66 research outputs found
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
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
Optimal pulse spacing for dynamical decoupling in the presence of a purely-dephasing spin-bath
Maintaining quantum coherence is a crucial requirement for quantum
computation; hence protecting quantum systems against their irreversible
corruption due to environmental noise is an important open problem. Dynamical
decoupling (DD) is an effective method for reducing decoherence with a low
control overhead. It also plays an important role in quantum metrology, where
for instance it is employed in multiparameter estimation. While a sequence of
equidistant control pulses (CPMG) has been ubiquitously used for decoupling,
Uhrig recently proposed that a non-equidistant pulse sequence (UDD) may enhance
DD performance, especially for systems where the spectral density of the
environment has a sharp frequency cutoff. On the other hand, equidistant
sequences outperform UDD for soft cutoffs. The relative advantage provided by
UDD for intermediate regimes is not clear. In this paper, we analyze the
relative DD performance in this regime experimentally, using solid-state
nuclear magnetic resonance. Our system-qubits are 13C nuclear spins and the
environment consists of a 1H nuclear spin-bath whose spectral density is close
to a normal (Gaussian) distribution. We find that in the presence of such a
bath, the CPMG sequence outperforms the UDD sequence. An analogy between
dynamical decoupling and interference effects in optics provides an intuitive
explanation as to why the CPMG sequence performs superior to any
non-equidistant DD sequence in the presence of this kind of environmental
noise.Comment: To be published in Phys. Rev. A. 15 pages, 16 figures. Presentation
of the work was improved. One Figure and some Refs. were adde
Stable Three-Axis Nuclear Spin Gyroscope in Diamond
We propose a sensitive and stable three-axis gyroscope in diamond. We achieve
high sensitivity by exploiting the long coherence time of the N14 nuclear spin
associated with the Nitrogen-Vacancy center in diamond, and the efficient
polarization and measurement of its electronic spin. While the gyroscope is
based on a simple Ramsey interferometry scheme, we use coherent control of the
quantum sensor to improve its coherence time as well as its robustness against
long-time drifts, thus achieving a very robust device with a resolution of
0.5mdeg/s/(Hz mm^3)^(1/2). In addition, we exploit the four axes of
delocalization of the Nitrogen-Vacancy center to measure not only the rate of
rotation, but also its direction, thus obtaining a compact three-axis
gyroscope.Comment: 5+3 page
Evolution-free Hamiltonian parameter estimation through Zeeman markers
We provide a protocol for Hamiltonian parameter estimation which relies only
on the Zeeman effect. No time-dependent quantities need to be measured, it
fully suffices to observe spectral shifts induced by fields applied to local
`markers'. We demonstrate the idea with a simple tight-binding Hamiltonian and
numerically show stability with respect to Gaussian noise on the spectral
measurements. Then we generalize the result to show applicability to a wide
range of systems, including quantum spin chains, networks of qubits, and
coupled harmonic oscillators, and suggest potential experimental
implementations.Comment: 5 pages, 2 figure
Dual-space Compressed Sensing
Compressed sensing (CS) is a powerful method routinely employed to accelerate
image acquisition. It is particularly suited to situations when the image under
consideration is sparse but can be sampled in a basis where it is non-sparse.
Here we propose an alternate CS regime in situations where the image can be
sampled in two incoherent spaces simultaneously, with a special focus on image
sampling in Fourier reciprocal spaces (e.g. real-space and k-space).
Information is fed-forward from one space to the other, allowing new
opportunities to efficiently solve the optimization problem at the heart of CS
image reconstruction. We show that considerable gains in imaging acceleration
are then possible over conventional CS. The technique provides enhanced
robustness to noise, and is well suited to edge-detection problems. We envision
applications for imaging collections of nanodiamond (ND) particles targeting
specific regions in a volume of interest, exploiting the ability of lattice
defects (NV centers) to allow ND particles to be imaged in reciprocal spaces
simultaneously via optical fluorescence and 13C magnetic resonance imaging
(MRI) respectively. Broadly this work suggests the potential to interface CS
principles with hybrid sampling strategies to yield speedup in signal
acquisition in many practical settings
Svetlichny's inequality and genuine tripartite nonlocality in three-qubit pure states
The violation of the Svetlichny's inequality (SI) [Phys. Rev. D, 35, 3066
(1987)] is sufficient but not necessary for genuine tripartite nonlocal
correlations. Here we quantify the relationship between tripartite entanglement
and the maximum expectation value of the Svetlichny operator (which is bounded
from above by the inequality) for the two inequivalent subclasses of pure
three-qubit states: the GHZ-class and the W-class. We show that the maximum for
the GHZ-class states reduces to Mermin's inequality [Phys. Rev. Lett. 65, 1838
(1990)] modulo a constant factor, and although it is a function of the three
tangle and the residual concurrence, large number of states don't violate the
inequality. We further show that by design SI is more suitable as a measure of
genuine tripartite nonlocality between the three qubits in the the W-class
states, and the maximum is a certain function of the bipartite entanglement
(the concurrence) of the three reduced states, and only when their certain sum
attains a certain threshold value, they violate the inequality.Comment: Modified version, 5 pages, 2 figures, REVTeX
Nanoscale Vector dc Magnetometry via Ancilla-Assisted Frequency Up-Conversion
Sensing static magnetic fields with high sensitivity and spatial resolution is critical to many applications in fundamental physics, bioimaging, and materials science. Even more beneficial would be full vector magnetometry with nanoscale spatial resolution. Several versatile magnetometry platforms have emerged over the past decade, such as electronic spins associated with nitrogen vacancy (NV) centers in diamond. Achieving vector magnetometry has, however, often required using an ensemble of sensors or degrading the sensitivity. Here we introduce a hybrid magnetometry platform, consisting of a sensor and an ancillary qubit, that allows vector magnetometry of static fields. While more generally applicable, we demonstrate the method for an electronic NV sensor and a nuclear spin qubit. In particular, sensing transverse fields relies on frequency up-conversion of the dc fields through the ancillary qubit, allowing quantum lock-in detection with low-frequency noise rejection. In combination with the Ramsey detection of longitudinal fields, our frequency up-conversion scheme delivers a sensitive technique for vector dc magnetometry at the nanoscale.National Science Foundation (U.S.) (Grant PHY1734011)United States. Army Research Office (Grant W911NF-11-1- 0400)United States. Army Research Office (Grant W911NF-15-1-0548
Nanoscale Vector dc Magnetometry via Ancilla-Assisted Frequency Up-Conversion
Sensing static magnetic fields with high sensitivity and spatial resolution is critical to many applications in fundamental physics, bioimaging, and materials science. Even more beneficial would be full vector magnetometry with nanoscale spatial resolution. Several versatile magnetometry platforms have emerged over the past decade, such as electronic spins associated with nitrogen vacancy (NV) centers in diamond. Achieving vector magnetometry has, however, often required using an ensemble of sensors or degrading the sensitivity. Here we introduce a hybrid magnetometry platform, consisting of a sensor and an ancillary qubit, that allows vector magnetometry of static fields. While more generally applicable, we demonstrate the method for an electronic NV sensor and a nuclear spin qubit. In particular, sensing transverse fields relies on frequency up-conversion of the dc fields through the ancillary qubit, allowing quantum lock-in detection with low-frequency noise rejection. In combination with the Ramsey detection of longitudinal fields, our frequency up-conversion scheme delivers a sensitive technique for vector dc magnetometry at the nanoscale.National Science Foundation (U.S.) (Grant PHY1734011)United States. Army Research Office (Grant W911NF-11-1- 0400)United States. Army Research Office (Grant W911NF-15-1-0548
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