Detecting multipartite entanglement

We discuss the problem of determining whether the state of several quantum mechanical subsystems is entangled. As in previous work on two subsystems we introduce a procedure for checking separability that is based on finding state extensions with appropriate properties and may be implemented as a semidefinite program. The main result of this work is to show that there is a series of tests of this kind such that if a multiparty state is entangled this will eventually be detected by one of the tests. The procedure also provides a means of constructing entanglement witnesses that could in principle be measured in order to demonstrate that the state is entangled.Comment: 9 pages, REVTE

Holonomic quantum computing in symmetry-protected ground states of spin chains

While solid-state devices offer naturally reliable hardware for modern classical computers, thus far quantum information processors resemble vacuum tube computers in being neither reliable nor scalable. Strongly correlated many body states stabilized in topologically ordered matter offer the possibility of naturally fault tolerant computing, but are both challenging to engineer and coherently control and cannot be easily adapted to different physical platforms. We propose an architecture which achieves some of the robustness properties of topological models but with a drastically simpler construction. Quantum information is stored in the symmetry-protected degenerate ground states of spin-1 chains, while quantum gates are performed by adiabatic non-Abelian holonomies using only single-site fields and nearest-neighbor couplings. Gate operations respect the symmetry, and so inherit some protection from noise and disorder from the symmetry-protected ground states.Comment: 19 pages, 4 figures. v2: published versio

Entanglement in a quantum annealing processor

Entanglement lies at the core of quantum algorithms designed to solve problems that are intractable by classical approaches. One such algorithm, quantum annealing (QA), provides a promising path to a practical quantum processor. We have built a series of scalable QA processors consisting of networks of manufactured interacting spins (qubits). Here, we use qubit tunneling spectroscopy to measure the energy eigenspectrum of two- and eight-qubit systems within one such processor, demonstrating quantum coherence in these systems. We present experimental evidence that, during a critical portion of QA, the qubits become entangled and that entanglement persists even as these systems reach equilibrium with a thermal environment. Our results provide an encouraging sign that QA is a viable technology for large-scale quantum computing.Comment: 13 pages, 8 figures, contact corresponding author for Supplementary Informatio

Magnetization Dynamics, Bennett Clocking and Associated Energy Dissipation in Multiferroic Logic

It has been recently shown that multiferroic logic - where logic bits are encoded in the magnetization orientation of a nanoscale magnetostrictive layer elastically coupled to a piezoelectric layer - can be Bennett clocked with small electrostatic potentials of few tens of mV applied to the piezoelectric layer. The potential generates stress in the magnetostrictive layer and rotates its magnetization by a large angle to carry out Bennett clocking. This method of clocking is far more energy-efficient than using spin transfer torque. In order to assess if such a clocking scheme can be also reasonably fast, we have studied the magnetization dynamics of a multiferroic logic array with nearest neighbor dipole coupling using the Landau-Lifshitz-Gilbert (LLG) equation. We find that switching delays of ~ 3 ns (clock rates of 0.33 GHz) can be achieved with proper design provided we clock non-adiabatically and dissipate ~48,000 kT (at room temperature) of energy per clock cycle per bit flip in the clocking circuit. This dissipation far exceeds the energy barrier separating the two logic states, which we assumed to be 32 kT to yield a bit error probability of . Had we used spin transfer torque to switch with the same ~ 3 ns delay, the energy dissipation would have been much larger (~ $6 \times 106$ kT). This shows that spin transfer torque, widely used in magnetic random access memory, is an inefficient way to switch a magnet, and multiferroic logic clocked with voltage-induced stress is a superior nanomagnetic logic scheme

High-rate quantum cryptography in untrusted networks

We extend the field of continuous-variable quantum cryptography to a network formulation where two honest parties connect to an untrusted relay by insecure quantum links. To generate secret correlations, they transmit coherent states to the relay where a continuous-variable Bell detection is performed and the outcome broadcast. Even though the detection could be fully corrupted and the links subject to optimal coherent attacks, the honest parties can still extract a secret key, achieving high rates when the relay is proximal to one party, as typical in public networks with access points or proxy servers. Our theory is confirmed by an experiment generating key-rates which are orders of magnitude higher than those achievable with discrete-variable protocols. Thus, using the cheapest possible quantum resources, we experimentally show the possibility of high-rate quantum key distribution in network topologies where direct links are missing between end-users and intermediate relays cannot be trusted.Comment: Theory and Experiment. Main article (6 pages) plus Supplementary Information (additional 13 pages

Fault tolerant architectures for superconducting qubits

In this short review, I draw attention to new developments in the theory of fault tolerance in quantum computation that may give concrete direction to future work in the development of superconducting qubit systems. The basics of quantum error correction codes, which I will briefly review, have not significantly changed since their introduction fifteen years ago. But an interesting picture has emerged of an efficient use of these codes that may put fault tolerant operation within reach. It is now understood that two dimensional surface codes, close relatives of the original toric code of Kitaev, can be adapted to effectively perform logical gate operations in a very simple planar architecture, with error thresholds for fault tolerant operation simulated to be 0.75%. This architecture uses topological ideas in its functioning, but it is not 'topological quantum computation' -- there are no non-abelian anyons in sight. I offer some speculations on the crucial pieces of superconducting hardware that could be demonstrated in the next couple of years that would be clear stepping stones towards this surface-code architecture.Comment: 28 pages, 10 figures. For the Nobel Symposium on Qubits for Quantum Information, submitted to Physica Scripta. v. 2 Corrections and small changes to reference

Fundamental limits to quantum channel discrimination

What is the ultimate performance for discriminating two arbitrary quantum channels acting on a finite-dimensional Hilbert space? Here we address this basic question by deriving a general and fundamental lower bound. More precisely, we investigate the symmetric discrimination of two arbitrary qudit channels by means of the most general protocols based on adaptive (feedback-assisted) quantum operations. In this general scenario, we first show how port-based teleportation can be used to simplify these adaptive protocols into a much simpler non-adaptive form, designing a new type of teleportation stretching. Then, we prove that the minimum error probability affecting the channel discrimination cannot beat a bound determined by the Choi matrices of the channels, establishing a general, yet computable formula for quantum hypothesis testing. As a consequence of this bound, we derive ultimate limits and no-go theorems for adaptive quantum illumination and single-photon quantum optical resolution. Finally, we show how the methodology can also be applied to other tasks, such as quantum metrology, quantum communication and secret key generation

Advances in photonic quantum sensing

Quantum sensing has become a mature and broad field. It is generally related with the idea of using quantum resources to boost the performance of a number of practical tasks, including the radar-like detection of faint objects, the readout of information from optical memories or fragile physical systems, and the optical resolution of extremely close point-like sources. Here we first focus on the basic tools behind quantum sensing, discussing the most recent and general formulations for the problems of quantum parameter estimation and hypothesis testing. With this basic background in our hands, we then review emerging applications of quantum sensing in the photonic regime both from a theoretical and experimental point of view. Besides the state-of-the-art, we also discuss open problems and potential next steps.Comment: Review in press on Nature Photonics. This is a preliminary version to be updated after publication. Both manuscript and reference list will be expande