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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3 Measurement and Control of Molecular Quantum Systems
3 Measurement and Control of Molecular Quantum Systems
Pages 63-136
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From page 63... ...
has demonstrated several DiVincenzo cri teria required for quantum computing, including high-fidelity quantum-state readout, detection, and on-demand entanglement of molecular qubits via a two-qubit gate sufficient for universal gate-based quantum computing. With these demonstrations, the time is ripe for investigations of OCCs as new QIS platforms.
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From page 64... ...
A relatively new approach of using optical cycling in small molecules has also gained great attention for addressing and manipulating electron spin states. For optical measurements of coherence and QIS applications, the focus has been directed toward ultrafast, time-resolved, nonlinear optical spectroscopy and spectroscopy with quantum light (e.g., entangled photons)
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Conventional NMR spectroscopy continues to find utility at the interface between chemistry and QIS as research has shifted toward the study of simpler monometallic systems and their potential use as spin qubits (Gimeno et al.
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The VIV example shows how the 1H dynamics and, hence, the nuclear spin patterning can ultimately influence the electronic coherence, suggesting important design principles for molecular spin qubits (see Figure 3-2)
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first asked whether coherence times in molecular magnets would be long enough to permit quantum information processing, reporting a phase memory time of several microseconds for a deuterated spin S = ½ {Cr7Ni} wheel complex. This marked the dawn of an era that extends to the present day; synthetic inorganic chemists have systematically explored the factors influencing relaxation times in molecular magnets (Gaita-Ariño et al.
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Timeline for some of the most relevant spin qubits made from transition metal ions in terms of quantum coherence time (T2)
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. As a result, electron spin-spin fluctuations are almost completely suppressed, and one can observe coherent electron spin dynamics in a highly concentrated crystalline sample, as has been demonstrated for several polynuclear Fe clusters (Takahashi et al.
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. As depicted in Figure 3-7, DEER has recently been deployed to measure electron–electron coupling within dimers of {Cr7Ni} spin qubits that have been proposed as molecular two-qubit gates (Ardavan et al.
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This technique is also useful in the study of molecular spin qubits (Ariciu et al. 2019; Atzori et al.
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inelastic neutron scattering (INS) spectroscopy, providing unprecedented insight into coherent spin dynamics (Figure 3-9)
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3.2 ENHANCE OPTICAL CONTROL IN MOLECULAR SYSTEMS IN QIS The diversity of molecular species, coupled with the precision available with from molecular synthesis techniques, offer the prospect of building molecular qubits that are uniquely tailored for applications ranging from quantum networks to quantum-enhanced sensing (Wasielewski et al.
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This can be harnessed to create quantum networks of distant molecular qubits that are entangled, which can be a resource for quantum communication (Togan et al.
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molecular qubits with atom-like structures or by finding molecules with particularly favorable cycling properties. Optical cycling, optical initialization of molecular bits, and optical quantum-state (spin)
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. Notably, with relevance to quantum information processing and quantum simulation, recent work with lasercooled diatomic molecules has demonstrated high-fidelity detection of arrays of single molecules held in optical tweezer traps (Anderegg et al.
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2021. 3.3 DEVELOP TECHNIQUES TO PROBE MOLECULAR QUBITS AT COMPLEX INTERFACES TO INFORM THEIR SYSTEMATIC CONTROL 3.3.1 State of the Art in Single-Molecule and Surface NMR The need to obtain microscopic information concerning nuclear spin dynamics and structural details associated with molecules deposited onto surfaces has driven the development of several novel nuclear resonance methods.
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The location in the magnetic field of this jump, therefore, provides a convenient readout of the nuclear state of the molecule. One can then perform logic operations on the nuclear spin via the application of selective microwave NMR pulses (see Figure 3-13)
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. 3.3.2 Single-Molecule EPR Signatures of individual molecular spins within an ensemble were first reported via optically detected EPR in 1993 (Köhler et al.
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2021) , emphasizing the tremendous potential of this relatively new method for coherent control of individual molecular qubits.
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Recently, rare-earth ions in oxide semiconductor hosts -- including silicon-compatible materials -- have emerged with encouraging properties as single quantum memories, with impressively long coherence times and single-shot photonic readout. 3.4 DEVELOP ENHANCED SPECTROSCOPIC AND MICROSCOPIC TECHNIQUES WITH NONCLASSICAL LIGHT 3.4.1 Quantum Light Spectroscopy and Entangled Photon Sources As chemists push forward in their quest for deeper knowledge of the important processes and mechanisms of molecular interactions, great pressure is placed upon the nature of the measurements used to probe such processes.
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The use of nonclassical sources of light such as squeezed states as well as entangled photons may provide an opportunity to exceed these quantum-imposed limits. Great enthusiasm exists for exploring quantum information in molecular systems.
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It is notable that the photons must still be interfered with or measured by coincidence counting circuits to determine these properties. Such techniques are promising because extremes such as measuring an X-ray photon using a visible light photon are possible, all while gaining the quantum advantages of the entangled photons.
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From page 84... ...
