134 research outputs found
Precision Limits in Quantum Metrology with Open Quantum Systems
The laws of quantum mechanics allow to perform measurements whose precision
supersedes results predicted by classical parameter estimation theory. That is,
the precision bound imposed by the central limit theorem in the estimation of a
broad class of parameters, like atomic frequencies in spectroscopy or external
magnetic field in magnetometry, can be overcome when using quantum probes.
Environmental noise, however, generally alters the ultimate precision that can
be achieved in the estimation of an unknown parameter. This tutorial reviews
recent theoretical work aimed at obtaining general precision bounds in the
presence of an environment. We adopt a complementary approach, where we first
analyze the problem within the general framework of describing the quantum
systems in terms of quantum dynamical maps and then relate this abstract
formalism to a microscopic description of the system's dissipative time
evolution. We will show that although some forms of noise do render quantum
systems standard quantum limited, precision beyond classical bounds is still
possible in the presence of different forms of local environmental
fluctuations.Comment: slightly modified versio
Controllable Non-Markovianity for a Spin Qubit in Diamond
We present a flexible scheme to realize non-artificial non-Markovian dynamics
of an electronic spin qubit, using a nitrogen-vacancy center in diamond where
the inherent nitrogen spin serves as a regulator of the dynamics. By changing
the population of the nitrogen spin, we show that we can smoothly tune the
non-Markovianity of the electron spin's dynamic. Furthermore, we examine the
decoherence dynamics induced by the spin bath to exclude other sources of
non-Markovianity. The amount of collected measurement data is kept at a minimum
by employing Bayesian data analysis. This allows for a precise quantification
of the parameters involved in the description of the dynamics and a prediction
of so far unobserved data points.Comment: 12 pages, 9 figure, including supplemental materia
A resource efficient approach for quantum and classical simulations of gauge theories in particle physics
Gauge theories establish the standard model of particle physics, and lattice
gauge theory (LGT) calculations employing Markov Chain Monte Carlo (MCMC)
methods have been pivotal in our understanding of fundamental interactions. The
present limitations of MCMC techniques may be overcome by Hamiltonian-based
simulations on classical or quantum devices, which further provide the
potential to address questions that lay beyond the capabilities of the current
approaches. However, for continuous gauge groups, Hamiltonian-based
formulations involve infinite-dimensional gauge degrees of freedom that can
solely be handled by truncation. Current truncation schemes require
dramatically increasing computational resources at small values of the bare
couplings, where magnetic field effects become important. Such limitation
precludes one from `taking the continuous limit' while working with finite
resources. To overcome this limitation, we provide a resource-efficient
protocol to simulate LGTs with continuous gauge groups in the Hamiltonian
formulation. Our new method allows for calculations at arbitrary values of the
bare coupling and lattice spacing. The approach consists of the combination of
a Hilbert space truncation with a regularization of the gauge group, which
permits an efficient description of the magnetically-dominated regime. We focus
here on Abelian gauge theories and use dimensional quantum
electrodynamics as a benchmark example to demonstrate this efficient framework
to achieve the continuum limit in LGTs. This possibility is a key requirement
to make quantitative predictions at the field theory level and offers the
long-term perspective to utilise quantum simulations to compute physically
meaningful quantities in regimes that are precluded to quantum Monte Carlo.Comment: 25 pages, 9 figures, journal versio
Adaptive estimation of quantum observables
The accurate estimation of quantum observables is a critical task in science. With progress on the hardware, measuring a quantum system will become increasingly demanding, particularly for variational protocols that require extensive sampling. Here, we introduce a measurement scheme that adaptively modifies the estimator based on previously obtained data. Our algorithm, which we call AEQuO, continuously monitors both the estimated average and the associated error of the considered observable, and determines the next measurement step based on this information. We allow both for overlap and non-bitwise commutation relations in the subsets of Pauli operators that are simultaneously probed, thereby maximizing the amount of gathered information. AEQuO comes in two variants: a greedy bucket-filling algorithm with good performance for small problem instances, and a machine learning-based algorithm with more favorable scaling for larger instances. The measurement configuration determined by these subroutines is further post-processed in order to lower the error on the estimator. We test our protocol on chemistry Hamiltonians, for which AEQuO provides error estimates that improve on all state-of-the-art methods based on various grouping techniques or randomized measurements, thus greatly lowering the toll of measurements in current and future quantum applications
