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Science

A Coherent Single-Hole Spin in a Semiconductor

Daniel Brunner, Brian D. Gerardot, Paul A. Dalgarno, Gunter Wüst, K. Karraï, Nick Stoltz, Pierre M. Petroff, Richard John Warburton

Published Jul 2, 2009• 345 citations
Publisher

Abstract

A Hole New Approach Quantum dots can behave as artificial atoms, exhibiting a ladder of quantized energy levels with the number of electrons added to the dot being controllable. They are thus being extensively studied for application in the likes of quantum information processing strategies. However, the electrons interact with their environment and quickly lose their coherence properties. Brunner et al. (p. 70 ; see the Perspective by Kolodrubetz and Petta ) now show that if the charge of the dot is manipulated so that it is positive; that is, populated with a single hole, then the coherence properties of the dot can be extended. The strategy of using holes instead of electrons may provide a solution to the decoherence problem.

DOI
10.1126/science.1173684

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A Hole New Approach Quantum dots can behave as artificial atoms, exhibiting a ladder of quantized energy levels with the number of electrons added to the dot being controllable. They are thus being extensively studied for application in the likes of quantum information processing strategies. However, the electrons interact with their environment and quickly lose their coherence properties. Brunner et al. (p. 70 ; see the Perspective by Kolodrubetz and Petta ) now show that if the charge of the dot is manipulated so that it is positive; that is, populated with a single hole, then the coherence properties of the dot can be extended. The strategy of using holes instead of electrons may provide a solution to the decoherence problem.
Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μs-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λ-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.
Semiconductor quantum dots tightly confine excited electron-hole pairs, called excitons, resulting in discrete energy levels similar to those of single atoms. Transition energies in the visible or near-infrared make quantum dots suitable for many applications in quantum optics and quantum information science, but to take advantage of all the properties of quantum dot emission, it is necessary to excite them coherently which has been a great challenge due to background scattering of the excitation laser. This dissertation presents the first coherent control of a single quantum dot with observation of its resonance fluorescence and decoherence phenomena. Strong continuous-wave excitation causes the dot to undergo several Rabi oscillations before emitting. These are visible as oscillations in the first- and second-order correlation functions of the emission, and the quantum dot states are "dressed", resulting in a Mollow triplet in the emission spectrum. Some resonantly excited dots, in addition to resonance fluorescence, also emit light from excited states several meV higher in energy. Such up-conversion fits existing theories of decoherence but has never been directly observed before. The up-conversion intensity is shown to be described well by a fairly simple three-level model with single-phonon absorption. The coherent phenomena of resonance fluorescence and the decoherence due to up-conversion paint a dual picture of single quantum dots wherein they can sometimes be treated as an ideal two-level system, but their interactions with the host crystal can lead to many complex behaviors.
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
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.
Physics There are several different approaches to quantum information processing currently under development, including (but not exhaustively) superconducting circuits, nitrogen vacancy defects in diamond, and semiconductor quantum dots. Each one relies on the basic building block of a qubit, a two-level quantum system, on which the information can be stored and manipulated. In the semiconductor quantum dot approach, manipulating the spin of a single electron is promising because it has all the requirements: being fast, controllable and stable. A downside, however, has been the issue of scalability. Any useful quantum processor will require a system of many qubits, but getting quantum dots to talk to each other has presented a challenge. Addressing a preliminary point of this issue, Schaibley et al. show that the light (a single photon) emitted from the excited state of a charged quantum dot is entangled with the spin of the electron on the dot. The authors argue that by making the single photons emitted from other distinct quantum dots interact, it may be possible to then entangle the spins on these quantum dots and thus realize a scalable architecture. Phys. Rev. Lett. 110 , 167401 (2013).