Fidelity Of Operations Research Articles

Scalable Architecture for Trapped-Ion Quantum Computing Using rf Traps and Dynamic Optical Potentials

Qubits based on ions trapped in linear radio-frequency traps form a successful platform for quantum computing, due to their high fidelity of operations, all-to-all connectivity, and degree of local control. In principle, there is no fundamental limit to the number of ion-based qubits that can be confined in a single 1D register. However, in practice, there are two main issues associated with long trapped-ion crystals, that stem from the “softening” of their modes of motion, upon scaling up: high heating rates of the ions’ motion and a dense motional spectrum; both impede the performance of high-fidelity qubit operations. Here, we propose a holistic, scalable architecture for quantum computing with large ion crystals that overcomes these issues. Our method relies on dynamically operated optical potentials that instantaneously segment the ion crystal into cells of a manageable size. We show that these cells behave as nearly independent quantum registers, allowing for parallel entangling gates on all cells. The ability to reconfigure the optical potentials guarantees connectivity across the full ion crystal and also enables efficient midcircuit measurements. We study the implementation of large-scale parallel multiqubit entangling gates that operate simultaneously on all cells and present a protocol to compensate for crosstalk errors, enabling full-scale usage of an extensively large register. We illustrate that this architecture is advantageous both for fault-tolerant digital quantum computation and for analog quantum simulations. Published by the American Physical Society 2024

Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits

Scalable quantum processors require high-fidelity universal quantum logic operations in a manufacturable physical platform. Donors in silicon provide atomic size, excellent quantum coherence and compatibility with standard semiconductor processing, but no entanglement between donor-bound electron spins has been demonstrated to date. Here we present the experimental demonstration and tomography of universal one- and two-qubit gates in a system of two weakly exchange-coupled electrons, bound to single phosphorus donors introduced in silicon by ion implantation. We observe that the exchange interaction has no effect on the qubit coherence. We quantify the fidelity of the quantum operations using gate set tomography (GST), and we use the universal gate set to create entangled Bell states of the electrons spins, with fidelity 91.3 ± 3.0%, and concurrence 0.87 ± 0.05. These results form the necessary basis for scaling up donor-based quantum computers.

Ancilla-free scheme for the photon-addition operation and its application to squeezing and coherence manipulation

The photon-addition operation is the inverse of the photon-subtraction operation and has many important applications in research on photon number statistics in quantum optics. In this study, we propose a scheme for a photon-addition operation with ancilla-free photonic manipulation. This is in strong contrast to conventional photon-addition operations in which a single-photon ancilla is an indispensable resource for photon addition. Moreover, our scheme helps break the exponential decay trend in the probability of success for photon addition operating in large Fock states. Thus, our scheme can be considered as a useful tool for manipulating the photon number of a bright coherent state or a strongly squeezed state in the near future. To be precise, we used the SU(1,1) beamsplitter to replace the conventional beamsplitter, and our work can be considered another important application of the SU(1,1) beamsplitter. The photon-addition operation is applied to both single- and two-mode quantum entangled states, and our results show that SU(1,1)-based photon addition is more powerful and efficient in terms of the fidelity and success probability of the photon-addition operation.

Measurement of cryoelectronics heating using a local quantum dot thermometer in silicon

Silicon technology offers the enticing opportunity for monolithic integration of quantum and classical electronic circuits. However, the power consumption levels of classical electronics may compromise the local chip temperature and hence the fidelity of qubit operations. Here, we utilize a quantum-dot-based thermometer embedded in an industry-standard silicon field-effect transistor (FET), to assess the local temperature increase produced by an active FET placed in close proximity. We study the impact of both static and dynamic operation regimes. When the FET is operated statically, we find a power budget of 45 nW at 100 nm separation whereas at 216 μm the power budget raises to 150 μW. When operated dynamically, we observe negligible temperature increase for the switch frequencies tested up to 10 MHz. Our work describes a method to accurately map out the available power budget at a distance from a solid-state quantum processor and indicate under which conditions cryoelectronics circuits may allow the operation of hybrid quantum-classical systems.

