Advancing Chemistry and Quantum Information Science An Assessment of Research Opportunities at the Interface of Chemistry and Quantum Information Science in the United States (2023) / Chapter Skim
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2 Design and Synthesis of Molecular Qubit Systems
2 Design and Synthesis of Molecular Qubit Systems
Pages 25-62
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• Organic multispin molecules offer long coherence times and several demonstrated strategies for using photons to prepare well-defined spin quantum states. • Optical cycling of molecules in the gas phase has seen much progress in identifying and predicting molecules with minimal vibrational branching, which could lead to new and improved molecular qubits.
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2.2 DEVELOPING AN UNDERSTANDING OF MOLECULAR STRUCTURE–PROPERTY RELATIONSHIPS NEEDED FOR QIS APPLICATIONS 2.2.1 Increasing Coherence Times in Molecular Qubits and Quantum Memories Developing molecular systems for QIS applications depends critically on establishing and maintaining quantum-state coherence through the fundamental properties of superposition and entanglement. As described in Box 2-1, an electron spin in a superposition state is very sensitive to its environment (Wasielewski et al.
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For example, if we place an electron spin in a magnetic field, the two relevant quantum states are ‘spin up' or ‘spin down,' based on the orientation of the magnetic moment of the electron parallel or anti-parallel to the field, respectively. An analogous two-state quantum property of photons is polarization.
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nuclear spin diffusion, (b) coupling to nearby electronic spins, (c)
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Future work on phonon engineering, a key research area, may enable room-temperature operation of molecular QIS systems, while increased coherence times at lower temperatures may be valuable for quantum interconnects essential to communications applications. 2.2.2 Creating Optically Addressable Molecular Qubits The history of harnessing inorganic chemistry to create quantum systems is extensive and has its origins in the field of molecular magnetism.
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iii) The system must provide a set of universal quantum logic gates that operate on one or two en tangled molecular qubits.
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The excited-state design needs to consider the dynamics of both the excited and ground spin states to enable photodriven spin polarization and ensure that the singlet state is higher in energy than the triplet state. The triplet state necessarily arises from the promotion of an electron from the lower energy level to the higher one, meaning it will be extremely sensitive to the ligand field strength.
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2.2.2b Creating Optically Addressable Molecular Qubits: Lanthanides and Actinides The study of quantum phenomena and the development of quantum information processing systems have benefited from understanding the chemistry and physics of f-electron compounds and materials. f-electron compounds include those formed from lanthanide or actinide elements.
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Complexes of actinide elements, where the 5f- and/or 6d-valence electron shells are partially filled, also present unique opportunities related to the manipulation of electron spins for QIS applications. One possible advantage of actinides is that the larger radial extent of the 5f principal quantum shell and indirect relativistic effects that are operative for these heavy atoms render the 5f electrons more accessible for covalent bonding interactions and modifications through structural chemistry, as compared to the 4f electrons.
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Within the United States, much of the work on transuranic isotopes is performed in the national laboratory system, largely due to both safety and security concerns; a smaller fraction is performed in certain academic laboratories where the necessary licensing and infrastructure are in place. 2.2.2c Creating Optically Addressable Molecular Qubits: Organic Multispin Qubits Molecular qubit design principles based on fully organic systems utilize the premise that decoherence in metal-based molecular spin-qubits has significant contributions from spin–orbit coupling and ZFS.
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. A second promising approach to photoinitialized molecular qubits having optical pumping and addressability properties similar to those of NV centers uses photoexcited, covalently linked chromophore-stable radical (C-R•)
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2.2.2d Creating Optically Addressable Molecular Qubits: Optical Cycling Centers Optically active molecules could provide high-fidelity quantum-state initialization through optical pumping and high-fidelity qubit readout through the detection of laser-induced fluorescence. Both purposes require the ability to scatter optical photons repeatedly (i.e., optically cycle)
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trapped in programmable optical tweezer arrays as a new molecular qubit platform. The electric dipolar interaction between OCCs allows two-qubit gates to be implemented.
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, a promising route for realizing multiple-spin entangled molecular qubits in organic systems is the photogeneration of high spin states in organic semiconductors using singlet fission (SF)
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state for QIS applications. 2.2.3 Beyond Qubits: Multispin Systems, Error-Mitigation and Error-Correction While challenges in the field of molecular QIS have focused primarily on the structural factors that control decoherence rates and relaxation dynamics of qubit states, significant challenges remain in the design and control
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. Effective quantum error correction need not assume that encoding and decoding of quantum states must be carried out without error, as would be the case for noisy quantum gates.
