Enrique Pacis Blair

Molecular quantum-dot cellular automata (QCA) may provide low-power, high-speed computational hardware for processing classical information. Simulation and modeling play an important role in the design of QCA circuits because fully coherent models of QCA scale exponentially with the number of devices, and such models are severely limited in size. For larger circuits, approximations become necessary. In the era of fault-tolerant quantum computation, however, it may become possible to model large QCA circuits without such limitations. This work explores the use of the noisy-intermediate scale quantum (NISQ) variational quantum eigensolver (VQE) method for estimating the ground state of QCA circuits. This is relevant because the computational result of a QCA calculation is encoded in the circuit’s ground state. In this study, VQE is used to model logic circuits, including binary wires, inverters, and majority gates. VQE models are performed by ideal simulators, noisy simulators, and actual quantum hardware. This study demonstrates that VQE may indeed be used to model molecular QCA circuits. It is observed that using modern NISQ hardware, results are still quite sensitive to noise, so measures should be taken to minimize noise. These include simplifying the ansatz circuit whenever possible and using low-noise hardware.

Simulating open quantum systems, which interact with external environments, presents significant challenges on noisy intermediate‐scale quantum (NISQ) devices due to limited qubit resources and noise. In this study, an efficient framework is proposed for simulating open quantum systems on NISQ hardware by leveraging a time‐perturbative Kraus operator representation of the system's dynamics. This approach avoids the computationally expensive Trotterization method and exploits the Lindblad master equation to represent time evolution in a compact form, particularly for systems satisfying specific commutation relations. The efficiency of this method is demonstrated by simulating quantum channels, such as the continuous‐time Pauli channel and damped harmonic oscillators, on NISQ trapped‐ion hardware, including IonQ Harmony and Quantinuum H1‐1. Additionally, hardware‐agnostic error mitigation techniques are introduced, including Pauli channel fitting and quantum depolarizing channel inversion, to enhance the fidelity of quantum simulations. These results show strong agreement between the simulations on real quantum hardware and exact solutions, highlighting the potential of Kraus‐based methods for scalable and accurate simulation of open quantum systems on NISQ devices. This framework opens pathways for simulating more complex systems under realistic conditions in the near term.

The simulation of quantum systems is one of the flagship applications of near‐term NISQ (noisy intermediate‐scale quantum) computing devices. Efficiently simulating the rich, non‐unitary dynamics of open quantum systems remains challenging on NISQ hardware. Current simulation methods for open quantum systems employ time‐stepped Trotter product formulas (“Trotterization”) which can scale poorly with respect to the simulation time and system dimension. Here, a new simulation method is proposed based on the derivation of a time‐perturbative Kraus operator series representation of the system. A class of open quantum systems is identified for which this method produces circuits of time‐independent depth, which may serve as a desirable alternative to Trotterization, especially on NISQ devices.

Molecular quantum-dot cellular automata (QCA) may provide high speed, low-power, classical information processing in the post-CMOS era. The readout of molecular QCA bits is challenging because the molecules may be much smaller than transistors and even single-electron transistors. This paper builds on a recent proposal for the spectroscopic readout of bits from asymmetric QCA molecules. Here, we propose circuits for fanning out a bit onto a large QCA circuit to increase the spectroscopic signal-to-noise ratio. As the number of molecules in a fanout circuit grows, the internal bias of each asymmetric cell accumulates, and the circuit may become stuck, tolerating only a very small internal bias. We also propose the use of an applied electric field to compensate for a candidate molecule’s internal bias, thereby restoring switchability, even when the internal bias is significant.

Molecular quantum‐dot cellular automata (QCA) is a low‐power computing paradigm that may offer ultra‐high device densities and THz‐speed switching at room temperature. A single mixed‐valence (MV) molecule acts as an elementary QCA device known as a cell. Cells coupled locally via the electrostatic field form logic circuits. However, previously‐synthesized ionic MV molecular cells are affected by randomly‐located, nearby neutralizing counterions that can bias device states or change device characteristics, causing incorrect computational results. Thisab initiostudy explores how non‐biasing counterions affect individual molecular cells. Additionally, we model two novel neutral, zwitterionic MV QCA molecules designed to avoid biasing and other undesirable counterionic effects. The location of the neutralizing counterion is controlled by integrating one counterion into each cell at a well‐defined, non‐biasing location. Each zwitterionic QCA candidate molecule presented here has a fixed, integrated counterion, which neutralizes the mobile charges used to encode the device state.

