Session EE02: V: Spin Qubits, Quantum Sensing, and Quantum Networking

Scalable donor-based electron spin qubit unit in silicon

Donor-based spin qubit has gained attention, for its long coherence time and great scalability on processing. With the development of scanning tunneling microscope (STM) lithography and micro-nano fabrication, large-scale donor-based spin qubit devices are promising. However, manufacturing is not the only challenge. There are two critical issues including the tunability of the two-qubit coupling and the addressability of computing qubits. In this talk, we propose a scalable unit of donor-based electron spin qubits that incorporates an ancilla donor. In particular, we introduce an asymmetric structure that exhibits great compatibility between the tunability of two-qubit coupling and the addressability, with the aid of so-called 'superexchange' and hyperfine interaction. And the fidelity of single-qubit and two-qubit gates can exceed the fault-tolerant threshold. Moreover, the asymmetric scheme can resist the valley oscillation of the tunneling coupling, with a nanoscale placement accuracy of donors. Consequently, the proposed scheme is a promising prototype for the large-scale fault-tolerant spin-based quantum processor.   

Nanoscale Electrical Tuning of Charged Excitons in Two-Dimensional Materials with 1-nm Gate

Tunable localization of charge carriers becomes a critical tool in creating and confining quantum dots within the complex landscape of low-dimensional materials. However, current methods for electrical tunability fall short in scalability and density, posing challenges for high-precision, solid-state semiconducting quantum platforms. Here, we engineer an architecture that achieves electrical tuning in monolayers of transition metal dichalcogenides (TMDCs) at the nanoscale. This platform requires minimal lithography and was made possible by the progress in the synthesis, transfer, and alignment of carbon nanotubes (CNTs) which enabled advanced electronics including 1-nm gates and high-density molybdenum disulfide (MoS₂) transistors. Our measurements confirmed the electrostatic doping effect, leading to exciton-trion conversion processes at 4k. We observed the photoluminescence (PL) spectral behavior of neutral excitons (X^o), charged trions (X^-), and charged biexcitons (XX^-) under various applied biases. Multiphysics simulations showed a doping radius around 15 nm with a pathway to reduce the size down to the Bohr radius limit. This trion state serves as a crucial interface between spin and photons, paving the way for quantum networks and long-range quantum communication. Realizing scalable arrays of indistinguishable and electrically tunable quantum emitters will open doors to diverse quantum photonic technologies, from quantum networks to sensing and simulation.    

Defining Mach's Principle in a Topological World

A very general statement of Mach's principle: Local physical laws are determined by the large-scale structure of the universe. It implies the existence of a background, causal (geometric) and/or acausal (topological). Discovery of the quantum Hall impedance opened a window for topology in experimental physics. Impedance matching governs amplitude and phase of energy flow, of information transmission. Topological impedances are of rotations, of Newton's bucket. They are acausal, communicate only relative phase, not a single measurement observable. Resultant motion is perpendicular to applied force, the gyroscope. Absence of an independent observer distinguishes quantum from classical. The simplest possible example is that of the background independent two-body problem, where rotation relative to large-scale structure of the universe is not observable. This suggests Mach's principle is applicable only to the scale-invariant topological impedances. We seek to define Mach's principle in light of this limitation.   

Spin-phonon entanglement in SiC optomechanical quantum oscillators

Scaling up quantum systems, especially solid-state spins, presents a significant challenge in the field of quantum information science. While spin-based quantum processors and networks show promise for remote entanglement and long-coherence quantum memory, the development of scalable schemes for large-scale spin entanglement remains elusive. In this study, we propose a hybrid spin-phonon architecture based on spin-embedded optomechanical crystal (OMC) cavities. This architecture combines integrated photonic and phononic accesses, allowing for the simultaneous entanglement of multiple spins. Remarkably, we proposed the hybrid spin-optomechanical system in a Raman-facilitated scheme which offers a coupling of spins to the vibration mode of simulated Silicon Carbide OMC cavities approaching MHz, enabling a fast and efficient spin-phonon entanglement with fidelity of 98%. By incorporating the Stimulated Raman Adiabatic Passage (STIRAP) protocol into the coupled tripod-phonon system, a two-qubit Controlled-Z gate with 97% fidelity is implemented by engineering the non-vanishing geometry phase in a strongly coupled spin-phonon state basis, which is optically dark and robust against the dominated loss from the spin excited-state decoherence, spectral diffusion and the additional two- level system decoherence of phononic cavity. Our work establishes a crucial platform for exploring the spin entanglement with potential scalability in addition to the optical link, which opens the path to investigate cavity quantum-acousto dynamics in the hybrid solid-state system.   