Local Control of a Single Nitrogen-Vacancy Center by Nanoscale Engineered Magnetic Domain Wall Motion.

Effective control and readout of qubits form the technical foundation of next-generation, transformative quantum information sciences and technologies. The nitrogen-vacancy (NV) center, an intrinsic three-level spin system, is naturally relevant in this context due to its excellent quantum coherence, high fidelity of operations, and remarkable functionality over a broad range of experimental conditions. It is an active contender for the development and implementation of cutting-edge quantum technologies. Here, we report magnetic domain wall motion driven local control and measurements of the NV spin properties. By engineering the local magnetic field environment of an NV center via nanoscale reconfigurable domain wall motion, we show that NV photoluminescence, spin level energies, and coherence time can be reliably controlled and correlated to the magneto-transport response of a magnetic device. Our results highlight the electrically tunable dipole interaction between NV centers and nanoscale magnetic structures, providing an attractive platform to realize interactive information transfer between spin qubits and nonvolatile magnetic memory in hybrid quantum spintronic systems.

Investigation of modified uni-traveling carrier photodiode for cryogenic microwave photonic links

Quantum devices present the potential for unparalleled computing and communications capabilities; however, the cryogenic temperatures required to successfully control and read out many qubit platforms can prove to be very challenging to scale. Recently, there has emerged an interest in using microwave photonics to deliver control signals down to ultracold stages via optical fiber, thereby reducing thermal load and facilitating dense wavelength multiplexing. Photodetectors can then convert this optical energy to electrical signals for qubit control. The fidelity of the quantum operations of interest therefore depend heavily upon the characteristics of the photodiode, yet experimental demonstrations of fiber-coupled photodetection systems at low temperatures are relatively few in number, leaving important open questions regarding how specific detectors may perform in real-world cryogenic settings. In this work, we examine a highly linear modified uni-traveling carrier photodiode (MUTC-PD) under C-band illumination (1530–1565 nm) at three temperature regimes (300 K, 80 K, and ∼4 K) and multiple bias conditions. Our findings of reduced responsivity but preserved bandwidth are consistent with previous studies, while our saturation tests suggest a variety of potential applications for MUTC-PDs in cryogenic microwave photonics with and without electrical bias. Overall, our results should provide a valuable foundation for the continued and expanding use of this detector technology in quantum information processing.

Influence of random telegraph noise on quantum bit gate operation

We consider the problem of analyzing spin-flip qubit gate operation in the presence of Random Telegraph Noise (RTN). Our compressive approach is the following. By using the Feynman disentangling operators method, we calculate the spin-flip probability of qubit driven by different kinds of composite pulses, e.g., Constant pulse (C-pulse), Quantum Well pulse (QW-pulse), and Barrier Potential pulse (BP-pulse) in the presence of RTN. When composite pulses and RTN act in the x-direction and z-direction respectively, we calculate the optimal time to achieve perfect spin-flip probability of qubit. We report that the highest fidelity of spin-flip qubit can be achieved by using C-pulse, followed by BP-pulse and QW-pulse. For a more general case, we have tested several pulse sequences for achieving high fidelity quantum gates, where we use the pulses acting in different directions. From the calculations, we find that high fidelity of qubit gate operation in the presence of RTN is achieved when QW-pulse, BP-pulse, and C-pulse act in the x-direction, y-direction, and z-direction, respectively. We extend our investigations for multiple QW and BP pulses while choosing the C-pulse amplitude constant in the presence of RTN. The results of calculation show that 98.5% fidelity can be achieved throughout the course of RTN that may be beneficial for quantum error correction.

