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Beyond classical computing : towards organic quantum information processing

Marc Warner

Published Nov 28, 2011• 0 citations
OpenAlex

Abstract

This thesis examines the potential of a class of organic molecules, porphyrin derivatives, for quantum information purposes. The experiments in this thesis follow a simple progression: they begin by exploring the time-independent characteristics of the molecules, then investigate the dynamics and decoherence, and end with a consideration of the potential for controllable coupling. Whilst this methodology is obviously motivated by interest in quantum information processing (QIP), it is hoped that the results will have relevance for a larger group of fields including organic electronics, solar energy and spintronics. QIP is at the forefront of science, requiring an understanding of the coherent properties of materials - an understanding deeper than has ever been required before. Since copper phthalocyanine is a technologically relevant material, with uses in organic electronics and solar cells, and possible applications in the developing field of spintronics, this deep understanding can be fed back to provide interesting new developments. The first experimental chapter, Chapter 4, is such an example, where work focussed on understanding a novel phase of CuPc (the \\eta-phase), through magnetometry and electron spin resonance led to the development of a method to measure the magnetic properties of liquids. It is hoped that this could open a path for solution based biological samples to be studied via magnetometry. A discussion of the details of the technique, the interpretation of the data, and a proof of principle are provided in this chapter. Another interesting technological development is discussed in Chapter 5, where the initial characterisation of the thin films of copper phthalocyanine (CuPc) led to a deeper understanding of their nanostructure, an important property for the development of solar cells. This consisted of continuous wave electron spin resonance (ESR) experiments on thin films of CuPc, varying both the percentage of copper and the orientation of the molecules, to allow the interpretation of the ESR spectra of mixed films of CuPc and C60, the mixture that is used in organic solar cells. I demonstrate that the CuPc and C60 form nano-clusters in these films, with a preferred orientation. Since the mixed films are known to be more efficient than a simple two layer device, this result provides new information, which can be used to improve the design of this type of solar cell. Chapter 6 consists of the measurement of the decoherence times of spins in CuPc films, and an understanding of the mechanisms of decoherence. Pulsed ESR is used on both powders and thin films of copper phthalocyanine and the field and temperature dependence is explored. Since it is unlikely that molecules in solution will ever be able to provide a scalable QIP solution, this move to the solid state represents a real advance. It is not trivial however, as the solid state can provide many additional decoherence routes. These times are the first step in proposing a material as a potential vehicle for QIP, and I demonstrate that the decoherence times are long enough that, given its other advantages, CuPc can be taken seriously as a candidate host for qubits. The important distinction of these measurements from those that have been made before on, for instance, molecular magnets is that these decoherence times are achieved in the solid state. The last results chapter is an attempt to make progress on a problem that is particularly relevant for molecular QIP. Since molecules can be made easily, they provide a attractive path to an individual qubit. The identical nature of the molecules comes at a price, though, as it makes it hard to address each qubit independently. Chapter 7 seeks to explore the physics of optically coupling two qubits (in this case TEMPO radicals) through a porphyrin linker. Steady state and time resolved optical absorption and pulsed and transient ESR are used to characterise the spin physics of the samples. I demonstrate that it is very likely a coupling can be controllably switched on (if not, as yet, o*). This chapter lays the foundations for a considerable number of future experiments, some of which are discussed at the end, that could demonstrate a truly switchable coherent coupling between qubits.

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Molecular spin qubits are promising candidates for quantum technologies, but their performance is limited by decoherence arising from diverse mechanisms. The complexity of the environment makes it challenging to identify the main source of noise and target it for mitigation. Here we present a systematic experimental and theoretical framework for analyzing the mechanisms of transverse relaxation in copper(II) phthalocyanine (CuPc) diluted into diamagnetic phthalocyanine hosts. Using pulsed EPR spectroscopy together with first-principles cluster correlation expansion simulations, we quantitatively separate the contributions from hyperfine-coupled nuclear spins, spin--lattice relaxation, and electron--electron dipolar interactions. Our detailed modeling shows that both strongly and weakly coupled nuclei contribute negligibly to $T_2$, while longitudinal dipolar interactions with electronic spins, through instantaneous and spectral diffusion, constitute the main decoherence channel even at moderate spin densities. This conclusion is validated by direct comparison between simulated spin-echo dynamics and experimental data. By providing a robust modeling and experimental approach, our work identifies favorable values of the electron spin density for quantum applications, and provides a transferable methodology for predicting ensemble coherence times. These insights will guide the design and optimization of molecular spin qubits for scalable quantum devices.

