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SCIENTIA SINICA Informationis, Volume 47, Issue 10: 1255-1276(2017) https://doi.org/10.1360/N112017-00118

Quantum computation based on semiconductor quantum dots

More info
  • ReceivedMay 20, 2017
  • AcceptedJun 30, 2017
  • PublishedOct 16, 2017

Abstract

Quantum computing is the outcome of the size of semiconductor chips breaking the limits of classical physics, and is a landmark technology in the post-Moore's law era. Making use of the quantum properties of electrons in semiconductor quantum dots is believed to be one of the most promising candidates for the realization of quantum computing. In recent years, a series of breakthroughs have been made, including the preparation and readout of qubits, and the manipulation of quantum logic gates. This paper first introduces the background and significance of research on semiconductor quantum dot-based quantum computing and then provides an overview of the developments regarding spin-, charge-, and few-electron-based qubits, as well as the long distance coherent coupling of qubits. Finally, we discuss future trends in semiconductor-based quantum computing.


Funded by

国家重点研发计划(2016YFA0301700)

国家自然科学基金(61674132,11674300)

国家杰出青年基金(11625419)

中国科学院战略性先导科技专项(B)(XDB01030100)

中央高校基本科研业务费专项资金


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  • Figure 1

    (Color online) The schematic view of a semiconductor quantum dot. (a) Surface gates are used to deplete electrons and define a double quantum dot in the 2DEG formed in the AlGaAs/GaAs heterostructure; (b) scanning electron microscope (SEM) image of a GaAs/AlGaAs double quantum dots integrated with two quantum point contacts

  • Figure 2

    (Color online) Schematic diagrams for spin-charge conversion. In the magnetic field, the spin-down electron has a higher energy than spin-up electron. If we put the fermi level in between the spin-up level and the spin-down level, the spin-down electron can tunnel back to reservoir while the spin-up one cannot. As a result, we can read out the spin direction by discriminating whether there is a current

  • Figure 3

    (Color online) The sequential flow of electrons at spinblockade using ESR (modified from [48]). This cycle can be described via the occupations (NL, NR) of the left and right dots as $(1,0)$-$(1,1)$-$(2,0)$-$(1,0)$. Starting from $(1,0)$, an electron tunnels from the source to form the $(1,1)$T+ triplet state. The electron in the right dot cannot tunnel to the left dot because of the Pauli exclusion principle and transport is blocked. With ESR, the electron on the right dot can tunnel to the left dot to form the (2,0) state. One of the two electrons then tunnels out to the drain to complete the cycle, yielding a finite leakage current

  • Figure 4

    (Color online) Visualized theoretical and experimental truth tables for a SWAP gate by Vandersypen's group (modified from [48]). The SWAP gate means the information of two input qubits are exchanged after the gate operation, and the experimental data shows that the spins are only partly exchanged, which is due to local nuclear fields

  • Figure 5

    (Color online) Schematics of (a) a doped heterostructure and (b) an undoped heterostructure. In the doped herterostructure, the electrons come from the doping layer and in low temperature, current can be formed automatically, while in the undoped heterostructure, a top gate is needed to from 2DEG (modified from [53])

  • Figure 6

    (Color online) False color image of the two devices whose single qubit fidelity reaches 99% by (a) Vandersypen's group (modified from [8]) and (b) Tarucha's group (modified from [9]). Both devices were fabricated on undoped Si/SiGe heterostructure with splitting gates, the locations of the dots are denoted by the circles in the image, respectively

  • Figure 7

    (Color online) Silicon two-qubit logic device fabricated by Dzurak's group, incorporating single electron transistor (SET) readout and selective qubit control (modified from [57]). Schematic (a) and SEM colored image (b) of the device; (c) stability diagram of the double quantum dot obtained by monitoring the current $I_{SET}$ through the capacitively coupled SET; (d) clear Rabi oscillations for both qubits are observed

  • Figure 8

    (Color online) Two different proposals for silicon based quantum computing. (a) Physical architecture to operate one unit module (modified from [11]), this device starts with an SOI wafer, where the top layers host the classical circuitry with bit lines and word lines, the isotopically enriched silicon-28 bottom layer holds the quantum circuit, and these are interconnected via floating gates. (b) Schematic diagram of the donor-dot array structure (modified from [10]). The combination of top gates and holes in the depletion gate form the quantum dots, half of which are occupied with data qubit electrons. These electrons can be moved to dots positioned above the donor measurement qubits, each with a back gate to control the exchange coupling between the donor electron and the electron in the quantum dot