Operational challenges and mitigation measures during the COVID-19 pandemic–Lessons from DELIVER

Catastrophic disruptions in care delivery threaten the operational efficiency and potentially the validity of clinical research efforts, in particular randomized clinical trials. Most recently, the COVID-19 pandemic affected essentially all aspects of care delivery and clinical research conduct. While consensus statements and clinical guidance documents have detailed potential mitigation measures, few real-world experiences detailing clinical trial adaptations to the COVID-19 pandemic exist, particularly among, large, global registrational cardiovascular trials. We outline the operational impact of COVID-19 and resultant mitigation measures in the Dapagliflozin Evaluation to Improve the LIVEs of Patients with Preserved Ejection Fraction Heart Failure (DELIVER) trial, one of the largest and most globally diverse experiences with COVID-19 of any cardiovascular clinical trial to date. Specifically, we address the needed coordination between academic investigators, trial leadership, clinical sites, and the supporting sponsor to ensure the safety of participants and trial staff, to maintain the fidelity of trial operations, and to prospectively adapt statistical analyses plans to evaluate the impact of COVID-19 and the pandemic at large on trial participants. These discussions included key operational issues such as ensuring delivery of study medications, adaptations to study visits, enhanced COVID-19 related endpoint adjudication, and protocol and analytical plan revisions. Our findings may have important implications for establishing consensus on prospective contingency planning in future clinical trials. gov: NCT03619213. GOV: NCT03619213.

Sensitivity of quantum gate fidelity to laser phase and intensity noise

The fidelity of gate operations on neutral atom qubits is often limited by fluctuations of the laser drive. Here, we quantify the sensitivity of quantum gate fidelities to laser phase and intensity noise. We first develop models to identify features observed in laser self-heterodyne noise spectra, focusing on the effects of white noise and servo bumps. In the weak-noise regime, characteristic of well-stabilized lasers, we show that an analytical theory based on a perturbative solution of a master equation agrees very well with numerical simulations that incorporate phase noise. We compute quantum gate fidelities for one- and two-photon Rabi oscillations and show that they can be enhanced by an appropriate choice of Rabi frequency relative to spectral noise peaks. We also analyze the influence of intensity noise with spectral support smaller than the Rabi frequency. Our results establish requirements on laser noise levels needed to achieve desired gate fidelities.

Scalable fast benchmarking for individual quantum gates with local twirling

With the development of controllable quantum systems, fast and practical characterization of multi-qubit gates has become essential for building high-fidelity quantum computing devices. The usual way to fulfill this requirement via randomized benchmarking demands complicated implementation of numerous multi-qubit twirling gates. How to efficiently and reliably estimate the fidelity of a quantum process remains an open problem. This work thus proposes a character-cycle benchmarking protocol and a character-average benchmarking protocol using only local twirling gates to estimate the process fidelity of an individual multi-qubit operation. Our protocols were able to characterize a large class of quantum gates including and beyond the Clifford group via the local gauge transformation, which forms a universal gate set for quantum computing. We demonstrated numerically our protocols for a non-Clifford gate—controlled- ( T X ) and a Clifford gate—five-qubit quantum error-correcting encoding circuit. The numerical results show that our protocols can efficiently and reliably characterize the gate process fidelities. Compared with the cross-entropy benchmarking, the simulation results show that the character-average benchmarking achieves three orders of magnitude improvements in terms of sampling complexity.

Exploring entanglement resource in Si quantum dot systems with operational quasiprobability approach

We characterize the quantum entanglement of the realistic two-qubit signals that are sensitive to charge noises. Our working example is the time response generated from a silicon double quantum dot (DQD) platform, where a single-qubit rotation and a two-qubit controlled-NOT operation are conducted sequentially in time to generate arbitrary entangled states. In order to characterize the entanglement of two-qubit states, we employ the marginal operational quasiprobability (OQ) approach that allows negative values of the probability function if a given state is entangled. While the charge noise, which is omnipresent in semiconductor devices, severely affects logic operations implemented in the DQD platform, causing huge degradation in fidelity of unitary operations as well as resulting two-qubit states, the pattern in the OQ-driven entanglement strength turns out to be quite invariant, indicating that the resource of quantum entanglement is not significantly broken though the physical system is exposed to noise-driven fluctuations in exchange interaction between quantum dots.