Quantum-dot cellular automata (QCA) is a nanoscale, transistor-less device technology. A single molecule may provide an elementary QCA device known as a cell. Molecular redox centers function as quantum dots, and the configuration of mobile charge on the dots encodes device states useful for classical computing. Molecular QCA may support ultra-high device densities and THz-scale switching speeds at room temperature. An applied electric field may be used to clock molecular QCA, providing power gain to boost weakened signals, as well as quasi-adiabatic device operation for minimal power dissipation in QCA devices and circuits. A zwitterionic, Y-shaped, three-dot molecule may function as a field-clocked QCA cell. We focus on the design of a counterion built into the center of the cell. Ab initio computations demonstrate that choice of counterion determines the number of mobile charges for encoding the device state on the three quantum dots. We use B 5 H 5 2 − or B 4 CH 5 − as the central counterionic linker for two different Y-shaped, three-dot QCA molecules. While both molecules support the desired device states, the number of trapped charges in the counterion determines the number of mobile holes on the molecular quantum dots. This, in turn, determines whether the device state is encoded by a hole or an electron. This choice of encoding determines how the molecular QCA cell responds to a clocking field. The two counterions studied here lead to two QCA molecules with opposite responses to the clock, similar to the complementary responses of PMOS and NMOS transistors to gated voltage control.

Quantum-dot cellular automata (QCA) is a paradigm for low-power, general-purpose, classical computing designed to overcome the challenges facing CMOS in the extreme limits of scaling. A molecular implementation of QCA offers nanometer-scale devices with device densities and operating speeds which may surpass CMOS device densities and speeds by several orders of magnitude, all at room temperature. Here, a proposal for electric field bit write-in to molecular QCA circuits is extended to QCA circuits clocked using the $z$ component of an applied electric field. Input electrodes, which may be much larger than the cells themselves, apply an input $y$ field component. The input field selects an input bit on a field-sensitive portion of the circuit, and a shift register transmits the input bit to downstream logic. The circuit is sensitive to the input field, since even a very weak input field selects a bit and drives the shift register as desired. The circuit also is robust: the coupling between the field-sensitive segment and the shift register fails at input field strengths much stronger than those needed to write a bit. A simple rotation of the the shift register cells eliminates this circuit failure, even under arbitrarily strong input fields. The modified circuit also tolerates a significant unwanted $x$ field component, which is used neither for clocking nor for input. The write-in of classical bits to molecular QCA circuits is one road-block that must be cleared in order to realize energy-efficient molecular computation using QCA. Write-in may be achieved simply by using relatively large electrodes and clocked molecular circuits that respond to a weak input field and tolerate very strong applied fields without failing.

Quantum-dot cellular automata (QCA) may provide low-power, general-purpose computing in the post-CMOS era. A molecular implementation of QCA features nanometer-scale devices and may support ∼THz switching speeds at room-temperature. Here, we explore the ability of molecular QCA circuits to tolerate unwanted applied electric fields, which may come from a variety of sources. One likely source of strong unwanted electric fields may be electrodes recently proposed for the write-in of classical bits to molecular QCA input circuits. Previous models have shown that the input circuits are sensitive to the applied field, and a coupled QCA wire can successfully transfer the input bit to downstream circuits despite strong applied fields. However, the ability of other QCA circuits to tolerate an applied field has not yet been demonstrated. Here, we study the robustness of various QCA circuits by calculating their ground state responses in the presence of an applied field. To do this, a circuit is built from several QCA molecules, each described as a two-state system. A circuit Hamiltonian is formed and diagonalized. All pairwise interactions between cells are considered, along with all correlations. An examination of the ground state shows that these QCA circuits may indeed tolerate strong unwanted electric fields. We also show that circuit immunity to the dominant unwanted field component may be obtained by choosing the orientation of constituent molecules. This suggests that relatively large electrodes used for bit write-in to molecular QCA need not disrupt the operation of nearby QCA circuits. The circuits may tolerate significant electric fields from other sources as well.

Random numbers are a valuable commodity in gaming and gambling, simulation, conventional and quantum cryptography, and in non-conventional computing schemes such as stochastic computing. We propose to generate a random bit using a position measurement of a single mobile charge on a coupled pair of quantum dots. True randomness of the measurement outcome is - depending on implementation - provided by statistical mechanics, or by quantum mechanics via Born's rule. A random bit string may be generated using a sequence of repeated measurements on the same double quantum dot (DQD) system. Any bias toward a "0" measurement or a "1" measurement may be removed or tuned as desired simply by adjusting the bias between the dots. Device tunability provides versatility, enabling this quantum random number generator (QRNG) to support applications in which no bias is desired, or where a tunable bias is desired. We discuss a metal-dot implementation as well as a molecular implementation of this QRNG. Basic quantum mechanical principles are used to study power dissipation and timing considerations for the generation of random bit strings. The DQD offers a small form factor and, in a metallic implementation, is usable in the case where cryogenic operations are desirable (as in the case of quantum computing). For room-temperature applications, a molecular DQD may be used.