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1. Quantum repeaters based on two species trapped ions

   https://doi.org/10.1088/1367-2630/ab2a45  

   Santra, Siddhartha ; Muralidharan, Sreraman ; Lichtman, Martin ; Jiang, Liang ; Monroe, Christopher ; Malinovsky, Vladimir S. ( July 2019 , New Journal of Physics)

   We examine the viability of quantum repeaters based on two-species trapped ion modules for long-distance quantum key distribution. Repeater nodes comprised of ion-trap modules of co-trapped ions of distinct species are considered. The species used for communication qubits has excellent optical properties while the other longer lived species serves as a memory qubit in the modules. Each module interacts with the network only via single photons emitted by the communication ions. Coherent Coulomb interaction between ions is utilized to transfer quantum information between the communication and memory ions and to achieve entanglement swapping between two memory ions. We describe simple modular quantum repeater architectures realizable with the ion-trap modules and numerically study the dependence of the quantum key distribution rate on various experimental parameters, including coupling efficiency, gate infidelity, operation time and length of the elementary links. Our analysis suggests crucial improvements necessary in a physical implementation for co-trapped two-species ions to be a competitive platform in long-distance quantum communication.

    

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2. Quantum control of spin qubits using nanomagnets

   https://doi.org/10.1038/s42005-022-01041-8  

   Niknam, Mohamad ; Chowdhury, Md. Fahim F. ; Rajib, Md Mahadi ; Misba, Walid Al ; Schwartz, Robert N. ; Wang, Kang L. ; Atulasimha, Jayasimha ; Bouchard, Louis-S. ( November 2022 , Communications Physics)

   Single-qubit gates are essential components of a universal quantum computer. Without selective addressing of individual qubits, scalable implementation of quantum algorithms is extremely challenging. When the qubits are discrete points or regions on a lattice, selectively addressing magnetic spin qubits at the nanoscale remains a challenge due to the difficulty of localizing and confining a classical divergence-free field to a small volume of space. Herein we propose a technique for addressing spin qubits using voltage-control of nanoscale magnetism, exemplified by the use of voltage control of magnetic anisotropy. We show that by tuning the frequency of the nanomagnet’s electric field drive to the Larmor frequency of the spins confined to a nanoscale volume, and by modulating the phase of the drive, single-qubit quantum gates with fidelities approaching those for fault-tolerant quantum computing can be implemented. Such single-qubit gate operations require only tens of femto-Joules per gate operation and have lossless, purely magnetic field control. Their physical realization is also straightforward using foundry manufacturing techniques.

    

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3. Voltage‐Gated Nanoparticle Transport and Collisions in Attoliter‐Volume Nanopore Electrode Arrays

   https://doi.org/10.1002/smll.201703248  

   Fu, Kaiyu ; Han, Donghoon ; Crouch, Garrison M. ; Kwon, Seung‐Ryong ; Bohn, Paul W. ( January 2018 , Small)

   Single nanoparticle analysis can reveal how particle‐to‐particle heterogeneity affects ensemble properties derived from traditional bulk measurements. High‐bandwidth, low noise electrochemical measurements are needed to examine the fast heterogeneous electron‐transfer behavior of single nanoparticles with sufficient fidelity to resolve the behavior of individual nanoparticles. Herein, nanopore electrode arrays (NEAs) are fabricated in which each pore supports two vertically spaced, individually addressable electrodes. The top ring electrode serves as a particle gate to control the transport of silver nanoparticles (AgNPs) within individual attoliter volume NEAs nanopores, as shown by redox collisions of AgNPs collisions at the bottom disk electrode. The AgNP‐nanoporeis system has wide‐ranging technological applications as well as fundamental interest, since the transport of AgNPs within the NEA mimics the transport of ions through cell membranes via voltage‐gated ion channels. A voltage threshold is observed above which AgNPs are able to access the bottom electrode of the NEAs, i.e., a minimum potential at the gate electrode is required to switch between few and many observed collision events on the collector electrode. It is further shown that this threshold voltage is strongly dependent on the applied voltage at both electrodes as well as the size of AgNPs, as shown both experimentally and through finite‐element modeling. Overall, this study provides a precise method of monitoring nanoparticle transport and in situ redox reactions within nanoconfined spaces at the single particle level.

