69,388 research outputs found
Realization of quantum walks with negligible decoherence in waveguide lattices
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects
Realization of quantum walks with negligible decoherence in waveguide lattices
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects
Molecular Spin Qudits for Quantum Algorithms
Presently, one of the most ambitious technological goals is the development
of devices working under the laws of quantum mechanics. One prominent target is
the quantum computer, which would allow the processing of information at
quantum level for purposes not achievable with even the most powerful computer
resources. The large-scale implementation of quantum information would be a
game changer for current technology, because it would allow unprecedented
parallelised computation and secure encryption based on the principles of
quantum superposition and entanglement. Currently, there are several physical
platforms racing to achieve the level of performance required for the quantum
hardware to step into the realm of practical quantum information applications.
Several materials have been proposed to fulfil this task, ranging from quantum
dots, Bose-Einstein condensates, spin impurities, superconducting circuits,
molecules, amongst others. Magnetic molecules are among the list of promising
building blocks, due to (i) their intrinsic monodispersity, (ii) discrete
energy levels (iii) the possibility of chemical quantum state engineering, and
(iv) their multilevel characteristics, leading to the so called Qudits (d > 2),
amongst others. Herein we review how a molecular multilevel nuclear spin qubit
(or qudit, where d = 4), known as TbPc2, gathers all the necessary requirements
to perform as a molecular hardware platform with a first generation of
molecular devices enabling even quantum algorithm operations.Comment: Chem. Soc. Rev., 2017, Advance Articl
Time-optimal universal quantum gates on superconducting circuits
Decoherence is inevitable when manipulating quantum systems. It decreases the
quality of quantum manipulations and thus is one of the main obstacles for
large-scale quantum computation, where high-fidelity quantum gates are needed.
Generally, the longer a gate operation is, the more decoherence-induced gate
infidelity will be. Therefore, how to shorten the gate time becomes an urgent
problem to be solved. To this end, time-optimal control based on solving the
quantum brachistochrone equation is a straightforward solution. Here, based on
time-optimal control, we propose a scheme to realize universal quantum gates on
superconducting qubits in a two-dimensional square lattice configuration, and
the two-qubit gate fidelity approaches 99.9\%. Meanwhile, we can further
accelerate the Z-axis gate considerably by adjusting the detuning of the
external driving. Finally, in order to reduce the influence of the dephasing
error, decoherence-free subspace encoding is also incorporated in our physical
implementation. Therefore, we present a fast quantum scheme which is promising
for large-scale quantum computation.Comment: v2 accepte
Determining eigenstates and thermal states on a quantum computer using quantum imaginary time evolution
The accurate computation of Hamiltonian ground, excited and thermal states on quantum computers stands to impact many problems in the physical and computer sciences, from quantum simulation to machine learning. Given the challenges posed in constructing large-scale quantum computers, these tasks should be carried out in a resource-efficient way. In this regard, existing techniques based on phase estimation or variational algorithms display potential disadvantages; phase estimation requires deep circuits with ancillae, that are hard to execute reliably without error correction, while variational algorithms, while flexible with respect to circuit depth, entail additional high-dimensional classical optimization. Here, we introduce the quantum imaginary time evolution and quantum Lanczos algorithms, which are analogues of classical algorithms for finding ground and excited states. Compared with their classical counterparts, they require exponentially less space and time per iteration, and can be implemented without deep circuits and ancillae, or high-dimensional optimization. We furthermore discuss quantum imaginary time evolution as a subroutine to generate Gibbs averages through an analogue of minimally entangled typical thermal states. Finally, we demonstrate the potential of these algorithms via an implementation using exact classical emulation as well as through prototype circuits on the Rigetti quantum virtual machine and Aspen-1 quantum processing unit
Modern Approaches to Topological Quantum Error Correction
The construction of a large-scale fault-tolerant quantum computer is an outstanding scientific and technological goal. It holds the promise to allow us to solve a variety of complex problems such as factoring large numbers, quick database search, and the quantum simulation of many-body quantum systems in fields as diverse as condensed matter, quantum chemistry, and even high-energy physics. Sophisticated theoretical protocols for reliable quantum information processing under imperfect conditions have been de-veloped, when errors affect and corrupt the fragile quantum states during storage and computations. Arguably, the most realistic and promising ap-proach towards practical fault-tolerant quantum computation are topologi-cal quantum error-correcting codes, where quantum information is stored in interacting, topologically ordered 2D or 3D many-body quantum systems. This approach offers the highest known error thresholds, which are already today within reach of the experimental accuracy in state-of-the-art setups. A combination of theoretical and experimental research is needed to store, protect and process fragile quantum information in logical qubits effectively so that they can outperform their constituting physical qubits. Whereas small-scale quantum error correction codes have been implemented, one of the main theoretical challenges remains to develop new and improve existing efficient strategies (so-called decoders) to derive (near-)optimal error cor-rection operations in the presence of experimentally accessible measurement information and realistic noise sources. One main focus of this project is the development and numerical implementation of scalable, efficient decoders to operate topological color codes. Additionally, we study the feasibility of im-plementing quantum error-correcting codes fault-tolerantly in near-term ion traps. To this end, we use realistic modeling of the different noise sources, computer simulations, and most modern quantum information approaches to quantum circuitry and noise suppression techniques
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