Towards simulating 2D effects in lattice gauge theories on a quantum computer
Gauge theories are the most successful theories for describing nature at its
fundamental level, but obtaining analytical or numerical solutions often
remains a challenge. We propose an experimental quantum simulation scheme to
study ground state properties in two-dimensional quantum electrodynamics (2D
QED) using existing quantum technology. The proposal builds on a formulation of
lattice gauge theories as effective spin models in arXiv:2006.14160, which
reduces the number of qubits needed by eliminating redundant degrees of freedom
and by using an efficient truncation scheme for the gauge fields. The latter
endows our proposal with the perspective to take a well-controlled continuum
limit. Our protocols allow in principle scaling up to large lattices and offer
the perspective to connect the lattice simulation to low energy observable
quantities, e.g. the hadron spectrum, in the continuum theory. By including
both dynamical matter and a non-minimal gauge field truncation, we provide the
novel opportunity to observe 2D effects on present-day quantum hardware. More
specifically, we present two Variational Quantum Eigensolver (VQE) based
protocols for the study of magnetic field effects, and for taking an important
first step towards computing the running coupling of QED. For both instances,
we include variational quantum circuits for qubit-based hardware, which we
explicitly apply to trapped ion quantum computers. We simulate the proposed VQE
experiments classically to calculate the required measurement budget under
realistic conditions. While this feasibility analysis is done for trapped ions,
our approach can be easily adapted to other platforms. The techniques presented
here, combined with advancements in quantum hardware pave the way for reaching
beyond the capabilities of classical simulations by extending our framework to
include fermionic potentials or topological terms
Simulating 2D lattice gauge theories on a qudit quantum computer
Particle physics underpins our understanding of the world at a fundamental
level by describing the interplay of matter and forces through gauge theories.
Yet, despite their unmatched success, the intrinsic quantum mechanical nature
of gauge theories makes important problem classes notoriously difficult to
address with classical computational techniques. A promising way to overcome
these roadblocks is offered by quantum computers, which are based on the same
laws that make the classical computations so difficult. Here, we present a
quantum computation of the properties of the basic building block of
two-dimensional lattice quantum electrodynamics, involving both gauge fields
and matter. This computation is made possible by the use of a trapped-ion qudit
quantum processor, where quantum information is encoded in different states
per ion, rather than in two states as in qubits. Qudits are ideally suited for
describing gauge fields, which are naturally high-dimensional, leading to a
dramatic reduction in the quantum register size and circuit complexity. Using a
variational quantum eigensolver, we find the ground state of the model and
observe the interplay between virtual pair creation and quantized magnetic
field effects. The qudit approach further allows us to seamlessly observe the
effect of different gauge field truncations by controlling the qudit dimension.
Our results open the door for hardware-efficient quantum simulations with
qudits in near-term quantum devices
The Amino-Terminus of Nitric Oxide Sensitive Guanylyl Cyclase α1 Does Not Affect Dimerization but Influences Subcellular Localization
BACKGROUND: Nitric oxide sensitive guanylyl cyclase (NOsGC) is a heterodimeric enzyme formed by an α- and a ÎČâ-subunit. A splice variant (C-αâ) of the αâ-subunit, lacking at least the first 236 amino acids has been described by Sharina et al. 2008 and has been shown to be expressed in differentiating human embryonic cells. Wagner et al. 2005 have shown that the amino acids 61-128 of the αâ-subunit are mandatory for quantitative heterodimerization implying that the C-αâ-splice variant should lose its capacity to dimerize quantitatively. METHODOLOGY/PRINCIPAL FINDINGS: In the current study we demonstrate preserved quantitative dimerization of the C-αâ-splice by co-purification with the ÎČâ-subunit. In addition we used fluorescence resonance energy transfer (FRET) based on fluorescence lifetime imaging (FLIM) using fusion proteins of the ÎČâ-subunit and the αâ-subunit or the C-αâ variant with ECFP or EYFP. Analysis of the respective combinations in HEK-293 cells showed that the fluorescence lifetime was significantly shorter (â0.3 ns) for αâ/ÎČâ and C-αâ/ÎČâ than the negative control. In addition we show that lack of the amino-terminus in the αâ splice variant directs it to a more oxidized subcellular compartment. CONCLUSIONS/SIGNIFICANCE: We conclude that the amino-terminus of the αâ-subunit is dispensable for dimerization in-vivo and ex-vivo, but influences the subcellular trafficking
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