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This thesis discusses various schemes and protocols for quantum information processing in mesoscopic systems with particular focus on using the spin of a particle as the bearer of information. The first chapter introduce various aspects of the field of quantum information used in this thesis such as qubits, entanglement, its quantification, quantum logic gates and entanglement swapping. In this chapter concepts such as AKLT states, decoherence and adiabatic elimination are introduced as they will be relevant in the thesis. In chapter 2 we introduce the Quantum Dots as the solid state system that will primarily be used as the hardware for the development of Quantum Information Processing (QIP). The different properties of quantum dots depending on their size are discussed. The exchange interaction between tunnel coupled quantum dots and the background of quantum computation in quantum dots is described. The principal sources of decoherence and the measurement techniques for spin qubits are presented. In chapter 3, carbon nanowires filled with N@C60 dimers are studied to analyse the entanglement between nuclear spins. The dimer is modelled as a two coupled nuclear spin- electron spin pair with a Heisenberg interaction. The entanglement have been studied depending on the temperature and the intensity of an external magnetic field. Witnessing the entanglement, and particularly bound entanglement are discussed. In chapter 4, the way to extract a singlet from a quantum dot is explored. The system that we model will be consisting of a triple dot and analyse the best way to get the singlet out, with each electron in a separate dot. The chief motivation is to create a singlet between separate dots in a time-scale much faster than that given by spinspin exchange interactions. In chapter 5, quantum logic gates in a triple dot system has been studied. Such gates have been widely studied in double and single quantum dots. Motivated by the advent of experimental set ups of triple dots, we have studied the natural quantum gates that came out of a triple dot system. There are still two spin quantum bits in the three dots and there is an empty intervening dot, which imparts the scheme some advantages, as well as a substantial difference from the class of schemes studied so far. In chapter 6, we model a large square dot. As we describe in chapter 2, the properties of the large dots make them behave with some interesting properties such as hosting Wigner molecules of electrons inside. We explore the application of these structures for quantum information processing. We show here how to get singlet/triplet measurement, entanglement swapping, and how to prepare a 1D AKLT state, using the square dot as a construction block of the system. Finally in chapter 7 conclusions and further work. Here we indicate the further work that could be done with the knowledge present in this thesis and motivated by future advances in the technology.

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•Oct 19, 2012•0 citations

Molecular electron-spin quantum computers and quantum information processing: pulse-based electron magnetic resonance spin technology applied to matter spin-qubits

