properties of the signal and noise at
hand. Quantum error correction schemes have therefore emerged as a
complementary technique without the same limitations. To date, however, they
have failed to correct the dominant noise type in many quantum sensors, which
couples to each qubit in a sensor in the same way as the signal. Here we show
how quantum error correction can correct for such noise, which dynamical
decoupling can only partially address. Whereas dynamical decoupling exploits
temporal noise correlations in signal and noise, our scheme exploits spatial
correlations. We give explicit examples in small quantum devices and
demonstrate a method by which error-correcting codes can be tailored to their
noise.Comment: 8 pages, 2 figures, RevTeX 4.1. v2: Updated to match published
versio
Quantum simulation via filtered Hamiltonian engineering: application to perfect quantum transport in spin networks
We propose a method for Hamiltonian engineering in quantum information
processing architectures that requires no local control, but only relies on
collective qubit rotations and field gradients. The technique achieves a
spatial modulation of the coupling strengths via a dynamical construction of a
weighting function combined with a Bragg grating. As an example, we demonstrate
how to generate the ideal Hamiltonian for perfect quantum information transport
between two separated nodes of a large spin network. We engineer a spin chain
with optimal couplings from a large spin network, such as naturally occurring
in crystals, while decoupling all unwanted interactions. For realistic
experimental parameters, our method can be used to drive perfect quantum
information transport at room-temperature. The Hamiltonian engineering method
can be made more robust under coherence and coupling disorder by a novel
apodization scheme. Thus the method is quite general and can be used engineer
the Hamiltonian of many complex spin lattices with different topologies and
interactions.Comment: v2: Extended robustness to decoherenc
Implementation of State Transfer Hamiltonians in Spin Chains with Magnetic Resonance Techniques
Nuclear spin systems and magnetic resonance techniques have provided a fertile platform for experimental investigation of quantum state transfer in spin chains. From the first observation of polarization transfer, predating the formal definition of quantum state transfer, to the realization of state transfer simulations in small molecules and in larger solid-state spin systems, the experiments have drawn on the strengths of nuclear magnetic resonance (NMR), in particular on its long history of well-developed control techniques. NMR implementations have been invaluable both as proof-of-principle demonstrations of quantum state transfer protocols and to explore dynamics occurring in real systems that go beyond what can be analytically solved or numerically simulated. In addition, control techniques developed in these systems to engineer the Hamiltonians required for transport can be adopted in potentially scalable quantum information processing architectures. In this contribution we describe recent results and outline future directions of research in magnetic-resonance based implementations of quantum state transfer in spin chains.National Science Foundation (U.S.) (Grant DMR-1005926)United States. Air Force Office of Scientific Researc
Quantifying precision loss in local quantum thermometry via diagonal discord
When quantum information is spread over a system through nonclassical
correlation, it makes retrieving information by local measurements
difficult---making global measurement necessary for optimal parameter
estimation. In this paper, we consider temperature estimation of a system in a
Gibbs state and quantify the separation between the estimation performance of
the global optimal measurement scheme and a greedy local measurement scheme by
diagonal quantum discord. In a greedy local scheme, instead of global
measurements, one performs sequential local measurement on subsystems, which is
potentially enhanced by feed-forward communication. We show that, for
finite-dimensional systems, diagonal discord quantifies the difference in the
quantum Fisher information quantifying the precision limits for temperature
estimation of these two schemes, and we analytically obtain the relation in the
high-temperature limit. We further verify this result by employing the examples
of spins with Heisenberg's interaction.Comment: 5+4 pages, 4 figures, We thank the referees and editors for helpful
opinions. Accepted by Phys. Rev. A (accepted version
Coherent state transfer via highly mixed quantum spin chains
Spin chains have been proposed as quantum wires in many quantum information
processing architectures. Coherent transmission of quantum information over
short distances is enabled by their internal dynamics, which drives the
transport of single-spin excitations in perfectly polarized chains. Given the
practical challenge of preparing the chain in a pure state, we propose to use a
chain that is initially in the maximally mixed state. We compare the transport
properties of pure and mixed-state chains, finding similarities that enable the
experimental study of pure-state transfer by its simulation via mixed-state
chains, and demonstrate protocols for the perfect transfer of quantum
information in these chains. Remarkably, mixed-state chains allow the use of
Hamiltonians which do not preserve the total number of excitations, and which
are more readily obtainable from the naturally occurring magnetic dipolar
interaction. We propose experimental implementations using solid-state nuclear
magnetic resonance and defect centers in diamond.Comment: 9 page
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