Multi-spin Systems as Spin Qubits
Taking advantage of their structural reproducibility and modularity with precision at the atomic level, we design molecular systems in which electron spins involved in photochemical processes following light excitation fulfill crucial requirements for functional qubits. We have shown that the pure initial quantum state in photogenerated spin-correlated radical pairs can be detected using electron paramagnetic resonance (EPR) spectroscopy. Our bottom-up synthetic tailoring yields systems with lifetime and spin state addressability appropriate to quantum gate operations using pulse-EPR techniques. More specifically, quantum teleportation, CNOT gate operation, and polarization transfer to a third spin have been demonstrated on radical-pair-based systems.
In the meantime, we construct molecular analogs of diamond defect nitrogen-vacancy (NV) centers, aiming to achieve optical readout, common in today’s field of quantum information processing. Upon photoexcitation and subsequent enhanced intersystem crossing due to the presence of a radical, the chromophore-radical systems we work with provide highly spin-polarized ground doublet and excited quartet spin states that are detectable in both traditional EPR spectroscopy and optically detected magnetic resonance (ODMR) spectroscopy. We seek to further explore the unique molecular degrees of freedom absent from the defect centers and implement quantum gate protocols in these systems.
Representative Publications:
Effect of the Time Delay between Spin State Preparation and Measurement on Electron Spin Teleportation in a Covalent Donor− Acceptor−Radical System. J. Phys. Chem. Lett. 2022, 13 (1), 156−160.
Mechanistic Study of Electron Spin Polarization Transfer in Covalent Donor-Acceptor-Radical Systems. Appl Magn Reson 2021.
Interaction of Photogenerated Spin Qubit Pairs with a Third Electron Spin in DNA Hairpins. J. Am. Chem. Soc. 2021, 143(12), 4625–4632.
Controlling the Dynamics of Three Electron Spin Qubits in a Donor–Acceptor–Radical Molecule Using Dielectric Environment Changes. J. Phys. Chem. Lett. 2021, 12(9), 2213–2218.
Photodriven quantum teleportation of an electron spin state in a covalent donor-acceptor-radical system. Nat. Chem. 2019, 11, 981-986.
Spin Qubit Pairs as Quantum Sensors
Small molecules consisting of covalently attached electron donors, chromophores, and electron acceptors can be photoinitiated to form spin qubit pairs (SQPs). SQPs of this form offer the advantages of synthetic scalability and tunability of quantum properties, such that molecular architectures with coherence times long enough to manipulate spins using electron paramagnetic resonance (EPR) techniques can be synthesized. The fact that SQPs are also ion pairs suggests that these systems are suitable candidates for local electric field sensors since the interaction them and an electric field should manifest itself in spin dynamics.
One powerful tool that enables the differentiation of subtle changes in electric fields (for example, on a monolayer surface) is out-of-phase electron spin echo envelope modulation (OOP-ESEEM). This phenomenon originates from the coherence properties of the entangled spins and is responsive to local electric field changes, which are manifested as modulations of spin-spin interactions between the ion pairs. We aim to adapt these radical pair systems for the purpose of quantum sensing.
An alternative strategy to probe small electric field changes relies on monitoring the nuclear quadruple moment of molecular qubits. The coupling of internal molecular dipoles of paramagnetic metal porphyrin complexes to external fields has been shown to result in substantial changes in nuclear quadruple splitting. Incorporation of building blocks with high polarizability is expected to contribute to high sensitivity to electric field gradient at the metal center nucleus, which manifests itself in changes in hyperfine and quadruple splittings detected through pulse electron-nuclear double resonance (pulse-ENDOR) spectroscopy.
Chirality-Induced Spin Selectivity
In a phenomenon called chirality-induced spin selectivity (CISS), the transmission of an electron through a chiral environment couples the electron’s linear and spin angular momentum, resulting in spin polarization. This effect has been demonstrated in self-assembled monolayers of double-stranded DNA, which act as spin filters of conduction electrons with the extent of spin polarization depending on the length of the sequence.