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Journal of Materials Chemistry•Jan 1, 2009•152 citations
This thesis examines the potential of a class of organic molecules, porphyrin derivatives, for quantum information purposes. The experiments in this thesis follow a simple progression: they begin by exploring the time-independent characteristics of the molecules, then investigate the dynamics and decoherence, and end with a consideration of the potential for controllable coupling. Whilst this methodology is obviously motivated by interest in quantum information processing (QIP), it is hoped that the results will have relevance for a larger group of fields including organic electronics, solar energy and spintronics. QIP is at the forefront of science, requiring an understanding of the coherent properties of materials - an understanding deeper than has ever been required before. Since copper phthalocyanine is a technologically relevant material, with uses in organic electronics and solar cells, and possible applications in the developing field of spintronics, this deep understanding can be fed back to provide interesting new developments. The first experimental chapter, Chapter 4, is such an example, where work focussed on understanding a novel phase of CuPc (the \eta-phase), through magnetometry and electron spin resonance led to the development of a method to measure the magnetic properties of liquids. It is hoped that this could open a path for solution based biological samples to be studied via magnetometry. A discussion of the details of the technique, the interpretation of the data, and a proof of principle are provided in this chapter. Another interesting technological development is discussed in Chapter 5, where the initial characterisation of the thin films of copper phthalocyanine (CuPc) led to a deeper understanding of their nanostructure, an important property for the development of solar cells. This consisted of continuous wave electron spin resonance (ESR) experiments on thin films of CuPc, varying both the percentage of copper and the orientation of the molecules, to allow the interpretation of the ESR spectra of mixed films of CuPc and C60, the mixture that is used in organic solar cells. I demonstrate that the CuPc and C60 form nano-clusters in these films, with a preferred orientation. Since the mixed films are known to be more efficient than a simple two layer device, this result provides new information, which can be used to improve the design of this type of solar cell. Chapter 6 consists of the measurement of the decoherence times of spins in CuPc films, and an understanding of the mechanisms of decoherence. Pulsed ESR is used on both powders and thin films of copper phthalocyanine and the field and temperature dependence is explored. Since it is unlikely that molecules in solution will ever be able to provide a scalable QIP solution, this move to the solid state represents a real advance. It is not trivial however, as the solid state can provide many additional decoherence routes. These times are the first step in proposing a material as a potential vehicle for QIP, and I demonstrate that the decoherence times are long enough that, given its other advantages, CuPc can be taken seriously as a candidate host for qubits. The important distinction of these measurements from those that have been made before on, for instance, molecular magnets is that these decoherence times are achieved in the solid state. The last results chapter is an attempt to make progress on a problem that is particularly relevant for molecular QIP. Since molecules can be made easily, they provide a attractive path to an individual qubit. The identical nature of the molecules comes at a price, though, as it makes it hard to address each qubit independently. Chapter 7 seeks to explore the physics of optically coupling two qubits (in this case TEMPO radicals) through a porphyrin linker. Steady state and time resolved optical absorption and pulsed and transient ESR are used to characterise the spin physics of the samples. I demonstrate that it is very likely a coupling can be controllably switched on (if not, as yet, o*). This chapter lays the foundations for a considerable number of future experiments, some of which are discussed at the end, that could demonstrate a truly switchable coherent coupling between qubits.
Molecular spin qubits are promising candidates for quantum technologies, but their performance is limited by decoherence arising from diverse mechanisms. The complexity of the environment makes it challenging to identify the main source of noise and target it for mitigation. Here we present a systematic experimental and theoretical framework for analyzing the mechanisms of transverse relaxation in copper(II) phthalocyanine (CuPc) diluted into diamagnetic phthalocyanine hosts. Using pulsed EPR spectroscopy together with first-principles cluster correlation expansion simulations, we quantitatively separate the contributions from hyperfine-coupled nuclear spins, spin--lattice relaxation, and electron--electron dipolar interactions. Our detailed modeling shows that both strongly and weakly coupled nuclei contribute negligibly to T2T_2T2​, while longitudinal dipolar interactions with electronic spins, through instantaneous and spectral diffusion, constitute the main decoherence channel even at moderate spin densities. This conclusion is validated by direct comparison between simulated spin-echo dynamics and experimental data. By providing a robust modeling and experimental approach, our work identifies favorable values of the electron spin density for quantum applications, and provides a transferable methodology for predicting ensemble coherence times. These insights will guide the design and optimization of molecular spin qubits for scalable quantum devices.
