Initialization Of Electron Spins Research Articles

Spin-Selective Resonant Tunneling Induced by Rashba Spin-Orbit Interaction in Semiconductor Nanowire

We consider a single electron confined within a quantum wire in a system of two electrostatically-induced QDs defined by nearby gates. The time-varying electric field, of single GHz frequency, perpendicular to the quantum wire, is used to induce the Rashba coupling and enable spin-dependent resonant tunneling of the electron between two adjacent potential wells with fidelity over 99.5%. This effect can be used for the high fidelity all-electrical electron-spin initialization or readout in the spin-based quantum computer. In contrast to other spin initialization methods, our technique can be performed adiabatically without increase in the energy of the electron. Our simulations are supported by a realistic self-consistent time-dependent Poisson-Schroedinger calculations.

Pulse-width-induced polarization enhancement of optically pumped N-V electron spin in diamond

The nitrogen-vacancy (N-V) center in diamond is a widely used platform for quantum information processing and sensing. The electron-spin state of the N-V center could be initialized, read out optically, and manipulated by resonate microwave fields. In this work, we analyze the dependence of electron-spin initialization on widths of laser pulses. We build a numerical model to simulate this process and to verify the simulation results in experiments. Both simulations and experiments reveal that shorter laser pulses are helpful to the electron-spin polarization. We therefore propose to use extremely short laser pulses for electron-spin initialization. In this new scheme, the spin-state contrast could be improved about 10% in experiments by using laser pulses as short as 4 ns in width. Furthermore, we provide a mechanism to explain this effect, which is due to the occupation time in the meta-stable spin-singlet states of the N-V center. Our new scheme is applicable in a broad range of N-V-based applications in the future.

All-electric single electron spin initialization

We propose a nanodevice for single-electron spin initialization. It is based on a gated planar semiconductor heterostructure with a quantum well and with potentials generated by voltages applied to local gates. Initially we insert an electron with arbitrary spin into the nanodevice. Next we perform a sequence of spin manipulations, after which the spin is set in a desired direction (e.g., the growth direction). The operations are done all-electrically, do not require any external fields and do not depend on the initial spin direction.

Fast and high-fidelity optical initialization of spin state of an electron in a semiconductor quantum dot using light-hole-trion states

We theoretically show that under the Faraday geometry fast and high-fidelity optical initialization of electron spin (ES) state in a semiconductor quantum dot (SQD) can be realized by utilizing the light-hole (LH)-trion states. Initialization is completed within the time scale of ten nanoseconds with high fidelity, and the initialization laser pulse can be linearly, right-circularly, or left-circularly polarized. Moreover, we demonstrate that the time required for initialization can be further shortened down to a few hundreds of picoseconds if we introduce a pillar-microcavity to promote the relaxation of a LH-trion state towards the desired ES state through the Purcell effect. We also clarify the role of heavy-hole and light-hole mixing induced transitions on the fidelity of ES state initialization.

Light- and heavy-hole trions bound to isoelectronic centers

Optical control strategies in semiconductor nanostructures have almost exclusively relied on heavy-hole exciton and trion states. Interestingly, optical access to both light- and heavy-hole trions may provide the missing resource for consolidating all single qubit operations in a single magnetic field configuration. In this work, we describe the conditions in which nearly degenerate light- and heavy-hole trions can be exploited for implementing electron spin initialization, control, and single-shot readout in systems of ${C}_{s}$ or higher symmetry. Then, we experimentally demonstrate that a previously unidentified nitrogen pair configuration in GaAs exhibiting ${C}_{s}$ symmetry binds both light- and heavy-hole excitons and negative trions. A detailed analysis of the energy and polarization of the emission allows determining the crystal-field and exchange coefficients, the electron, hole, and exciton Zeeman coefficients, and the light- and heavy-hole character of all states. This analysis also reveals that light- and heavy-hole trion states provide the $\mathrm{\ensuremath{\Lambda}}$ level structure necessary for initialization and control along with an energetically protected cycling transition compatible with single-shot readout.

Resonant optical control of the electrically induced spin polarization by periodic excitation

We show that the electron spin polarization generated by an electrical current may have its direction controlled and magnitude amplified by periodic optical excitation. The electrical and optical spin control methods were combined and implemented in a two-dimensional electron gas. By Kerr rotation in an external transverse magnetic field, we demonstrate unexpected long-lived coherent spin oscillations of the current-induced signal in a system with large spin-orbit interaction. Using a single linearly polarized pulse for spin manipulation and detection, we found a strong dependence on the pulse optical power and sample temperature indicating the relevance of the hole spin in the electron spin initialization. The signal was mapped in a Hall bar as function of the position relative to the injection contact. Finally, the presence of an in-plane spin polarization was directly verified by rotating the experimental geometry.

Single-shot initialization of electron spin in a quantum dot using a short optical pulse

We propose a technique to initialize an electron spin in a semiconductor quantum dot with a single short optical pulse. It relies on the fast depletion of the initial spin state followed by a preferential, Purcell-accelerated desexcitation towards the desired state thanks to a micropillar cavity. We theoretically discuss the limits on initialization rate and fidelity, and derive the pulse area for optimal initialization. We show that spin initialization is possible using a single optical pulse down to a few tens of picoseconds wide.

Proposal for high-speed and high-fidelity electron-spin initialization in a negatively charged quantum dot coupled to a microcavity in a weak external magnetic field

We describe a proposal for fast electron spin initialization in a negatively charged quantum dot coupled to a microcavity without the need for a strong magnetic field. We employ two-photon excitation to access trion states that are spin forbidden by one-photon excitation. Our simulation shows a maximum initialization speed of 1.3 GHz and maximum fidelity of 99.7% with realistic system parameters.

Spin-polarization dynamics in InGaAs quantum dots during pulsed electrical spin-injection

We investigate the fidelity of electron spin initialization in quantum dots utilizing nanosecond-pulsed electrical spin injection through a semimagnetic spin aligner in a spin light-emitting diode. At the onset of the electroluminescence signal, the circular polarization degree of the emitted light, corresponding to the spin polarization degree, is distinctively higher than under constant-current excitation. The observed spin-polarization dynamics are attributed to state filling effects. Additional contributions due to spin-flip mechanisms within the optically active region are identified.

Parallel preparation of highly spin-polarized electrons in single InAs∕GaAs quantum dots

Initialization of electron spins in semiconductor quantum dots (QDs) is a major prerequisite for a successful implementation of such QDs in quantum information applications. It is essential that the initialization is achieved for many individually separable dots in parallel. Here the authors show that exactly this can be accomplished with near-unity fidelity by electrical spin injection from the diluted magnetic semiconductor ZnMnSe into InAs∕GaAs quantum dots. The deviation from unity is smaller than 0.13, more precise determination is limited by the signal-to-noise ratio of their setup. They demonstrate the robust concurrent initialization of several quantum dots with the same high fidelity.

Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.

A spin solo

Quantum computers could solve problems insurmountable to conventional computers. The missing ingredient for quantum computing with electron spins is now available — the rotation of a single spin. One of the most promising approaches to realizing a practical quantum bit is to use spin states of single electrons in semiconductor quantum dots. A detailed proposal for such spin qubits was published about ten years ago; since then, all the necessary ingredients to realize it, such as long-lived spin states and the initialization of electron spins, have been reported one by one. Now the last, most difficult step has been accomplished, that of driving coherent spin oscillations for individual electrons, which is necessary to carry out quantum operations. This is achieved by applying oscillating magnetic fields that are resonant with the precession frequencies of the electron spins, generated on-chip near a double quantum dot system. All is now ready to design universal quantum logic gates.