Recent theoretical studies that modeled coherent electron transfer between a donor and acceptor molecule linked by a chiral bridge suggest that electron paramagnetic resonance (EPR) techniques can detect this striking effect. While the CISS effect may afford new opportunities to control the properties of spin qubit pairs (SQPs), experimental studies illustrating the effect of CISS on SQP spin polarization and coherence have never been performed. We aim to use EPR techniques to characterize the role of the CISS effect in the dynamics of SQPs formed by chromophore-linked DNA hairpins and small molecules.
Representative Publications:
Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science. J Am Chem Soc 2021, 143 (38), 15508-15529.
Tracking Photoinduced Charge Separation in DNA: from Start to Finish. Acc Chem Res 2018, 51(8), 1746-1754.
Charge Separation and Recombination Pathways in Diblock DNA Hairpins. J Phys Chem B 2019, 123(7), 1545-1553.
Quantum Transduction between Electron and Nuclear Spins
Quantum transduction is a coherent exchange of quantum properties and quantum information between systems. Studies of quantum transduction are essential to advancing the field of quantum information science. Transduction between electronic and nuclear coherence is useful since the nuclear spins are highly confined and substantially less sensitive to their environment, enabling coherence lifetimes orders of magnitudes longer than those of electronic spins.
To better understand electron-nuclear spin coherence transduction, we utilize the well-established photogenerated electron spin qubit pairs as a model system, where a stable radical with a 13C label is further incorporated. By carrying out two-qubit gates using pulse-EPR and pulse-ENDOR spectroscopies, we aim for transduction of information between the initial optically initialized spin pair and the stable radical electron spin, and that between the nuclear and electronic spins on the stable radical. Another direction in the investigation of electron-nuclear transduction focuses on the transduction of nuclear spin information in the quantum teleportation system we previously established. In particular, in the covalent donor-acceptor-radical system where a series of photo-initiated, thermodynamically driven, entanglement swapping, electron transfer processes transfer the electron spin information from the radical to the donor, we include a 13C nuclear spin in the stable radical moiety. The ultrafast quantum teleportation is expected to result in the readout of the nuclear spin information as a component of the teleported electron spin information, in other words, transduction between a nuclear spin and a physically remote electron spin.
Representative Publications:
Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science. J Am Chem Soc 2021, 143 (38), 15508-15529.
Exploiting Chemistry and Molecular Systems for Quantum Information Science. Nat Rev Chem 2020,4 (9), 490–504.
Quantum Transduction between Photons and Electron Spins
Quantum transduction is a coherent exchange of quantum properties and quantum information between systems. Studies of quantum transduction are essential to advancing the field of quantum information science. Achieving photon-to-electron-spin quantum transduction will allow to couple multiple physically separated qubit systems performing quantum logic gates operations. As the first step, we utilize our synthetic organic chemistry expertise to design and produce covalent donor-acceptor organic molecules capable of producing spin-correlated radical pairs that act as two-qubit systems. High tunability of the organic chromophores allows to achieve desired photophysical parameters optimal for studying the interaction of these molecules with quantum light.
Quantum transduction experiments require a source of entangled photons. We are constructing a bright source of degenerate entangled photon pairs at 680 nm through the process of spontaneous parametric down conversion (SPDC). These photons are entangled with respect to their frequency, momentum, and polarization. Working in the single photon pair limit, it is possible to use the quantum properties of light as an additional control knob in QIS experiments. Our entangled photon setup will be employed in light-matter quantum transduction experiments, where the tunability of the entangled state provides a high degree of control over the light degrees of freedom in the interaction. Additionally, the nonclassical bandwidth characteristics allow simultaneous resolution in frequency and time beyond the classical limit in spectroscopic measurements.
Representative Publications:
Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science. J Am Chem Soc 2021, 143 (38), 15508-15529.
Exploiting Chemistry and Molecular Systems for Quantum Information Science. Nat Rev Chem 2020,4 (9), 490–504.