  • Figure 9

    (Color online) Two devices for hole spin qubits. (a) Simplified schematic of a silicon-on-insulator nanowire field-effect transistor with two gates by Franceschi's group (modified from [27]); (b) top view of an ambipolar silicon quantum dot device with its two different working modes: electron mode and hole mode (modified from [63])

  • Figure 10

    (Color online) SEM image of milestones of the DQD devices for electron charge qubits. (a) The device by Hirayama's first realized few-electron qubits (modified from [64]); (b) the device by Petta's group first realized single electron charge qubits (modified from [65]); (c) the device made by Petta's group first estimated T$_2$ as high as 7 ns (modified from [66]); (d) the device fabricated by Guo's group for universal manipulation of single charge qubits (modified from [28])

  • Figure 11

    (Color online) Schematics of the devices for two charge qubit gate. (a) and (b) are the device fabricated by Guo's group on GaAs quantum dot and the CNOT gate illustration on it (modified from [29]); (c) the two-qubit device made by Erikkson's group on Si/SiGe quantum dot (modified from [31])

  • Figure 12

    (Color online) The control cycle for Singlet-Triplet qubits (modified from [35]). Experiments generally consists of preparation, singlet separation, evolution of various kinds, and projection onto the (0,2) singlet state (measurement). Projective measurement is based on the spin-blockaded transition of T states onto $(0,2)$S, whereas S states proceed freely, allowing S to be distinguished from T by the charge sensor during the measurement step

  • Figure 13

    (Color online) Device design fabricated by Hughes Research Laboratories (modified from [36]). (a) Device cross-section showing undoped heterostructure, dielectric and gate stack; (b) SEM image of actual device before dielectric isolation and field gate deposition, the two-electron $(1,1)$ state is superimposed on the micrograph

  • Figure 14

    (Color online) Energy levels as a function of detuning for the lowest energy states for exchange qubit (modified from [77]). The red and blue levels form the logical subspace inside 111, with the logical states $|$0$\rangle$ and $|$1$\rangle$ denoted at the detuning at which they are the eigenstates of the system

  • Figure 15

    (Color online) Schematics of quantum dots for exchange qubits by (a) Marcus's group (modified from [38]) and (b) Hughes Research Laboratories (modified from [39]). (a) was made by traditional splitting gates in GaAs quantum dots and (b) was made by overlapping gates in Si/SiGe quantum dots

  • Figure 16

    (Color online) Schematics for the hybrid qubits (modified from [80]). (a) Introducing tunneling between the dots induces transitions between $|$0$\rangle$ and $|$1$\rangle$. Starting from $|$0$\rangle$, in which the electrons in the left dot are in a singlet, if an electron tunnels from the left dot to the right dot, and then the other electron tunnels back to the left dot, the spins in the left dot will end up in a triplet. The actual process yields transitions between $|$0$\rangle$ and $|$1$\rangle$. (b) and (c) are schematics illustrating independent tuning of the coupling between the electron in the singly occupied dot and the singlet and triplet states in the doubly occupied dot via the barrier height and relative energies in the two dots

  • Figure 17

    (Color online) Schematics of different devices by (a) Kontos's group (modified from [89]), (b) Petta's group (modified from [90]) and (c) Wallraff's group (modified from [91]) for realizing the strong coupling between the quantum dot and cavity

  • Figure 18

    (Color online) Schematics of various methods for long distance coupling. (a) The quantum dots are connected by a 4 $\mu$m channel and electrons are transported by utlizing surface acoustic waves (modified from [93]); (b) the quantum dots are separated by 6 $\mu$m, and the coupling is realized by an Aharonov-Bohm ring connected to two-channel wires (modified from [94]); (c) the long distance coupling is realized by virtual occupation of discrete states of quantum dots located between the distant dots (modified from [41,43]); (d) two semiconducting quantum dots are tunnel coupled to a two-dimensional superconducting film (modified from [95])

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