Devitalizing noise-driven instability of entangling logic in silicon devices with bias controls

The quality of quantum bits (qubits) in silicon is highly vulnerable to charge noise that is omnipresent in semiconductor devices and is in principle hard to be suppressed. For a realistically sized quantum dot system based on a silicon-germanium heterostructure whose confinement is manipulated with electrical biases imposed on top electrodes, we computationally explore the noise-robustness of 2-qubit entangling operations with a focus on the controlled-X (CNOT) logic that is essential for designs of gate-based universal quantum logic circuits. With device simulations based on the physics of bulk semiconductors augmented with electronic structure calculations, we not only quantify the degradation in fidelity of single-step CNOT operations with respect to the strength of charge noise, but also discuss a strategy of device engineering that can significantly enhance noise-robustness of CNOT operations with almost no sacrifice of speed compared to the single-step case. Details of device designs and controls that this work presents can establish practical guideline for potential efforts to secure silicon-based quantum processors using an electrode-driven quantum dot platform.

Reducing circuit complexity in optical quantum computation using 3D architectures.

Integrated photonic architectures based on optical waveguides are one of the leading candidates for the future realisation of large-scale quantum computation. One of the central challenges in realising this goal is simultaneously minimising loss whilst maximising interferometric visibility within waveguide circuits. One approach is to reduce circuit complexity and depth. A major constraint in most planar waveguide systems is that beamsplitter transformations between distant optical modes require numerous intermediate SWAP operations to couple them into nearest neighbour proximity, each of which introduces loss and scattering. Here, we propose a 3D architecture which can significantly mitigate this problem by geometrically bypassing trivial intermediate operations. We demonstrate the viability of this concept by considering a worst-case 2D scenario, where we interfere the two most distant optical modes in a planar structure. Using femtosecond laser direct-writing technology we experimentally construct a 2D architecture to implement Hong-Ou-Mandel interference between its most distant modes, and a 3D one with corresponding physical dimensions, demonstrating significant improvement in both fidelity and efficiency in the latter case. In addition to improving fidelity and efficiency of individual non-adjacent beamsplitter operations, this approach provides an avenue for reducing the optical depth of circuits comprising complex arrays of beamsplitter operations.

Gate fidelity, dephasing, and ‘magic’ trapping of optically trapped neutral atom

The fidelity of the gate operation and the coherence time of neutral atoms trapped in an optical dipole trap are figures of merit for the applications. The motion of the trapped atom is one of the key factors which influences the gate fidelity and coherence time. The motion has been considered as a classical oscillator in analyzing the influence. Here we treat the motion of the atom as a quantum oscillator. The population on the vibrational states of the atom are considered in analyzing the gate fidelity and decoherence. We show that the fidelity of a coherent rotation gate is dramatically limited by the temperature of a thermally trapped atom. We also show that the dephasing between the two hyperfine states due to the thermal motion of the atom could rephase naturally if the differential frequency shift is stable and the vibrational states do not change. The decoherence due to the fluctuations of the trap laser intensity is also discussed. Both the gate fidelity and coherence time can be dramatically enhanced by cooling the atom into vibrational ground states and/or by using a blue-detuned trap. More importantly, we propose a ‘magic’ trapping condition by preparing the atom into specific vibrational states.

Toward the Speed Limit of High-Fidelity Two-Qubit Gates.

Most implementations of quantum gate operations rely on external control fields to drive the evolution of the quantum system. Generating these control fields requires significant efforts to design the suitable control Hamiltonians. Furthermore, any error in the control fields reduces the fidelity of the implemented control operation with respect to the ideal target operation. Achieving sufficiently fast gate operations at low error rates remains therefore a huge challenge. In this Letter, we present a novel approach to overcome this challenge by eliminating, for specific gate operations, the time-dependent control fields entirely. This approach appears useful for maximizing the speed of the gate operation while simultaneously eliminating relevant sources of errors. We present an experimental demonstration of the concept in a single nitrogen-vacancy center in diamond at room temperature.

