Quantum-gate implementation in permanently coupled AF spin rings without need of local fields

We propose a scheme for the implementation of quantum gates which is based on the qubit encoding in antiferromagnetic molecular rings. We show that a proper engineering of the intercluster link would result in an effective coupling that vanishes as far as the system is kept in the computational space, while it is turned on by a selective excitation of specific auxiliary states. These are also shown to allow the performing of single- and two-qubit gates without an individual addressing of the rings by means of local magnetic fields.Comment: To appear in Physical Review Letter

Magnetothermal properties of molecule-based materials

We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecular magnets. Perspectives of this field are discussed as well.Comment: 12 pages text + 14 pages figures, Submitted as "feature article" to Journal of Materials Chemistr

Spin-enhanced magnetocaloric effect in molecular nanomagnets

An unusually large magnetocaloric effect for the temperature region below 10 K is found for the Fe-14 molecular nanomagnet. This is to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states. We also show that the highly symmetric Fe-14 cluster core, resulting in small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below T-N=1.87 K

Magnetocaloric effect in hexacyanochromate Prussian blue analogs

We report on the magnetocaloric properties of two molecule-based hexacyanochromate Prussian blue analogs, nominally CsNi[Cr(CN)_6](H_2O) and Cr_3[Cr(CN)_6]_2x12(H_2O). The former orders ferromagnetically below Tc=90 K, whereas the latter is a ferrimagnet below Tc=230 K. For both, we find significantly large magnetic entropy changes DSm associated to the magnetic phase transitions. Notably, our studies represent the first attempt to look at molecule-based materials in terms of the magnetocaloric effect for temperatures well above the liquid helium range.Comment: 4 pages, 6 figure

Entanglement in a molecular three-qubit system

We study the entanglement properties of a molecular three-qubit system described by the Heisenberg spin Hamiltonian with anisotropic exchange interactions and including an external magnetic field. The system exhibits first order quantum phase transitions by tuning two parameters, $x$ and $y$, of the Hamiltonian to specific values. The three-qubit chain is open ended so that there are two types of pairwise entanglement : nearest-neighbour (n.n.) and next-nearest-neighbour (n.n.n.). We calculate the ground and thermal state concurrences, quantifying pairwise entanglement, as a function of the parameters $x$, $y$ and the temperature $T$. The entanglement threshold and gap temperatures are also determined as a function of the anisotropy parameter $x$. The results obtained are of relevance in understanding the entanglement features of the recently engineered molecular $Cr_{7}Ni$-$Cu^{2+}$-$Cr_{7}Ni$ complex which serves as a three-qubit system at sufficiently low temperatures.Comment: 9 pages, 13 figures, revtex

Microwave photon detectors based on semiconducting double quantum dots

Detectors of microwave photons find applications in different fields ranging from security to cosmology. Due to the intrinsic difficulties related to the detection of vanishingly small energy quanta ¯hω, significant portions of the microwave electromagnetic spectrum are still uncovered by suitable techniques. No prevailing technology has clearly emerged yet, although different solutions have been tested in different contexts. Here, we focus on semiconductor quantum dots, which feature wide tunability by external gate voltages and scalability for large architectures. We discuss possible pathways for the development of microwave photon detectors based on photon-assisted tunneling in semiconducting double quantum dot circuits. In particular, we consider implementations based on either broadband transmission lines or resonant cavities, and we discuss how developments in charge sensing techniques and hybrid architectures may be beneficial for the development of efficient photon detectors in the microwave range

YBCO microwave resonators for strong collective coupling with spin ensembles

Coplanar microwave resonators made of 330 nm-thick superconducting YBCO have been realized and characterized in a wide temperature ($T$, 2-100 K) and magnetic field ($B$, 0-7 T) range. The quality factor $Q_L$ exceeds 10$^4$ below 55 K and it slightly decreases for increasing fields, remaining 90$\%$ of $Q_L(B=0)$ for $B=7$ T and $T=2$ K. These features allow the coherent coupling of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate this, collective strong coupling was achieved by using DPPH organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap splitting across the single-mode cavity resonance at 7.75 GHz, where clear anticrossings are observed with a splitting as large as $\sim 82$ MHz at $T=2$ K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.Comment: to appear in Appl. Phys. Let

Microwave dual-mode resonators for coherent spin-photon coupling

We implement superconducting Yttrium barium copper oxide planar resonators with two fundamental modes for circuit quantum electrodynamics experiments. We first demonstrate good tunability in the resonant microwave frequencies and in their interplay, as emerges from the dependence of the transmission spectra on the device geometry. We then investigate the magnetic coupling of the resonant modes with bulk samples of 2,2-diphenyl-1-picrylhydrazyl organic radical spins. The transmission spectroscopy performed at low temperature shows that the coherent spin-photon coupling regime with the spin ensembles can be achieved by each of the resonator modes. The analysis of the results within the framework of the input-output formalism and by means of entropic measures demonstrates coherent mixing of the degrees of freedom corresponding to two remote spin ensembles and, with a suitable choice of the geometry, the approaching of a regime with spin-induced mixing of the two photon modes

Inhomogeneous magnetism in the doped kagome lattice of LaCuO2.66

The hole-doped kagome lattice of Cu2+ ions in LaCuO2.66 was investigated by nuclear quadrupole resonance (NQR), electron spin resonance (ESR), electrical resistivity, bulk magnetization and specific heat measurements. For temperatures above ~180 K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexisting non-magnetic sites, free spins and frozen moments reveals an intrinsically inhomogeneous magnetism. Numerical simulations of a 1/3-diluted kagome lattice rationalize this magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation threshold