This thesis discusses various schemes and protocols for quantum information processing in mesoscopic systems with particular focus on using the spin of a particle as the bearer of information. The first chapter introduce various aspects of the field of quantum information used in this thesis such as qubits, entanglement, its quantification, quantum logic gates and entanglement swapping. In this chapter concepts such as AKLT states, decoherence and adiabatic elimination are introduced as they will be relevant in the thesis. In chapter 2 we introduce the Quantum Dots as the solid state system that will primarily be used as the hardware for the development of Quantum Information Processing (QIP). The different properties of quantum dots depending on their size are discussed. The exchange interaction between tunnel coupled quantum dots and the background of quantum computation in quantum dots is described. The principal sources of decoherence and the measurement techniques for spin qubits are presented. In chapter 3, carbon nanowires filled with N@C60 dimers are studied to analyse the entanglement between nuclear spins. The dimer is modelled as a two coupled nuclear spin- electron spin pair with a Heisenberg interaction. The entanglement have been studied depending on the temperature and the intensity of an external magnetic field. Witnessing the entanglement, and particularly bound entanglement are discussed. In chapter 4, the way to extract a singlet from a quantum dot is explored. The system that we model will be consisting of a triple dot and analyse the best way to get the singlet out, with each electron in a separate dot. The chief motivation is to create a singlet between separate dots in a time-scale much faster than that given by spinspin exchange interactions. In chapter 5, quantum logic gates in a triple dot system has been studied. Such gates have been widely studied in double and single quantum dots. Motivated by the advent of experimental set ups of triple dots, we have studied the natural quantum gates that came out of a triple dot system. There are still two spin quantum bits in the three dots and there is an empty intervening dot, which imparts the scheme some advantages, as well as a substantial difference from the class of schemes studied so far. In chapter 6, we model a large square dot. As we describe in chapter 2, the properties of the large dots make them behave with some interesting properties such as hosting Wigner molecules of electrons inside. We explore the application of these structures for quantum information processing. We show here how to get singlet/triplet measurement, entanglement swapping, and how to prepare a 1D AKLT state, using the square dot as a construction block of the system. Finally in chapter 7 conclusions and further work. Here we indicate the further work that could be done with the knowledge present in this thesis and motivated by future advances in the technology.
Electron spin resonance (ESR) experiments are used to assess the possibilities for processing quantum information in the electronic and nuclear spins of endohedral fullerenes. It is shown that ¹⁵N@C₆₀ can be used for universal two-qubit quantum computing. The first step in this scheme is to initialize the nuclear and electron spins that each store one qubit. This was achieved with a magnetic field of 8.6 T at 3 K, by applying resonant RF and microwave radiation. This dynamic nuclear polarization technique made it possible to show that the nuclear T₁ time of ¹⁵N@C����� is on the order of twelve hours at 4.2 K. The electronic T₂ is the limiting decoherence time for the system. At 3.7 K, this can be extended to 215 μs by using amorphous sulphur as the solvent. Pulse sequences are described that could perform all single-qubit gates to the two qubits independently, as well as CNOT gates. After these manipulations, the value of the qubits should be measured. Two techniques are demonstrated for this, by measuring the nuclear spin. Sc@C₈₂ could also be useful for quantum computation. By comparing ESR measurements with density functional theory calculations, it is shown how the orientation of a Sc@C₈₂ molecule in an applied magnetic field affects the molecule's Zeeman and hyperfine coupling. Hence the g- and A-tensors are written in the coordinate frame of the molecule. Pulsed ESR measurements show that the decoherence time at 20 K is 13 μs, which is 20 times longer than had been previously reported. Carbon nanotubes have been filled with endohedral fullerenes, forming 1D arrays that could lead to a scalable quantum computer. N@C₆₀ and Sc@C₈₂ have been used for this filling in various concentrations. ESR measurements of these samples are consistent with simulations of the dipolar coupling.
Molecular spin systems are promising candidates for quantum information processing and nanoscale sensing, yet their characterization at room temperature remains challenging due to fast spin decoherence. In this work, we use T1T_1T1​ relaxometry of shallow nitrogen-vacancy (NV) centers in diamond to probe the electron spin ensemble of a polycrystalline copper phthalocyanine (CuPc) thin film. In addition to unequivocally identifying the NV-CuPc interaction thanks to its hyperfine spectrum, we further extract key parameters of the CuPc spin ensemble, including its correlation time and local lattice orientation, that cannot be measured in bulk electron resonance experiments. The analysis of our experimental results confirms that electron-electron interactions dominate the decoherence dynamics of CuPc at room temperature. Additionally, we demonstrate that the CuPc-enhanced NV relaxometry can serve as a robust method to estimate the NV depth with ∼1\sim1∼1~nm precision. Our results establish NV centers as powerful probes for molecular spin systems, providing insights into molecular qubits, spin bath engineering, and hybrid quantum materials, and offering a potential pathway toward their applications such as molecular-scale quantum processors and spin-based quantum networks.