Magnetic relaxometry study of cytochrome C using nitrogen vacancy centers in diamond

Cytochrome C (Cyt-C) is a water-soluble protein with a single heme group, pivotal in the mitochondrial electron transport chain, where the heme group remains in the Fe⁺³ paramagnetic state, generating fluctuating magnetic fields [1]. Detecting these stochastic fields is challenging, yet the nitrogen-vacancy (NV) center offers a unique opportunity to measure these weakly random magnetic fields through spin-lattice (T₁) relaxometry [2]. In this study, we perform NV-T₁ relaxometry measurements on a diamond chip doped with a 8-nm thick NV-layer without the presence of Cyt-C, yielding a relaxation time of approximately 1.2 ms. Subsequently, by varying the concentration of Cyt-C from 2.6 µM to 54 µM on the diamond surface, we observe a reduction in the T₁ time to 850 µs and 150 µs, respectively. This decrease is attributed to spin-noise originating from Fe⁺³ spins within the Cyt-C proteins [3]. Additionally, we conduct imaging of Cyt-C nanoclustered proteins on a microstructured diamond chip equipped with gratings, enabling us to detect the presence of 1.44 × 10⁶ to 1.7 × 10⁷ Fe⁺³ spins per µm² [3]. [1] I. Bertini, et al., Chem. Rev. 106 (1), 90–115 (2006). [2] P. Wang, et al., Sci. Adv. 5(4), eaau8038 (2019). [3] S. Lamichhane, et al., arXiv:2310.08605 (2023).   

An optimization framework for deterministic generation of photonic graph states

Photonic graph states are key resources in quantum computing and communications, but are challenging to realize due to the difficulty of photon-photon interactions. Deterministic methods of multi-photon entanglement creation leverage quantum emitter qubits, e.g., quantum dots, to establish and transfer entanglement to photons, and can minimize the resource overheads seen in probabilistic methods. However, there are still limitations in which quantum circuits, i.e., the necessary sequence of quantum gates required to create a target state, can be experimentally implemented due to constraints such as emitter coherence times and CNOT connectivity. While devising optimized state-generating circuits is crucial to experimentally realize photonic graph states, it is a highly non-trivial task. We introduce an optimization method for deterministic generation of photonic graph states considering cost metrics such as circuit depth and gate count which is based on the local Clifford (LC) equivalency of quantum states. Applied to the special case of repeater graph states, we achieve a reduction of up to 50% in the number of CNOT gates. The LC-enhanced circuit design brings us closer to the experimental realization of generation protocols for large photonic graph states, which are crucial for loss and error tolerant quantum protocols, and long range quantum communications.   

Scheduling Compact Error Correcting Codes in Entanglement Distribution Networks

A fundamental challenge in generating and distributing high-fidelity

entanglement in quantum repeater networks is the management

and suppression of quantum errors. In this work, we develop an

approach for extending the effective coherence time of shared

entanglement between nodes by utilizing compact quantum error

correcting (QEC) codes, which are scheduled and run locally on

resource-constrained repeaters, to maximize the fidelity of the shared state,

while also accounting for the incurred gate noise from QEC circuit

operations. This local QEC approach stabilizes the distributed state

via local operations, so independent execution on the nodes requires

no additional synchronization or protocol overhead. Simulation

results show significant improvements in the effective coherence

time relative to idle decay, suggesting this approach could be useful

with several quantum networking protocols such as entanglement

generation, swapping, purification, and teleportation.   

Optimal non-local Franson bi-photon quantum interferometry via high-finesse cavities in quantum communications

The Franson interferometer has been a crucial tool for assessing a range of quantum applications, such as time-energy entanglement distribution, quantum key distribution, and quantum networks. Specifically, mode-locked biphoton frequency combs (BFCs) with discrete comb-like temporal correlations have significantly contributed to enhancing time-energy entanglement in these quantum applications. Nevertheless, we've noted that the visibility of Franson interference recurrence in BFCs tends to decrease as the cavity round trips are extended, resulting in a weakening of time-energy entanglement. In our research, we have identified the cavity finesse F as a pivotal parameter for optimizing non-local Franson biphoton interferometry and BFC time correlations. In our initial observations, BFCs with free-spectral ranges of 5.03 GHz and 15.15 GHz, each filtered with a cavity finesse F of 11.14, displayed a decay pattern of Franson interference recurrence and reduced time-energy entanglement. However, when we utilized a higher cavity finesse of 45.92 and filtered a 15.11 GHz BFC, we achieved an approximately 3.13-fold improvement in Franson interference visibility compared to the Franson visibility associated with a cavity finesse of 11.14, specifically at the sixth time bin. With a finesse of F = 200, we anticipate achieving near-optimal Franson interference recurrence and a time-bin Schmidt number of around 16 effective modes in a similar free-spectral range. Our experimental setup provides flexibility in adjusting cavity parameters, making it suitable for various quantum applications, including high-dimensional quantum information processing and robust quantum communications.   

Highly-efficient multimode superconducting integrated quantum memory

Fault-tolerant quantum computing and quantum internet require quantum memory as an essential building block of a future quantum information processing platform. Superconducting circuits quantum electrodynamics (cQED) is among the leading realizations of noisy intermediate-scale quantum computers. Meanwhile there is a strong motivation to break the wall of nearest-neighbor qubit coupling using enhanced cQED architecture with integrated quantum memory. Compared with traditional superconducting qubits, high-quality factor resonators have a superior potential for quantum state storage due to their long lifetime efficient thermalization, no extra fridge control lines and ability to couple multiple qubits. We experimentally demonstrate microwave quantum storage for two spectral modes of microwave radiation in on-chip system of eight coplanar superconducting resonators. Single mode storage shows a power efficiency of up to 60±3% at single photon energy and more than 75±8% at higher intensity. The noiseless character of the storage is confirmed by coherent state quantum process tomography.