Searcharxiv⌕ Search

arXiv subjects

Daniel Burgarth

Publications and source records attributed to Daniel Burgarth.

At least 109 records · Page 6Linked to original sources
arXiv2011

Quantum System Identification

The aim of quantum system identification is to estimate the ingredients inside a black box, in which some quantum-mechanical unitary process takes place, by just looking at its input-output behavior. Here we establish a basic and general framework for quantum system identification, that allows us to classify how much knowledge about the quantum system is attainable, in principle, from a given experimental setup. Prior knowledge on some elements of the black box helps the system identification. We present an example in which a Bell measurement is more efficient to identify the system. When the topology of the system is known, the framework enables us to establish a general criterion for the estimability of the coupling constants in its Hamiltonian.

quant-ph↗
arXiv2010

Local quantum control of Heisenberg spin chains

Motivated by some recent results of quantum control theory, we discuss the feasibility of local operator control in arrays of interacting qubits modeled as isotropic Heisenberg spin chains. Acting on one of the end spins, we aim at finding piecewise-constant control pulses that lead to optimal fidelities for a chosen set of quantum gates. We analyze the robustness of the obtained results f or the gate fidelities to random errors in the control fields, finding that with faster switching between piecewise-constant controls the system is less susceptible to these errors. The observed behavior falls into a generic class of physical phenomena that are related to a competition between resonance- and relaxation-type behavior, exemplified by motional narrowing in NMR experiments. Finally, we discuss how the obtained optimal gate fidelities are altered when the corresponding rapidly-varying piecewise-constant control fields are smoothened through spectral filtering.

quant-ph↗
arXiv2010

Controlling qubit arrays with anisotropic XXZ Heisenberg interaction by acting on a single qubit

We investigate anisotropic $XXZ$ Heisenberg spin-1/2 chains with control fields acting on one of the end spins, with the aim of exploring local quantum control in arrays of interacting qubits. In this work, which uses a recent Lie-algebraic result on the local controllability of spin chains with "always-on" interactions, we determine piecewise-constant control pulses corresponding to optimal fidelities for quantum gates such as spin-flip (NOT), controlled-NOT (CNOT), and square-root-of-SWAP ($\sqrt{\textrm{SWAP}}$). We find the minimal times for realizing different gates depending on the anisotropy parameter $Δ$ of the model, showing that the shortest among these gate times are achieved for particular values of $Δ$ larger than unity. To study the influence of possible imperfections in anticipated experimental realizations of qubit arrays, we analyze the robustness of the obtained results for the gate fidelities to random variations in the control-field amplitudes and finite rise time of the pulses. Finally, we discuss the implications of our study for superconducting charge-qubit arrays.

quant-ph↗
arXiv2010

Quantum Defragmentation Algorithm

In this addendum of our paper [D. Burgarth and V. Giovannetti, Phys. Rev. Lett. 99, 100501 (2007)] we prove that during the transformation that allows one to enforce control by relaxation on a quantum system, the ancillary memory can be kept at a finite size, independently from the fidelity one wants to achieve. The result is obtained by introducing the quantum analog of defragmentation algorithms which are employed for efficiently reorganizing classical information in conventional hard-disks. Our result also implies that the reduced dynamics in any noisy system can be simulated with finitely many resources.

quant-ph↗
arXiv2010

Indirect Quantum Tomography of Quadratic Hamiltonians

A number of many-body problems can be formulated using Hamiltonians that are quadratic in the creation and annihilation operators. Here, we show how such quadratic Hamiltonians can be efficiently estimated indirectly, employing very few resources. We find that almost all properties of the Hamiltonian are determined by its surface, and that these properties can be measured even if the system can only be initialised to a mixed state. Therefore our method can be applied to various physical models, with important examples including coupled nano-mechanical oscillators, hopping fermions in optical lattices, and transverse Ising chains.

quant-ph↗
arXiv2010

Efficient Generation of a Maximally Entangled State by Repeated On- and Off-Resonant Scattering of Ancilla Qubits

A scheme for preparing two fixed non-interacting qubits in a maximally entangled state is presented. By repeating on- and off-resonant scattering of ancilla qubits, the state of the target qubits is driven from an arbitrary initial state into the singlet state with probability 1 (perfect efficiency). Neither the preparation nor the post-selection of the ancilla spin state is required. The convergence from an arbitrary input state to the unique fixed point (mixing property) is proved rigorously, and its robustness is investigated, by scrutinizing the effects of imperfections in the incident wave of the ancilla, such as mistuning to a resonant momentum, imperfect monochromatization, and fluctuation of the incident momentum, as well as detector efficiency.

quant-ph↗
arXiv2009

Quantum Data Bus in Dipolar Coupled Nuclear Spin Qubits

We implement an iterative quantum state transfer exploiting the natural dipolar couplings in a spin chain of a liquid crystal NMR system. During each iteration a finite part of the amplitude of the state is transferred and by applying an external operation on only the last two spins the transferred state is made to accumulate on the spin at the end point. The transfer fidelity reaches one asymptotically through increasing the number of iterations. We also implement the inverted version of the scheme which can transfer an arbitrary state from the end point to any other position of the chain and entangle any pair of spins in the chain, acting as a full quantum data bus.

