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Chahan M. Kropf

Publications and source records attributed to Chahan M. Kropf.

11 recordsLinked to original sources
arXiv2019

Measurement-Protected Quantum Key Distribution

In the distribution of quantum states over a long distance, not only are quantum states corrupted by interactions with an environment but also a measurement setting should be re-aligned such that detection events can be ensured for the resulting states. In this work, we present measurement-protected quantum key distribution where a measurement is protected against the interactions quantum states experience during the transmission, without the verification of a channel. As a result, a receiver does not have to revise the measurement that has been prepared in a noiseless scenario since it would remain ever optimal. The measurement protection is achieved by applications of local unitary transformations before and after the transmission, that leads to a supermap transforming an arbitrary channel to a depolarization one. An experimental demonstration is presented with the polarization encoding on photonic qubits. It is shown that the security bounds for prepare-and-measure protocols can be improved, for instance, errors up to 20.7% can be tolerated in the Bennett-Brassard 1984 protocol.

quant-ph↗
arXiv2019

Protecting quantum coherences from static noise and disorder

Quantum coherences are paramount resources for applications, such as quantum-enhanced light-harvesting or quantum computing, which are fragile against environmental noise. We here derive generalized quantum master equations using perturbation theory in order to describe the effective ensemble-averaged time-evolution of finite-size quantum systems subject to static noise on all time scales. We then analyse the time-evolution of the coherences under energy broadening noise in a variety of systems characterized by both short and long-range interactions, by strongly correlated and fully uncorrelated noise -- a single qubit, a lattice model with on-site disorder and a potential ladder, and bosons in a double-well potential with random interaction strength -- and show that couplings can partially protect the system from the ensemble-averaging induced loss of coherence. Our work suggests that suitably tuned couplings could be employed to counteract part of the dephasing detrimental to quantum applications. Conversely, tailored noise distributions can be utilized to reach target non-equilibrium quantum states.

quant-ph↗
arXiv2019

Electric-field assisted optimal quantum transport of photo-excitations in polar heterostructures

Transition-metal-oxide (TMO) heterostructures are promising candidates for building photon-harvesting devices which can exploit optimal quantum transport of charge excitations generated by light absorption. Here we address the explicit role of an electric field on the quantum transport properties of photo-excitations subject to dephasing in one-dimensional chains coupled to a continuum of states acting as a sink. We show that the average transfer time to the sink is optimized for suitable values of both the coupling strength to the sink and the electric field, thus fully exploiting the coherence-enhanced efficiency in the quantum transport regime achievable in few monolayers TMO heterostructures. The optimal coupling to the continuum remains approximately the same as that in absence of electric field and is characterizing the Superradiant Transition. On the other hand, the optimal electric field for which we provide estimates using an analytical expression is dependent on the initial state.

quant-ph↗
arXiv2019

Deep neural network for X-ray photoelectron spectroscopy data analysis

In this work, we characterize the performance of a deep convolutional neural network designed to detect and quantify chemical elements in experimental X-ray photoelectron spectroscopy data. Given the lack of a reliable database in literature, in order to train the neural network we computed a large ($>$100 k) dataset of synthetic spectra, based on randomly generated materials covered with a layer of adventitious carbon. The trained net performs as good as standard methods on a test set of $\approx$ 500 well characterized experimental X-ray photoelectron spectra. Fine details about the net layout, the choice of the loss function and the quality assessment strategies are presented and discussed. Given the synthetic nature of the training set, this approach could be applied to the automatization of any photoelectron spectroscopy system, without the need of experimental reference spectra and with a low computational effort.

cond-mat.dis-nn↗
arXiv2019

Channel Coding of a Quantum Measurement

In this work, we consider the preservation of a measurement for quantum systems interacting with an environment. Namely, a method of preserving an optimal measurement over a channel is devised, what we call channel coding of a quantum measurement in that operations are applied before and after a channel in order to protect a measurement. A protocol that preserves a quantum measurement over an arbitrary channel is shown only with local operations and classical communication without the use of a larger Hilbert space. Therefore, the protocol is readily feasible with present day's technologies. Channel coding of qubit measurements is presented, and it is shown that a measurement can be preserved for an arbitrary channel for both i) pairs of qubit states and ii) ensembles of equally probable states. The protocol of preserving a quantum measurement is demonstrated with IBM quantum computers.