    

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4. Electric-double-layer-gated transistors based on two-dimensional crystals: recent approaches and advances

   https://doi.org/10.1088/2515-7639/ab8270  

   Xu, Ke ; Fullerton-Shirey, Susan K. ( May 2020 , Journal of Physics: Materials)

   Electric-double-layer (EDL) gated transistors use ions in an electrolyte to induce charge in the channel of the transistor by field-effect. Because a sub-nanometer gap capacitor is created at the electrolyte/channel interface, large capacitance densities (∼µF cm⁻²) corresponding to high sheet carrier densities (10¹⁴cm⁻²) can be induced, exceeding conventional gate dielectrics by about one order of magnitude. Because it is an interfacial technique, EDL gating is especially effective on two-dimensional (2D) crystals, which—at the monolayer limit—are basically interfaces themselves. Both solid polymer electrolytes and ionic liquids are routinely used as ion-conducting gate dielectrics, and they have provided access to regimes of transport in 2D materials that would be inaccessible otherwise. The technique, now widely used, has enabled the 2D crystal community to study superconductivity, spin- and valleytronics, investigate electrical and structural phase transitions, and create abruptp-njunctions to generate tunneling, among others. In addition to using EDL gating as a tool to investigate properties of the 2D crystals, more recent efforts have emerged to engineer the electrolyte to add new functionality and device features, such as synaptic plasticity, bistability and non-volatility. Example of potential applications include neuromorphic computing and non-volatile memory. This review focuses on using ions forelectrostaticcontrol of 2D crystal transistors both to uncover basic properties of 2D crystals, and also to add new device functionalities.

    

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5. Random Quantum Circuits Transform Local Noise into Global White Noise

   https://doi.org/10.1007/s00220-024-04958-z  

   Dalzell, Alexander M. ; Hunter-Jones, Nicholas ; Brandão, Fernando G. S. L. ( March 2024 , Communications in Mathematical Physics)

   We study the distribution over measurement outcomes of noisy random quantum circuits in the regime of low fidelity, which corresponds to the setting where the computation experiences at least one gate-level error with probability close to one. We model noise by adding a pair of weak, unital, single-qubit noise channels after each two-qubit gate, and we show that for typical random circuit instances, correlations between the noisy output distribution$$p_{\text {noisy}}$$$p_{\text{noisy}}$and the corresponding noiseless output distribution$$p_{\text {ideal}}$$$p_{\text{ideal}}$shrink exponentially with the expected number of gate-level errors. Specifically, the linear cross-entropy benchmarkFthat measures this correlation behaves as$$F=\text {exp}(-2s\epsilon \pm O(s\epsilon ^2))$$$F=\text{exp}(-2sϵ\pm O\left(s{ϵ}^2\right))$, where$$\epsilon $$$ϵ$is the probability of error per circuit location andsis the number of two-qubit gates. Furthermore, if the noise is incoherent—for example, depolarizing or dephasing noise—the total variation distance between the noisy output distribution$$p_{\text {noisy}}$$$p_{\text{noisy}}$and the uniform distribution$$p_{\text {unif}}$$$p_{\text{unif}}$decays at precisely the same rate. Consequently, the noisy output distribution can be approximated as$$p_{\text {noisy}}\approx Fp_{\text {ideal}}+ (1-F)p_{\text {unif}}$$$p_{\text{noisy}}\approx F{p}_{\text{ideal}}+(1-F)p_{\text{unif}}$. In other words, although at least one local error occurs with probability$$1-F$$$1-F$, the errors are scrambled by the random quantum circuit and can be treated as global white noise, contributing completely uniform output. Importantly, we upper bound the average total variation error in this approximation by$$O(F\epsilon \sqrt{s})$$$O\left(Fϵ\sqrt{s}\right)$. Thus, the “white-noise approximation” is meaningful when$$\epsilon \sqrt{s} \ll 1$$$ϵ\sqrt{s}\ll 1$, a quadratically weaker condition than the$$\epsilon s\ll 1$$$ϵs\ll 1$requirement to maintain high fidelity. The bound applies if the circuit size satisfies$$s \ge \Omega (n\log (n))$$$s\ge \Omega (nlog(n\left)\right)$, which corresponds to onlylogarithmic depthcircuits, and if, additionally, the inverse error rate satisfies$$\epsilon ^{-1} \ge {\tilde{\Omega }}(n)$$${ϵ}^{-1}\ge \tilde{\Omega }\left(n\right)$, which is needed to ensure errors are scrambled faster thanFdecays. The white-noise approximation is useful for salvaging the signal from a noisy quantum computation; for example, it was an underlying assumption in complexity-theoretic arguments that noisy random quantum circuits cannot be efficiently sampled classically, even when the fidelity is low. Our method is based on a map from second-moment quantities in random quantum circuits to expectation values of certain stochastic processes for which we compute upper and lower bounds.

    

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