quant-ph↗
arXiv2009

Scalable quantum computation via local control of only two qubits

We apply quantum control techniques to control a large spin chain by only acting on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on the latter and swap operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.

quant-ph↗
arXiv2009

Local controllability of quantum networks

We give a sufficient criterion that guarantees that a many-body quantum system can be controlled by properly manipulating the (local) Hamiltonian of one of its subsystems. The method can be applied to a wide range of systems: it does not depend on the details of the couplings but only on their associated topology. As a special case, we prove that Heisenberg and Affleck-Kennedy-Lieb-Tasaki chains can be controlled by operating on one of the spins at their ends. In principle, arbitrary quantum algorithms can be performed on such chains by acting on a single qubit.

quant-ph↗
arXiv2009

Dynamics of nonequilibrium thermal entanglement

The dynamics of a simple spin chain (2 spins) coupled to bosonic baths at different temperatures is studied. The analytical solution for the reduced density matrix of the system is found. The dynamics and temperature dependence of spin-spin entanglement is analyzed. It is shown that the system converges to a steady-state. If the energy levels of the two spins are different, the steady-state concurrence assumes its maximum at unequal bath temperatures. It is found that a difference in local energy levels can make the steady-state entanglement more stable against high temperatures.

quant-ph↗
arXiv2009

Indirect Hamiltonian Identification through a small gateway

Identifying the nature of interactions in a quantum system is essential in understanding any physical phenomena. Acquiring information on the Hamiltonian can be a tough challenge in many-body systems because it generally requires access to all parts of the system. We show that if the coupling topology is known, the Hamiltonian identification is indeed possible indirectly even though only a small gateway to the system is used. Surprisingly, even a degenerate Hamiltonian can be estimated by applying an extra field to the gateway.

quant-ph↗
arXiv2008

Coupling strength estimation for spin chains despite restricted access

Quantum control requires full knowledge of the system many-body Hamiltonian. In many cases this information is not directly available due to restricted access to the system. Here we show how to indirectly estimate all the coupling strengths in a spin chain by measuring one spin at the end of the chain. We also discuss the efficiency of this "quantum inverse problem" and give a numerical example.

quant-ph↗
arXiv2008

A Protocol For Cooling and Controlling Composite Systems by Local Interactions

We discuss an explicit protocol which allows one to externally cool and control a composite system by operating on a small subset of it. The scheme permits to transfer arbitrary and unknown quantum states from a memory on the network ("upload access") as well as the inverse ("download access"). In particular it yields a method for cooling the system.

quant-ph↗
arXiv2008

Communication Through a Quantum Link

A chain of interacting spin behaves like a quantum mediator (quantum link) which allows two distant parties that control the ends of the chain to exchange quantum messages. We show that over repeated uses without resetting the study of a quantum link can be connected to correlated quantum channels with finite dimensional environment (finite memory quantum channel). Then, using coding arguments for such kind of channels and results on mixing channels we present a protocol that allows us to achieve perfect information transmission through a quantum link.

quant-ph↗
arXiv2007

Mediated Homogenization

Homogenization protocols model the quantum mechanical evolution of a system to a fixed state independently from its initial configuration by repeatedly coupling it with a collection of identical ancillas. Here we analyze these protocols within the formalism of "relaxing" channels providing an easy to check sufficient condition for homogenization. In this context we describe mediated homogenization schemes where a network of connected qudits relaxes to a fixed state by only partially interacting with a bath. We also study configurations which allow us to introduce entanglement among the elements of the network. Finally we analyze the effect of having competitive configurations with two different baths and we prove the convergence to dynamical equilibrium for Heisenberg chains.

quant-ph↗
arXiv2007

Quantum state transfer and time-dependent disorder in Quantum Chains

One of the most basic tasks required for Quantum Information Technology is the ability to connect different components of a Quantum Computer by quantum wires that obey the superposition principle. Since superpositions can be very sensitive to noise this turns out to be already quite difficult. Recently, it was suggested to use chains of permanently coupled spin-1/2 particles (quantum chains) for this purpose. They have the advantage that no external control along the wire is required during the transport of information, which makes it possible to isolate the wire from sources of noise. We first give an introduction to basic quantum state transfer and review existing advanced schemes by other authors. We then show a new result that demonstrates the stability of the scheme [1] against disorder that is approximately constant during one application of the channel, but time-dependent with respect to multiple applications.

quant-ph↗
arXiv2007

Full control by locally induced relaxation

We demonstrate a scheme for controlling a large quantum system by acting on a small subsystem only. The local control is mediated to the larger system by some fixed coupling Hamiltonian. The scheme allows to transfer arbitrary and unknown quantum states from a memory on the large system (``upload access'') as well as the inverse (``download access''). We study sufficient conditions of the coupling Hamiltonian and give lower bounds on the fidelities for downloading and uploading.

quant-ph↗