Dissipation and gate timing errors in SWAP operations of qubits

We examine how dissipation and gate timing errors affect the fidelity of a sequence of SWAP gates on a chain of interacting qubits in comparison to noise in the interqubit interaction. Although interqubit interaction noise and gate timing errors are always present in any qubit platform, dissipation is a special case that can arise in multivalley semiconductor spin qubit systems, such as Si-based qubits, where dissipation may be used as a general model for valley leakage. In our Hamiltonian, each qubit is coupled via Heisenberg exchange to every other qubit in the chain, with the strength of the exchange interaction decreasing exponentially with distance between the qubits. Dissipation is modeled through the term $-i\gamma\mathbf{1}$ in the Hamiltonian, and $\gamma$ is chosen so as to be consistent with the experimentally observed intervalley tunneling in Si. We show that randomness in the dissipation parameter should have little to no effect on the SWAP gate fidelity in the currently fabricated Si circuits. We introduce quasistatic noise in the interqubit interaction and random gate timing error and average the fidelities over 10,000 realizations for each set of parameters. The fidelities are then plotted against $J_\text{SWAP}$, the strength of the exchange coupling corresponding to the SWAP gate. We find that dissipation decreases the fidelity of the SWAP operation -- though the effect is small compared to that of the known noise in the interqubit interaction -- and that gate timing error creates an effective optimal value of $J_\text{SWAP}$, beyond which infidelity begins to increase.

Noise-reducing encoding strategies for spin chains

We present an encoding technique that reduces the effects of noise on quantum spin systems whose operation is driven by Hamiltonian evolution. This technique is widely applicable, being most relevant to the scenarios where there are insufficient qubits to permit full-scale error correction. Instead, our technique can be implemented over small numbers of qubits and still leads to noticeable improvements in the fidelity of operations. The encoding scheme is easy to implement, flexible with respect to choice of Hamiltonian, and close to optimal.

Biomarker-Responsive Fluorescent Probes for In-Vivo Imaging of Liver Injury.

Liver injury-related diseases have aroused widespread concern due to their extreme unpredictability, acute onset, and severe consequences. Nowadays, the clinical prediction and assessment of liver injury mainly focus on histopathological and serological approaches, which require a tedious process and sometimes invasive biopsy. Over the past decades, fluorescence imaging techniques have emerged for the diagnosis of diseases owing to their noninvasiveness, high fidelity and ease of operation. With regard to liver injury, fluorescent probes have been delicately designed to respond to a variety of endogenous biomolecules to precisely offer comprehensive information about the lesion site. Herein, we make a brief summary and discussion about the design strategies and applications of recently reported fluorescent biosensors responsive to a series of biomarkers involved in liver injury. The potential prospects and remaining challenges are also discussed to promote the progression in this field.

High-fidelity CNOT gate for spin qubits with asymmetric driving using virtual gates

Recent experiments have demonstrated two-qubit fidelities above 99%, however, theoretically, the fidelity of CNOT operations is limited by off-resonant driving described by off-diagonal terms in the system Hamiltonian. Here we investigate these off-diagonal contributions and we propose a fidelity improvement of several orders of magnitude by using asymmetric driving. Therefore, we provide a description of an ac virtual gate based on a simple capacitance model which not only enables a high fidelity CNOT but also crosstalk reduction when scaling up spin qubit devices to larger arrays.

Controlled-NOT operation of SiN-photonic circuit using photon pairs from silicon-photonic circuit

We demonstrate controlled-NOT (CNOT) operation of a silicon nitride photonic integrated circuit using path-entangled photon pairs from a silicon-photonic integrated circuit as logical inputs. The CNOT gate is constructed by combining several silicon-nitride Mach–Zehnder interferometers for a linear-optical quantum gate. We package each of the two photonic circuits with electrical wire bonding and optical fiber bonding for stable measurement. The logical-basis fidelity of CNOT operation is measured better than 81% for the fully packaged photonic circuits.