Direct evidence of quantum coherence in a single-molecule magnet in a frozen solution is reported with coherence times as long as T{2}=630+/-30 ns. We can strongly increase the coherence time by modifying the matrix in which the single-molecule magnets are embedded. The electron spins are coupled to the proton nuclear spins of both the molecule itself and, interestingly, also to those of the solvent. The clear observation of Rabi oscillations indicates that we can manipulate the spin coherently, an essential prerequisite for performing quantum computations.
Direct evidence of quantum coherence in a single-molecule magnet in a frozen solution is reported with coherence times as long as T{2}=630+/-30 ns. We can strongly increase the coherence time by modifying the matrix in which the single-molecule magnets are embedded. The electron spins are coupled to the proton nuclear spins of both the molecule itself and, interestingly, also to those of the solvent. The clear observation of Rabi oscillations indicates that we can manipulate the spin coherently, an essential prerequisite for performing quantum computations.
Molecular magnets are ideal systems to probe the realm that borders quantum and classical physics, as well as to study decoherence phenomena in nanoscale systems. The control of the quantum behavior of these materials and their structural characteristics requires synthesis of new complexes with desirable properties which will allow probing of the fundamental aspects of nanoscale physics and quantum information processing. Of particular interest among the magnetic molecular materials are single-molecule magnets (SMMs) and antiferromagnetic (AFM) molecular wheels in which the spin state of the molecule is known to behave quantum mechanically at low temperatures. In previous experiments the dynamics of the magnetic moment of the molecules is governed by incoherent quantum tunneling. Short decoherence times are mainly due to interactions between molecular magnets within the crystal and interactions of the electronic spin with the nuclear spin of neighboring ions within the molecule. This decoherence problem has imposed a limit to the understanding of the molecular spin dynamics and the sources of decoherence in condensed matter systems. Particularly, intermolecular dipolar interactions within the crystal, which shorten the coherence times in concentrated samples, have stymied progress in this direction. Several recent works have reported a direct measurement of the decoherence time in molecular magnets. This has been done by eliminating the dephasing created by dipolar interactions between neighboring molecules. This has been achieved by a) a dilution of the molecules in a liquid solution to decrease the dipolar interaction by separating the molecules, and b) by polarizing the spin bath by applying a high magnetic field at low temperatures. Unfortunately, both approaches restrict the experimental studies of quantum dynamics. For example, the dilution of molecular magnets in liquid solution causes a dispersion of the molecular spin orientation and anisotropy axes, while the large fields required to polarize the spin bath overcome the anisotropy of the molecular spin. In this thesis I have explored two methods to overcome dipolar interactions in molecular magnets: a) studying the dynamics of molecular magnets in single crystals where the separation between magnetic molecules is obtained by chemical doping or where the high crystalline quality allows observations intrinsic to the quantum mechanical nature of the tunneling of the spin, and b) studying the electronic transport through an individual magnetic molecule which has been carefully placed in a single-electron transistor device. I have used EPR microstrip resonators to measure Fe17Ga molecular wheels within single crystals of Fe18 AFM wheels, as well as demonstrating, for the first time in a Single Molecule Magnet, the complete suppression of a Quantum Tunneling of the Magnetization transition forbidden by molecular symmetry.
Single molecular magnets (SMMs) are among the potential systems for quantum memory and quantum information processing. Quantum coherence and oscillation are critical for these applications. The ground-state quantum coherence and Rabi oscillations of the SMM V15 ($[{\mathrm{V}}_{15}^{\mathrm{IV}}{\mathrm{As}}_{6}^{\mathrm{III}}{\mathrm{O}}_{42}({\mathrm{H}}_{2}\mathrm{O}){]}^{6\ensuremath{-}}$) have been studied in this context. We have affirmatively measured at 2.4 K the Rabi quantum oscillations and coherence time T2 for the ground states of the V15 ion of collective spin S=1/2, in addition to confirming the previously reported results for the S=3/2 excited states. The oscillations of S=3/2 and S=1/2 states are of different frequencies, and so can be separately selected for purposive manipulations. T2 of $188\ifmmode\pm\else\textpm\fi{}4\text{ }\text{ }\mathrm{ns}$ (S=3/2) and $149\ifmmode\pm\else\textpm\fi{}10\text{ }\text{ }\mathrm{ns}$ (S=1/2) are much less than ${T}_{1}\ensuremath{\sim}12\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ and are further extendible via various approaches for qubit implementations.
