Science

Double Quantum Dot as a Quantum Bit

David Peter Divincenzo

Published Sep 29, 2005• 85 citations

Abstract

Quantum dots are microscopic structures that can trap single electrons for study and manipulation. Such devices are of interest as potential logic elements in a future quantum computer in which electron spins are used as quantum bits. The problem is that the spins on the nuclei of the device material itself can interfere with the electron spins. In his Perspective, [DiVincenzo][1] discusses results reported in the same issue by [ Petta et al. ][2] in which a double quantum dot structure was used to avoid the effect of the background spins. By swapping the quantum spin states back and forth between the dots, the researchers were able to greatly increase the quantum coherence time of the stored states. The results show that fully electric (as opposed to magnetic) control of the spin states offers the possibility of using such double dots in a quantum computer. [1]: http://www.sciencemag.org/cgi/content/full/309/5744/2173 [2]: http://www.sciencemag.org/cgi/content/short/309/5744/2180

DOI: 10.1126/science.1118921

Related papers

Coupling Qubits by Waves on the Electron Sea

L. I. Glazman, R. C. Ashoori

There is great interest in exploiting the unique behavior of quantum systems for computation, but building real devices has proved difficult. One approach is to use electron spin to represent the qubits (quantum bits) in a computational device. In their Perspective, [Glazman and Ashoori][1] discuss results reported in the same issue by [ Craig et al .][2] on using quantum dots as quantum logic elements. The dots act as artificial atoms, with the number of electrons on each subject to control. The electron spins in a pair of quantum dots interact with each other, and the Kondo effect enables detection of the state of the spin pair. This ability to control and detect the interaction between the spins may one day find use in quantum computation. [1]: http://www.sciencemag.org/cgi/content/full/304/5670/524 [2]: http://www.sciencemag.org/cgi/content/short/304/5670/565

ScienceApr 22, 200429 citations

Controlling single qubit gate fidelity in double quantum dots

J. Mosakowski, T. Ferrus, D. A. Williams +3 more

Quantum computation offers ways of solving problems which are unreachable for classical computers [1]. Double quantum dots are of great interest as candidates for implementing solid state quantum bits (qubits) because of the long electron coherence times. The use of geometrically isolated structures (with the detectors coupled capacitively) improves the electrical isolation of the dots, however, it makes the detection more challenging [2].

Silicon Nanoelectronics Workshop (SNW)Jun 1, 20140 citations

Achieving quantum computation with quantum dot spin qubits

Sebastian Mehl, David Peter Divincenzo

A two-level quantum system is the building block of a quantum computer. This pair of quantum states defines the computational unit of a quantum computer, and it is called a quantum bit or qubit in analogy to the bit that is the binary unit of a classical computer. The spin of a single electron naturally defines such a two-level quantum system. A quantum dot can be tuned to the single-electron regime, and an array of singly occupied quantum dots provides a multi-qubit register. This thesis describes the multielectron encoding of a spin qubit using a pair or a trio of quantum dots, and it analyzes the coherence properties and manipulation protocols of this system. The singlet-triplet qubit, coded using the singlet state and the spinless triplet state of a pair of singly occupied quantum dots, can be controlled all electrically when applying voltages to gates close to the quantum dot structure. Rapid, subnanosecond modifications of the double quantum dot's charge configuration have been realized successfully. A small magnetic field gradient across the double quantum dot enables universal qubit control. Two modifications of the normal singlet-triplet encoding are introduced. (1) The six-electron configuration of a double quantum dot encodes a singlet-triplet qubit in the same way as for the two-electron double quantum dot. Two electrons at each quantum dot are irrelevant for the qubit manipulations because they are paired in a singlet state; the remaining two electrons encode the qubit. The qubit's wave function is immune to charge noise at moderate out-of-plane magnetic fields. (2) A singlet-triplet qubit encoded using two quantum dots of different sizes has an orbital state degeneracy of the singlet state and the spinless triplet state at finite out-of-plane magnetic fields. Spin-orbit interactions lift this state degeneracy, while the magnitude of the state coupling is determined by the size difference of the QDs. This setup enables the manipulations of singlet-triplet qubits without the need for magnetic field gradients. Finally, two-qubit gates between singlet-triplet qubits are proposed that use mediated exchange interactions via one quantum state. These operations are well controlled and highly noise insensitive. Orbital interactions alone can control spin qubits coded in a three electron Hilbert space. For example, the exchange-only qubit is encoded using three singly occupied quantum dots. The exchange interactions of two quantum dot pairs need to be modified to manipulate this qubit. The noise sensitivity of the exchange-only qubit is discussed. Alternatively, a three-electron qubit at a double quantum dot can be operated when single electrons are transferred between the quantum dots. Fast, subnanosecond manipulations of the double quantum dot's charge configuration are required to realize single-qubit gates. A novel two-qubit pulse gate for the three-electron double quantum dot qubit is proposed.

Jan 1, 20140 citations

Fingerprinting Spin Qubits

J. Carlos Egues

Electrons not only have mass and charge, they also have magnetic properties directly related to their intrinsic spin. These spins can combine into quantum states of different spin parity, and such states may be useful as qubits in future quantum computers. As [Egues][1] discusses in his Perspective, [Engel and Loss][2] report in this issue a method for fingerprinting the spin states of electrons contained in quantum dot structures. By allowing the electrons to leak from one dot to another, and then using a nanowire to sense the presence of the spin states, the authors were able to perform a nondestructive spin-parity measurement. Such a spin-parity detector should permit the manipulation of quantum dot qubits for quantum computation. [1]: http://www.sciencemag.org/cgi/content/full/309/5734/565 [2]: http://www.sciencemag.org/cgi/content/short/309/5734/586

ScienceJul 22, 20056 citations

Multi-bit dark state memory: Double quantum dot as an electronic quantum memory

Eran Aharon, Roni Pozner, Efrat Lifshitz +1 more

Quantum dot clusters enable the creation of dark states which preserve electrons or holes in a coherent superposition of dot states for a long time. Various quantum logic devices can be envisioned to arise from the possibility of storing such trapped particles for future release on demand. In this work, we consider a double quantum dot memory device, which enables the preservation of a coherent state to be released as multiple classical bits. Our unique device architecture uses an external gating for storing (writing) the coherent state and for retrieving (reading) the classical bits, in addition to exploiting an internal gating effect for the preservation of the coherent state.

Journal of Applied PhysicsDec 28, 20165 citations

Semiconductor quantum dots for electron spin qubits

Wilfred G. van der Wiel, Michael P. Stopa, Tetsuo Kodera +2 more

We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.

New Journal of PhysicsFeb 21, 200637 citations