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Strategies to shorten the manipulation times for gate operations involve taking advantage of electron–nuclear coupling (hyperfine interactions) to perform operations on nuclear spin states at rates much shorter than the decoherence times (Castro et al.
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(I = 5/2) can encode a d = 6 qudit that can be exploited for coherent control of the nuclear-spin degrees of freedom by nuclear magnetic resonance and via hyperfine coupling to electron spin (S = ½)
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2.3 INVESTIGATING THE INTERACTIONS OF MOLECULAR QUBITS WITH THEIR ENVIRONMENTS Synthetic chemistry can play a key role in the design of molecules with so-called clock transitions (GaitaAriño et al.
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Transitions that connect extrema of the curves (red lines) are the clock transitions.
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With transition linewidths of ~10 MHz at low temperatures, organic chromophores can function as single-photon sources with long coherence times that are scalable and compatible with diverse integrated platforms. In addition, such chromophores can be used as transducers for the optical readout of electrical and/or magnetic fields and material properties for quantum sensing with single-quantum resolution (Dickerson, Guo, Zhu, et al.
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. As a result, only one of the four two-spin states is populated in the weak-coupling limit, with vanishing entanglement between the hole and electron spin (Figure 2-12a)
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In an orthogonal area, tethering molecules to inert substrates, such as hexagonal boron nitride, would enable the measurement of properties at a spatial scale too small to resolve using other means and could open up new directions within condensed matter physics. As described previously, the initial step in understanding and creating more complex molecular systems for QIS applications is progressing beyond individual molecular qubits to two, three, and ultimately many qubits.
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found that the electron spins have a negligible effect on coherence times, a finding attributed to the distinct resonance frequencies. Coherence times are governed, instead, by the distance to nuclear spins on the other qubit's ligand framework (Figure 2-14)
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. Clock transitions were described earlier in this chapter and are particularly powerful for mitigating the effects of magnetically noisy environments such as those found in MOFs.
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Extensions of this strategy may be able to transfer a spin state coherently across the 10–1,000 nm distances that are important for quantum interconnects. 2.7 FABRICATING SCALABLE MOLECULAR QUANTUM ARCHITECTURES BASED ON MOLECULAR QUBITS The exquisite ability to manipulate inorganic materials at the nanoscale can potentially address many of the bottlenecks for all-molecular systems.
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The synthesis, fabrication, characterization, and understanding of nanomaterials are mature with applications ranging from microelectronics and energy storage to renewable energy -- these same approaches can be used to accelerate discovery and control in molecular qubits systems. Chemical inclusion or covalent design of qubits/qudits into carbon-based materials offer promising strategies for development of multiqubit arrays.
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creating optically addressable molecular qubits (e.g., transition metal complexes, lanthanides, organic-based multispin qubits, and optical cycling centers)
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2022. "Enhancing Spin Coherence in Optically Addressable Molecular Qubits through Host-Matrix Control." Physical Review X 12(3)
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2013. "Electron Spin Polarization Transfer from Photogenerated Spin-Correlated Radical Pairs to a Stable Radical Observer Spin." Journal of Physical Chemistry A 117(25)
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2018. "Progress Towards Creating Optically Addressable Molecular Qubits." Chemical Communications 54(98)
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2021. "Surface Chemical Trapping of Optical Cycling Centers." Physical Chemistry Chemical Physics 23(1)
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2003. "Light-Induced Electron Spin Polarization in Rigidly Linked, Strongly Coupled Triplet-Doublet Spin Pairs." Chemical Physics Letters 379(5,6)
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2020. "Electron Spin Polarization Generated by Transport of Singlet and Quintet Multi excitons to Spin-Correlated Triplet Pairs During Singlet Fissions." Chemical Science 11(11)
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2007. "Environmental Effects on Electron Spin Relaxation in N@C60." Physical Review B 76(8)
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1994. "Electron Spin Echo Envelope Modulation Due to Exchange and Dipolar Interactions in a Spin-Correlated Radical Pair." Chemical Physics Letters 219:283–290.
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2012. "Chemical Engineering of Molecular Qubits." Physical Review Letters 108(10)
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2022. "Multivalent Optical Cycling Centers: Towards Control of Polyatomics with Multi-Electron Degrees of Freedom." Physical Chemistry Chemical Physics.
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