quant-ph↗
arXiv2019

Towards high-temperature coherence-enhanced transport in few-atomic layers heterostructures

The possibility to exploit quantum coherence to strongly enhance the efficiency of charge transport in solid state devices working at ambient conditions would pave the way to disruptive technological applications. In this work, we tackle the problem of the quantum transport of photogenerated electronic excitations subject to dephasing and on-site Coulomb interactions. We show that the transport to a continuum of states representing metallic collectors can be optimized by exploiting the "superradiance" phenomena. We demonstrate that this is a coherent effect which is robust against dephasing and electron-electron interactions in a parameters range that is compatible with actual implementation in few monolayers transition-metal-oxide (TMO) heterostructures.

quant-ph↗
arXiv2018

Preserving Measurements for Optimal State Discrimination over Quantum Channels

In this work, we consider optimal state discrimination for a quantum system that interacts with an environment, i.e., states evolve under a quantum channel. We show the conditions on a quantum channel and an ensemble of states such that a measurement for optimal state discrimination is preserved. In particular, we show that when an ensemble of states with equal {\it a priori} probabilities is given, an optimal measurement can be preserved over any quantum channel by applying local operations and classical communication, that is, by manipulating the quantum states before and after the channel application. Examples are provided for illustration. Our results can be readily applied to quantum communication protocols over various types of noise.

quant-ph↗
arXiv2015

Anomalous longitudinal relaxation of nuclear spins in CaF$_2$

We consider the effect of non-secular resonances for interacting nuclear spins in solids which were predicted theoretically to exist in the presence of strong static and strong radio-frequency magnetic fields. These resonances imply corrections to the standard secular approximation for the nuclear spin-spin interaction in solids, which, in turn, should lead to an anomalous longitudinal relaxation in nuclear magnetic resonance experiments. In this article, we investigate the feasibility of the experimental observation of this anomalous longitudinal relaxation in calcium fluoride (CaF$_2$) and conclude that such an observation is realistic.

cond-mat.mes-hall↗
arXiv2013

Absence of exponential sensitivity to small perturbations in nonintegrable systems of spins 1/2

We show that macroscopic nonintegrable lattices of spins 1/2, which are often considered to be chaotic, do not exhibit the basic property of classical chaotic systems, namely, exponential sensitivity to small perturbations. We compare chaotic lattices of classical spins and nonintegrable lattices of spins 1/2 in terms of their magnetization responses to imperfect reversal of spin dynamics known as Loschmidt echo. In the classical case, magnetization exhibits exponential sensitivity to small perturbations of Loschmidt echoes, which is characterized by twice the value of the largest Lyapunov exponent of the system. In the case of spins 1/2, magnetization is only power-law sensitive to small perturbations. Our findings imply that it is impossible to define Lyapunov exponents for lattices of spins 1/2 even in the macroscopic limit. At the same time, the above absence of exponential sensitivity to small perturbations is an encouraging news for the efforts to create quantum simulators. The power-law sensitivity of spin 1/2 lattices to small perturbations is predicted to be measurable in nuclear magnetic resonance experiments.

cond-mat.stat-mech↗
arXiv2012

Recoupling the non-secular part of nuclear spin-spin interaction in solids

NMR spin-spin relaxation in solids in strong magnetic fields is normally described only with the help of the secular part of the full spin-spin interaction Hamiltonian. This approximation is associated with the averaging of the spin-spin interaction over the fast motion of spins under the combined action of the static and the radio-frequency (rf) fields. Here we report a set of conditions when the averaging over the above fast motion preserves some of the non-secular terms entering the full interaction Hamiltonian. These conditions relate the value of the static magnetic field with the frequency and the amplitude of the rf field. When the above conditions are satisfied, the effective recoupled spin-spin interaction Hamiltonian has an unconventional form with tunable parameters. These tunable Hamiltonian offers interesting possibilities to manipulate nuclear spins in solids and can shed new light on the fundamental properties of the nuclear spin-spin relaxation phenomenon.

cond-mat.stat-mech↗