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Mendeley Data Showcase

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1970
2024
1970 2024
2233 results
  • SQuADDS_DB
    • Dataset
  • qubits
    Full Changelog: https://github.com/thespacedoctor/qubits/compare/v0.3.3...v0.3.4
    • Software/Code
  • Encoding One Logical Qubit Into Six Physical Qubits
    We discuss two methods to encode one qubit into six physical qubits. Each of our two examples corrects an arbitrary single-qubit error. Our first example is a degenerate six-qubit quantum error-correcting code. We explicitly provide the stabilizer generators, encoding circuits, codewords, logical Pauli operators, and logical CNOT operator for this code. We also show how to convert this code into a non-trivial subsystem code that saturates the subsystem Singleton bound. We then prove that a six-qubit code without entanglement assistance cannot simultaneously possess a Calderbank-Shor-Steane (CSS) stabilizer and correct an arbitrary single-qubit error. A corollary of this result is that the Steane seven-qubit code is the smallest single-error correcting CSS code. Our second example is the construction of a non-degenerate six-qubit CSS entanglement-assisted code. This code uses one bit of entanglement (an ebit) shared between the sender and the receiver and corrects an arbitrary single-qubit error. The code we obtain is globally equivalent to the Steane seven-qubit code and thus corrects an arbitrary error on the receiver\'s half of the ebit as well. We prove that this code is the smallest code with a CSS structure that uses only one ebit and corrects an arbitrary single-qubit error on the sender\'s side. We discuss the advantages and disadvantages for each of the two codes.
    • Video
  • Electrical Manipulation of Donor Spin Qubits in Silicon and Germanium
    Many proposals for quantum information devices rely on electronic or nuclear spins in semiconductors because of their long coherence times and compatibility with industrial fabrication processes. One of the most notable qubits is the electron spin bound to phosphorus donors in silicon, which offers coherence times exceeding seconds at low temperatures. These donors are naturally isolated from their environments to the extent that silicon has been coined a "semiconductor vacuum". While this makes for ultra-coherent qubits, it is difficult to couple two remote donors so quantum information proposals rely on high density arrays of qubits. Here, single qubit addressability becomes an issue. Ideally one would address individual qubits using electric fields which can be easily confined. Typically these schemes rely on tuning a donor spin qubit onto and off of resonance with a magnetic driving field. In this thesis, we measure the electrical tunability of phosphorus donors in silicon and use the extracted parameters to estimate the effects of electric-field noise on qubit coherence times. Our measurements show that donor ionization may set in before electron spins can be sufficiently tuned. We therefore explore two alternative options for qubit addressability. First, we demonstrate that nuclear spin qubits can be directly driven using electric fields instead of magnetic fields and show that this approach offers several advantages over magnetically driven spin resonance. In particular, spin transitions can occur at half the spin resonance frequency and double quantum transitions (magnetic-dipole forbidden) can occur. In a second approach to realizing tunable qubits in semiconductors, we explore the option of replacing silicon with germanium. We first measure the coherence and relaxation times for shallow donor spin qubits in natural and isotopically enriched germanium. We find that in isotopically enriched material, coherence times can exceed 1 ms and are limited by a single-phonon T1 process. At lower frequencies or lower temperatures the qubit coherence times should substantially increase. Finally, we measure the electric field tunability of donors in germanium and find a four order-of-magnitude enhancement in the spin-orbit Stark shift and confirm that the donors should be tunable by at least 4 times the electron spin ensemble linewidth (in isotopically enriched material). Germanium should therefore also be more sensitive to electrically driven nuclear magnetic resonance. Based on these results germanium is a promising alternative to silicon for spin qubits.
    • Document
  • Two-qubit gates with ion-implanted phosphorus donors in silicon
    In the era of classical computation, humanity has advanced greatly in scientific, economic and social aspects. As today’s computers continue to develop, a new type of processing power slowly emerges. Worldwide efforts are directed at harvesting the power of quantum mechanics to create a universal quantum processor, able to solve tasks impossible for even the most powerful classical supercomputers. Multiple, radically different architectures are being considered for quantum computation and it is not clear which one will prevail. This thesis takes advantage of the great expertise in phosphorus donor spin qubit technology, developed by researchers at UNSW Sydney, and aims at progressing the state of the art to the point of realizing high-fidelity two- qubit logic operations. In quantum computation, one must be mindful of all the noise sources that could affect the information processing. Prior to quantum logic experiments, this thesis uncovers an insight into the nuclear spin dynamics of the residual 29Si atoms in the vicinity of the qubit. The hypothesis of a controllable nuclear freezing is supported by experimental and modelling efforts, expanding the microscopic understanding of the coherence times of electron spin qubits. In this thesis, we demonstrate an embryonic realization of the exchange based resonant CROT two qubit gate, in an experiment on weakly coupled phosphorus donor electron spins. The nuclear spins of the 31P atoms provide a natural source of detuning for the two qubits. The coherent control of the target qubit is conditional on the control qubit spin state, thus implementing a CROT two qubit gate. Finally, an implementation of quantum logic between two 31P nuclear spin is demonstrated in a 2P cluster with an electron hyperfine coupled to both nuclear spins. Two data qubits are encoded on the nuclear spins and the electron spin is utilized as an ancilla qubit for quantum non demolition readout, and to mediate a two-qubit logic operation between the nuclei. We employ, for the first time in a semiconductor two-qubit system, gate set tomography for single and two qubit gate fidelity estimation and optimization.