A unique aspect of chemistry that is different from quantum information is the ability to create entangled photons over a broad range of central wavelengths. New proposals to use lithium tantalate have been published, offering the ability to pump SPDC down to ~350 nm where lithium niobate begins to suffer from self-absorption issues (Figure 3-17)
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For example, in some cases the use of entangled photons may suppress background signals and enhance others. In one proposed experiment, time–energy entangled photons produced by SPDC are employed to calculate vibrational hyper-Raman (HR)
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(f) Resonantly enhanced sumfrequency generation with entangled photons.
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Earlier reports described coherent dynamics and dephasing processes in these systems. More recently, the HOM experiment was utilized to extract a dephasing time in an organic of ~102 fs upon coherent excitation and quantum interference with a path of entangled photons in the HOM interferometer.
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In this quantum interference effect, the photons that pass through the imaged object are never detected. The entangled photons that never interacted with the object provide the image.
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Another approach toward the use of entangled photons in molecular and biological microscopy involves TPA (Eshun et al. 2022; Mukamel et al.
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stimulated Raman signal, and (c) signal-to-noise ratio (SNR)
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3.4.4 Measurement of Coherence in Molecular Systems The measurement of coherence in chemical systems has a long history. The development of better methods to resolve more information about the molecular system is a significant part of modern physical chemistry.
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92 ADVANCING CHEMISTRY AND QUANTUM INFORMATION SCIENCE an ambient-temperature solution. Here, the broadband femtosecond pulses (red curve)
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Such gains warrant future research into coherence as a potential force for enhancing function. 3.4.6 High-Finesse Cavities and Nanophotonics for Molecular Qubit Systems In addition to understanding and designing molecular systems with favorable optical properties, a different and complementary approach is to engineer optical control by modifying the molecule–light coupling, by changing the vacuum field of the light itself.
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. Shown in red are the two cysteine residues, C49 and C353, that are known to steer and quench excitations in oxidizing conditions and tune vibronic coupling for enhanced energy transfer in reducing conditions.
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For atomic systems, this has been proposed as a way to produce entanglement and teleportation of quantum information over long distances. The same principles apply to optically active molecular systems.
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While considerable basic science interest certainly exists, there are also many possibilities for applications in QIS using molecular polaritons as quantum resources, particularly in the case of quantum transduction. Vibrational polaritons are especially promising in this context because of the relatively long coherence times achievable using molecular vibrations coupled to resonant optical cavities.
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By comparing the TPA spectra with classical and quantum light, Gu and Mukamel (2020) have demonstrated that entangled photons can create two-photon excitations in polaritonic systems that are drastically different from the classical two-photon excitations.
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of time-resolved/transient EPR spectroscopy will be essential to elucidating the CISS-induced spin dynamics.
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to study spin–electric coupling in molecular qubit candidates. The method is analogous to the pulsed ELDOR and DEER techniques, except that a pump electric field pulse is employed in order to modulate the coherent spin dynamics via the magnetoelectric effect.
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2] 9− molecules and the experimental configuration in which the sample is subjected to a static applied magnetic field, B0, and pulsed electric fields, E (in addition to microwave electron paramagnetic resonance pulses)
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The remarkable precision, placement, and tunability of molecular qubits and potential memories make molecular quantum systems powerful candidates to accelerate research in science and engineering (Baek et al.
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3.7 DEVELOP MOLECULAR QUANTUM TRANSDUCTION SCHEMES THAT TAKE ADVANTAGE OF ENTANGLED PHOTONS AND ENTANGLED ELECTRON AND NUCLEAR SPINS Quantum transduction refers to the transfer of quantum states between different forms of quantum subsystems to connect qubits, support quantum networks, or allow for quantum sensing. Current quantum transduction schemes range from electron spins, nuclear spins, bosons, acoustic modes, and microwaves to optical photons.
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Molecular qubits can be connected through linkages that are only a few nanometers long, potentially leading to higher-density qubit arrays than those based on isolated atoms or defects. These scaffolds can facilitate the control of the entangling interactions between qubits, such as the spin exchange coupling between two electron spin qubits (Olshansky et al.
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To achieve spatially dense measurements with high time resolution using quantum sensors, one will need to integrate the measurements with complementary classical sensing modalities that can provide nanoscale chemical information across an entire cell. These methods will exploit the exquisite sensitivity of enzymes to the local biochemical environment.
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electric fields, and (c) space–time perturbations.
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2019. 3.9 USE BIO-INSPIRED QUANTUM PROCESSES TO DEVELOP NEW QUANTUM TECHNOLOGIES Strategies to develop platforms for quantum sensing can be found within the realm of biology.
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advanced spectroscopy of entangled and classical interactions for molecular qubits. This section discusses these instrumentation classes as emerging tools and needs.
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A need exists to adapt current atomic resolution methods to be suitable for molecular qubits. Atomic resolution instruments fall into two main categories: scanning probe microscopy and electron imaging techniques.
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These types of measurements would give clues as to how to delineate and achieve multiple qubit interactions in molecular quantum sensors and other QIS systems. 3.10.1b Control and Detection of Ensembles of Molecular Qubits The application of molecular qubits in quantum scenarios requires the delineation of interactions in an ensemble.
