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1. A universal quantum gate set for transmon qubits with strong ZZ interactions

   Junling Long, Tongyu Zhao ( March 2021 , ArXivorg)

   null (Ed.)

   High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time tg=53.8 ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography. 

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2. Comparing Quantum Computing Platforms

   https://doi.org/10.1007/978-3-030-98012-2_32  

   Agarwal, Siddh ; Jaoude, Gio Abou ; Leider, Avery ; Tappert, Charles C. ( January 2022 , Advances in Information and Communication: Proceedings of the 2022 Future of Information and Communication Conference (FICC), Volume 1)

   Arai, Kohei (Ed.)

   This research compares and contrasts two commonly available quantum computing platforms available today to academic researchers: the IBM Q-Experience and the University of Maryland's IonQ. Hands-on testing utilized the implementation of a simple two qubit circuit and tested the Pauli X, Y, and Z single-qubit gates as well as the CNOT 2+ qubit gate and compared the results, as well as the user experience. The user experience and the interface must be straightforward to help the user's understanding when planning quantum computing training for new knowledge workers in this exciting new field. Additionally, we demonstrate how a quantum computer's results, when the output is read in the classical computer, loses some of its information, since the quantum computer is operating in more dimensions than the classical computer can interpret. This is demonstrated with the ZX and XZ gates which appear to give the same result; however, using the mathematics of matrix notation, the phase difference between the two answers is revealed in their vectors, which are 180 degrees apart. 

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3. Spacetime-Efficient Low-Depth Quantum State Preparation with Applications

   https://doi.org/10.22331/q-2024-02-15-1257  

   Gui, Kaiwen ; Dalzell, Alexander M. ; Achille, Alessandro ; Suchara, Martin ; Chong, Frederic T. ( February 2024 , Quantum)

   We propose a novel deterministic method for preparing arbitrary quantum states. When our protocol is compiled into CNOT and arbitrary single-qubit gates, it prepares an$N$-dimensional state in depth$O(\log (N))$and$\mathit{\text{spacetime allocation}}$(a metric that accounts for the fact that oftentimes some ancilla qubits need not be active for the entire circuit)$O(N)$, which are both optimal. When compiled into the$\{\mathrm{H},\mathrm{S},\mathrm{T},\mathrm{C}\mathrm{N}\mathrm{O}\mathrm{T}\}$gate set, we show that it requires asymptotically fewer quantum resources than previous methods. Specifically, it prepares an arbitrary state up to error$ϵ$with optimal depth of$O(\log (N)+\log (1/ϵ))$and spacetime allocation$O(N\log (\log (N)/ϵ))$, improving over$O(\log (N)\log (\log (N)/ϵ))$and$O(N\log (N/ϵ))$, respectively. We illustrate how the reduced spacetime allocation of our protocol enables rapid preparation of many disjoint states with only constant-factor ancilla overhead –$O(N)$ancilla qubits are reused efficiently to prepare a product state of$w$$N$-dimensional states in depth$O(w+\log (N))$rather than$O(w\log (N))$, achieving effectively constant depth per state. We highlight several applications where this ability would be useful, including quantum machine learning, Hamiltonian simulation, and solving linear systems of equations. We provide quantum circuit descriptions of our protocol, detailed pseudocode, and gate-level implementation examples using Braket.

    

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4. Mitigating CNOT Errors via Noise-aware Token Swapping

   https://doi.org/10.1109/QCE57702.2023.00044  

   Sharma, Asim ; Banerjee, Avah ( September 2023 , 2023 IEEE International Conference on Quantum Computing and Engineering (QCE))

   Due to the limited decoherence time of qubits in the Noisy Intermediate-Scale Quantum (NISQ) era, optimizing the size and depth of logical quantum circuits for efficient implementation on sparsely connected physical architectures is crucial. In this work, we extend the Approximate Token Swapping (ATS) algorithm by incorporating qubit priority and the size of permutation cycles to be routed. We refer to this algorithm as Priority-ATS (PATS), which also considers CNOT gate errors while constructing the routing schedule for the qubits. We provide theoretical justification for the superior effectiveness of PATS over ATS and experimentally demonstrate its ability to improve the fidelity of the output state. Furthermore, we use depolarizing error channels to model the noisy CNOT gates, where each adjacent qubit pair has a distinct error rate. By employing realistic error rates, we showcase the robust improvement in output fidelity when using PATS as opposed to a noise-oblivious ATS routing scheme. 

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5. Demonstration of a two-bit controlled-NOT quantum-like gate using classical acoustic qubit-analogues

   https://doi.org/10.1038/s41598-022-18314-5  

   Runge, Keith ; Hasan, M. Arif ; Levine, Joshua A. ; Deymier, Pierre A. ( December 2022 , Scientific Reports)

   Abstract The Controlled-NOT (CNOT) gate is the key to unlock the power of quantum computing as it is a fundamental component of a universal set of gates. We demonstrate the operation of a two-bit C-NOT quantum-like gate using classical qubit acoustic analogues, called herein logical phi-bits. The logical phi-bits are supported by an externally driven nonlinear acoustic metamaterial composed of a parallel array of three elastically coupled waveguides. A logical phi-bit has a two-state degree of freedom associated with the two independent relative phases of the acoustic wave in the three waveguides. A simple physical manipulation involving the detuning of the frequency of one of the external drivers is shown to operate on the complex vectors in the Hilbert space of pairs of logical phi-bits. This operation achieves a systematic and predictable C-NOT gate with unambiguously measurable input and output. The possibility of scaling the approach to more phi-bits is promising. 

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