    • Other
  • Photon-mediated interactions between semiconductor qubits
    The realization of a coherent interface between distant charge or spin qubits in semiconductor quantum dots is an open challenge for quantum information processing. Here we demonstrate both resonant and non-resonant photon-mediated coherent interactions between double quantum dot charge qubits separated by several tens of micrometers. We present clear spectroscopic evidence of the collective enhancement of the resonant coupling of two qubits. With both qubits detuned from the resonator, we observe exchange coupling between the qubits mediated by virtual photons. In both instances pronounced bright and dark states governed by the symmetry of the qubit-field interaction are found. Our observations are in excellent quantitative agreement with master equation simulations. The extracted two-qubit coupling strengths significantly exceed the linewidths of the combined resonator-qubit system. This indicates that this approach is viable for creating photon-mediated two qubit gates in quantum dot based systems.
    • Collection
  • Realization of Three-Qubit Quantum Error Correction with Superconducting Circuits
    Quantum computers promise to solve certain problems exponentially faster than possible classically but are challenging to build because of their increased susceptibility to errors. Remarkably, however, it is possible to detect and correct errors without destroying coherence by using quantum error correcting codes [1]. The simplest of these are the three-qubit codes, which map a one-qubit state to an entangled three-qubit state and can correct any single phase-flip or bit-flip error of one of the three qubits, depending on the code used [2]. Here we demonstrate both codes in a superconducting circuit by encoding a quantum state as previously shown [3,4], inducing errors on all three qubits with some probability, and decoding the error syndrome by reversing the encoding process. This syndrome is then used as the input to a three-qubit gate which corrects the primary qubit if it was flipped. As the code can recover from a single error on any qubit, the fidelity of this process should decrease only quadratically with error probability. We implement the correcting three-qubit gate, known as a conditional-conditional NOT (CCNot) or Toffoli gate, using an interaction with the third excited state of a single qubit, in 63 ns. We find 85 +/- 1% fidelity to the expected classical action of this gate and 78 +/- 1% fidelity to the ideal quantum process matrix. Using it, we perform a single pass of both quantum bit- and phase-flip error correction with 76 +/- 0.5% process fidelity and demonstrate the predicted first-order insensitivity to errors. Concatenating these two codes and performing them on a nine-qubit device would correct arbitrary single-qubit errors. When combined with recent advances in superconducting qubit coherence times [5,6], this may lead to scalable quantum technology.
    • Video
  • Overlapping Qubits
    An ideal system of n qubits has 2^n dimensions. This exponential grants power, but also hinders characterizing the system's state and dynamics. We study a new problem: the qubits in a physical system might not be independent. They can "overlap," in the sense that an operation on one qubit slightly affects the others. We show that allowing for slight overlaps, n qubits can fit in just polynomially many dimensions. (Defined in a natural way, all pairwise overlaps can be <= epsilon in n^{O(1/epsilon^2)} dimensions.) Thus, even before considering issues like noise, a real system of n qubits might inherently lack any potential for exponential power. On the other hand, we also provide an efficient test to certify exponential dimensionality. Unfortunately, the test is sensitive to noise. It is important to devise more robust tests on the arrangements of qubits in quantum devices.
    • Document
  • Dataset Accompanying: Simultaneous single-qubit driving of semiconductor spin qubits at the fault-tolerant threshold
    The promise of quantum information technology hinges on the ability to control large numbers of qubits with high fidelity. Quantum dots define a promising platform due to their compatibility with semiconductor manufacturing. Moreover, high-fidelity operations above 99.9\% have been realized with individual qubits \cite{Yoneda2018,Yang2019a,Hendrickx2021}, though their performance has been limited to 98.67\% when driving two qubits simultaneously \cite{Xue2019}. \textcolor{red}{Here we present single-qubit randomized benchmarking in a two-dimensional array of spin qubits, finding native gate fidelities as high as 99.992(1)\%. Furthermore, we benchmark single qubit gate performance while simultaneously driving two and four qubits. To do this, we develop a novel benchmarking technique called $N$-copy randomized benchmarking, designed for simple experimental implementation, while providing a good estimate of the simultaneous qubit gate fidelity. We find two- and four-copy randomized benchmarking fidelities as high as 99.905(8)\% and 99.34(4)\% respectively. We also find that two-copy benchmarking of next-nearest neighbour pairs can return fidelities within the error margin of their single qubit cases, indicating that cross talk can be highly local in the absence of an exchange interaction.} These characterizations of the single-qubit gate quality and the ability to operate simultaneously are crucial aspects for scaling up germanium based quantum information technology.
    • Dataset
  • Electrical two-qubit gates within a pair of clock-qubit magnetic molecules
    Enhanced coherence in HoW10 molecular spin qubits has been demonstrated by use of clock-transitions (CTs). More recently it was shown that, while operating at the CTs, it was possible to use an electrical field to selectively address HoW10 molecules pointing in a given direction, within a crystal that contains two kinds of identical but inversion-related molecules. Herein we theoretically explore the possibility of employing the electric field to effect entangling two-qubit quantum gates within a 2-qubit Hilbert space resulting from dipolar coupling of two CT-protected HoW10 molecules in a diluted crystal. We estimate the thermal evolution of T1, T2, find that CTs are also optimal operating points from the point of view of phonons, and lay out how to combine a sequence of microwave and electric field pulses to achieve coherent control within a switchable two-qubit operating space between symmetric and asymmetric qubit states that are protected both from spin-bath and from phonon-bath decoherence. This two-qubit gate approach presents an elegant correspondence between physical stimuli and logical operations, meanwhile avoiding any spontaneous unitary evolution of the qubit states. Finally, we found a highly protected 1-qubit subspace resulting from the interaction between two clock molecules.
    • Dataset
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