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. While a tedious technique, copious research has been put into this topic, and artificially creating arrays of molecular qubits could provide a fruitful equivalent to an atomic ion trap.
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However, instrumentation development on the ensemble level presents an opportunity to help understand how molecular qubits operate and transduce quantum information. Although microwaves have been used for the control and readout of most molecular qubits to date, how a molecule decoheres and its quantum coupling are often explored using ultrafast lasers.
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However, these techniques are not yet optimized for measurements of molecular qubits. Ultracold atoms and optical lattice research is currently focused in the physics community, and extension to molecular systems beyond a few atoms will enhance this area of research greatly.
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Indeed, one can already find examples of extremely low-cost components relevant to transient EPR spectroscopy. Another area of rapid progress is the development of advanced on-chip EPR spectrometers that employ tiny permanent magnets (Hassan et al.
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The facility is also staffed with internationally renowned EPR support scientists. Therefore, it serves as a model facility for carrying out research on molecular spin qubits of the kinds discussed in Chapter 2, with more than two-thirds of its users drawn from the chemistry community.
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From page 115... ...
Examples include optical excitation and detection, and integrated scanning probe techniques. Aside from major user facilities, a strong case can be made for establishing regional user hubs that support mid-scale instrumentation, which the chemistry QIS community relies on for advancing its research -- for example, instrumentation that would be out of reach for all but the most research-intensive universities.
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From page 116... ...
Therefore, a steady funding stream is needed to develop and sustain support for chemistry QIS instrumentation at national facilities, as has been the case for other large-scale entities such as synchrotrons. 3.11.3a Magnet Laboratories As noted in earlier sections of this report, strong spin–orbit coupling and ligand field effects can give rise to situations where metal-based spin qubits are EPR-silent at the conventional low frequencies (<50 GHz)
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Near-field X-ray microscopy using zone plates has also progressed significantly, and its element-specific images could be used for investigating coupled or individual molecular qubits (Shapiro et al.
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It goes without saying that continued efforts must be pursued to reduce reliance on such cryogenic approaches, by furnishing researchers with closed-cycle refrigeration systems. 3.12 SUMMARY OF RESEARCH PRIORITIES AND RECOMMENDATION The following fundamental research priorities have been identified by the committee and extensively discussed in Chapter 3 as those that the Department of Energy and NSF should prioritize within the target research area of "measurement and control of molecular quantum systems." Research Priorities: Develop new approaches and techniques for addressing and controlling multiple electron and • nuclear spins and optical cycling centers in molecular systems.
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• Develop molecular quantum transduction schemes that take advantage of entangled photons as well as entangled electrons and nuclear spins. • Advance quantum sensing techniques to further understand biological systems.
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2017. "Molecular Qubits Based on Potentially Nuclear-Spin-Free Nickel Ions." Physical Chemistry Chemical Physics 19(3)
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2020. "Decoherence in Molecular Electron Spin Qubits: Insights from Quantum Many-Body Simulations." Physical Review Letters 11(6)
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2021. "Franck Condon Tuning of Optical Cycling Centers by Organic Functionalization." Physical Review Letters 126(12)
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2016. "A Modular Design of Molecular Qubits to Implement Universal Quantum Gates." Nature Communications 7(1)
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2017. "Portraying Entanglement Between Molecular Qubits with Four Dimensional Inelastic Neutron Scattering." Nature Communications 8(1)
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2021. "Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science." Journal of the American Chemical Society 143(38)
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2020. "Toward Ultracold Organic Chemistry: Prospects of Laser Cooling Large Organic Molecules." Physical Review Letters 11(16)
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2021. "Initializing 214 Pure 14-Qubit Entangled Nuclear Spin States in a Hyperpolarized Molecular Solid." Journal of Physical Chemistry Letters 12(14)
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2019. "Electric Field Control of Spins in Molecular Magnets." Physical Review Letters 122(3)
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2022. "Pathway Toward Optical Cycling and Laser Cooling of Functionalized Arenes." Physical Review Letters 13(30)
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2004. "Nuclear Spin Dynamics in the Quantum Regime of a Single-Molecule Magnet." Physical Review Letters 93(19)
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2009. "Nonlinear Spectroscopy with Entangled Photons: Manipulating Quantum Pathways of Matter." Physical Review A 79(3)
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2009. "Molecular Electron-Spin Quantum Computers and Quantum Information Processing: Pulse-Based Electron Magnetic Resonance Spin Technology Applied to Matter Spin-Qubits." Journal of Materials Chemistry 19(22)
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2009. "Radiative Force from Optical Cycling on a Diatomic Molecule." Physical Review Letters 103(22)
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2022. "Quantum Light-Enhanced Two-Photon Imaging of Breast Cancer Cells." Physical Review Letters 13(12)
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2020. "Exploiting Chemistry and Molecular Systems for Quantum Information Science." Nature Reviews Chemistry 4(9)
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2020. "Interferometric Two-Photon-Absorption Spectroscopy with Three Entangled Photons." Applied Physics Letters 116(17)
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