Ultrafast pulses allow observation of molecular dynamics with femtosecond time resolution through pump probe experiments. However, averaging over an ensemble of molecules tends to wash out phase sensitive information, necessary to probe quantum effects, due to the intrinsic inhomogeneity in molecular conformations, orientations and interactions that lead to unique potential energy landscapes for each molecule. It is therefore important to go beyond the ensemble average when looking at quantum dynamics of organic systems at room temperature, and resolve the behaviour of specific molecules on an individual basis. In this thesis, we show the creation, manipulation and observation of ultrafast coherent effects in single molecules at room temperature, and resolve a certain measure of environmental influence on the specific dynamics of each molecule. Moreover, we apply this insight to investigate a functional light harvesting biosystem, and lay the basis for a technique that has the time and space resolution required to observe these systems in vivo. In chapter 1, we introduce the concepts and techniques the research in this thesis is built on. In chapter 2, we treat the possibility of controlling ultrafast pulses at the high-NA diffraction limit, and come to conclusions about the procedure to follow there that hold for all pulse-shaping experiments. We show in proof of principle experiments that we can control the ultrafast characteristics of optical pulses in nanometric excitation volumes. In chapters 3 and 4 we report the creation, detection and control of ultrafast quantum dynamics in single organic molecules at room temperature. We show that manipulation of superposition states is possible in these systems within a coherence dephasing time of ~50 fs. This leads to the first observation of rabi-oscillations in room temperature single molecules, to ultrafast operation of an organic qubit, and to the creation of non-stationary superposition states (vibrational wavepackets). We probe the influence of the local environment on the composition and dynamics of these wavepackets and show we can optimize the state preparation protocol for each individual molecule in its own nanoenvironment, leading to high fidelity coherent control. In these chapters we lay out the proof of principle work of detecting the quantumdynamics of a complex system in interaction with its environment at room temperature. In chapter 5 we discuss application of these techniques to the investigation of long lived coherence in photosynthetic systems. We show that electronic coherence between different rings of the LH2 system persists to time scales of 100s of femtoseconds at room temperature. Moreover we show that the energy transfer pathways in LH2 adapt to environmentally induced changes in the molecule and that the nature of the transfer remains coherent for each pathway, providing strong evidence that coherent energy transfer is the optimum process for energy transfer in photosynthesis. Finally, in chapter 6 we take the technical development one step further and report on the creation of a framework based on plasmonic antennas that allows for control of the amplitude-phase characteristics in nanometric sized hotspot fields. We show for the first time that the ultrafast characteristics of plasmonic hotspots can directly be engineered through design of the plasmonic system and experimentally demonstrate two much-anticipated examples: a sub-diffraction resolution phase shaper and an ultrafast plasmonic switch for pump probe experiments. The results presented in this thesis form the first creation and observation of ultrafast coherent dynamics in individual molecular systems at room temperature. This is a necessary step to be able to do true quantum tomography in complex systems, resolve the influence of the environment on molecular dynamics, and investigate the physics that determines evolutionary optimization and functionality in biomolecules.
Pulse-based Electron–Nuclear and ELectron–electron DOuble Resonance (ENDOR/ELDOR) techniques have been applied to molecular spins in order to implement ensemble electron spin-qubit based quantum computers/computing (QC) and quantum information processing (QIP) in the solid state. Pulsed ENDOR-based QC/QIP experiments for super dense coding (SDC) have for the first time been carried out by the use of molecular electron- and nuclear-spin entities such as the stable malonyl radical as matter spin-qubits. The spin-qubit manipulation technology for quantum gate operations in this work is based on the time-proportional-phase-increment (TPPI) technique, enabling us to distinguish between the phases of spin-qubit based entangled states. The TPPI technique, as firstly applied by Mehring et al. (M. Mehring, J. Mende and W. Scherer, Phys. Rev. Lett., 2003, 90, 153001), has illustrated the establishment of quantum entanglement between electron- and nuclear-spin states and mutual interconversion between the electron–nuclear Bell states. The electron-spin 4π-periodicity in phase shows up in the QC/QIP experiments, explicitly and experimentally illustrating the electron-spin spinor nature for the first time. Tripartite QC experiments have been made, showing the occurrence of separable states. Also, the development of novel electron-spin technology to manipulate multi-electron spin-qubits is described. In this work, the pulsed coherent-dual ELDOR for QC/QIP has for the first time been implemented by invoking a novel microwave dual phase-rotation technique. Thus, applications of the coherent-dual ELDOR to molecular electron spin-qubit systems are also discussed, emphasising designing the molecular two electron-qubit systems appropriate for QC/QIP. g- and/or hyperfine A-tensor engineering approaches give us the two- and multi-electron-qubit systems, which have been a materials challenge to implement matter spin-qubit based QC/QIP. The targeted matter spin-qubits can be used to facilitate selective resonant microwave excitations achieved by the pulsed ELDOR technique. In addition to DiVincenzo's five criteria, general requisites for scalable electron spin-qubit systems as 1D periodic robust spin structures are described. According to the requisites, double- or triple-stranded helicates embedding open-shell metal cations are proposed instead of organic molecular spin-qubits.