Coherence time is an essential parameter for quantum sensing, quantum information, and quantum computation. In this work, we demonstrate electron spin coherence times as long as 0.1 s for an ensemble of rubidium atoms trapped in a solid parahydrogen matrix. We explore the underlying physics limiting the coherence time. The properties of these matrix isolated atoms are very promising for future applications, including quantum sensing of nuclear spins. If combined with efficient single-atom readout, this would enable NMR and magnetic resonance imaging of single molecules cotrapped with alkali-metal atom quantum sensors within a parahydrogen matrix.
Rabi oscillations in the V15 single molecule magnet embedded in the surfactant (CH3)2[CH3(CH2)16CH2]2N+ have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency ${\ensuremath{\Omega}}_{R}$ falls in the vicinity of the Larmor frequency of protons ${\ensuremath{\omega}}_{N}$. The experiments are interpreted by a model showing that the damping (or Rabi) time ${\ensuremath{\tau}}_{R}$ is directly associated with decoherence caused by electron-nuclear cross relaxation in the rotating reference frame. This decoherence induces energy dissipation in the range ${\ensuremath{\omega}}_{N}\ensuremath{-}{\ensuremath{\sigma}}_{e}<{\ensuremath{\Omega}}_{R}<{\ensuremath{\omega}}_{N}$, where ${\ensuremath{\sigma}}_{e}$ is the mean superhyperfine field induced by protons at V15. Weaker decoherence without dissipation takes place outside this window. Specific estimations suggest that this rapid cross relaxation in a resonant microwave field, observed for the first time in V15, should also take place, e.g., in Fe8 and Mn12.
The ability to measure and manipulate the quantum state of a single spin in a solid state system opens up a broad range of powerful applications ranging from nano probes to quantum information processing. Over the past few years, a number of ground breaking experiments have been conducted on paramagnetic nitrogen vacancy defect centers in diamond, including up to 3-qubit entanglement, single-shot-readout of a nuclear spin, selective access of two coupled centers and magnetic, electric and thermographic sensing at the nanoscale with promising properties in some cases exceeding most currently known techniques. Yet, almost all of the experiments conducted so far focus on centers deeply buried in the diamond, which is undesirable especially for sensing applications, but also in terms of scalability and electronic access to the information. In this work we focus on the spin physics of centers only few nanometers below the surface, where the unique properties of the center are strongly influenced by the external environment. We coherently manipulated and detected single spins at ambient temperature by combining optically detected magnetic resonance with single molecule spectroscopy using a confocal microscope. We measured the spin dephasing behavior and the phase relaxation time and examined the primary sources of decoherence. Furthermore, we observed effects of water, atmosphere and paramagnetic molecules on the coherence and readability of spins in nanodiamonds and demonstrated ways to preserve the coherence using external magnetic fields and a number of different dynamic decoupling pulse sequences. In addition, we measured couplings to proximal nuclear spins by continuous wave spectroscopy and to molecules on the surface by double electron electron resonance. Also, we demonstrate execution of the refined Deutsch-Jozsa algorithm on a nitrogen vacancy center in a nanodiamond. Finally, we propose new technological approach for realtime measurements of magnetic fields using a single spin in diamond. In test measurements using external fields and spatially resolved measurements of the field from a magnetic microwire a sensitivity of a few uT / sqrt(Hz) was achieved.
Over the past two decades, many quantum systems have been explored as candidate qubits for quantum information processing. Qubits based on nuclear spins enjoy exceptionally long coherence times, especially compared to other solid-state platforms. However, this usually comes at the cost of slow gate operations and weak interqubit interactions, aspects which have so far prevented the development of multi-qubit processors comprised of solid-state nuclear spins. An exception to this is the TbPc2 single-molecule magnet, a nuclear spin qubit which features a remarkable combination of long-lived coherence and fast electrical control. Despite this promise, it has remained an open question how to couple such qubits together to create the entanglement needed for quantum information applications. Given the sensitivity of the TbPc2 molecule to electric fields via the hyperfine Stark effect, it is natural to consider the possibility of using a superconducting transmission line resonator to mediate a long-distance coupling between multiple TbPc2 qubits. We use recent experimental results on single-qubit Rabi oscillations to deduce the strength and nature of the hyperfine Stark effect, information we then use to estimate the qubit-resonator coupling and to design two-qubit entangling gates with fidelities exceeding 99%. Our results suggest that the qubit-resonator interaction is near the edge of the strong coupling regime and could potentially pass into it with further improvements in device designs and components.