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Transition metal dichalcogenide (TMD) semiconductor heterostructures are actively explored as a new platform for quantum optoelectronic systems. Most state of the art devices make use of insulating hexagonal boron nitride (hBN) that acts as a wide-bandgap dielectric encapsulating layer that also provides an atomically smooth and clean interface that is paramount for proper device operation. We report the observation of large, through-hBN photocurrents that are generated upon optical excitation of hBN encapsulated MoSe2 and WSe2 monolayer devices. We attribute these effects to Auger recombination in the TMDs, in combination with an asymmetric band offset between the TMD and the hBN. We present experimental investigation of these effects and compare our observations with detailed, ab-initio modeling. Our observations have important implications for the design of optoelectronic devices based on encapsulated TMD devices. In systems where precise charge-state control is desired, the out-of-plane current path presents both a challenge and an opportunity for optical doping control. Since the current directly depends on Auger recombination, it can act as a local, direct probe of both the efficiency of the Auger process as well as its dependence on the local density of states in integrated devices.

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use these insights to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.

Taking electrical control Excitons—bound pairs of electrons and holes in a solid—can, in principle, be used as information carriers. However, their lifetime is limited because the electrons and holes tend to quickly recombine. One way to extend this lifetime is to physically separate electrons and holes—for example, by having them reside in different layers of a van der Waals heterostructure. Jauregui et al. used this strategy to form long-lived interlayer excitons in a heterostructure made out of monolayers of molybdenum diselenide (MoSe 2 ) and tungsten diselenide (WSe 2 ). Through electrical control of the layers in the heterostructure, the researchers further increased exciton lifetime and formed and manipulated charged excitons. Science , this issue p. 870

Long-distance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)-wavelength photonic qubit. Electron spins in III-V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation and photon emission, but entanglement between a single quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantum-dot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates, the present technique advances the III-V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.

Electron spin in quantum dots are extensively studied as a qubit for quantum information processing. However, the coherence of electron spin is deleteriously influenced by nuclear spin. Quantum-dot holes are a potential alternative. Full control over hole-spin qubits is now achieved using picosecond lasers.

Many proposed photonic quantum networks rely on matter qubits to serve as memory elements1,2. The spin of a single electron confined in a semiconductor quantum dot forms a promising matter qubit that may be interfaced with a photonic network3. Ultrafast optical spin control allows gate operations to be performed on the spin within a picosecond timescale4,5,6,7,8,9,10,11,12,13,14, orders of magnitude faster than microwave or electrical control15,16. One obstacle to storing quantum information in a single quantum dot spin is the apparent nanosecond-timescale dephasing due to slow variations in the background nuclear magnetic field15,16,17. Here we use an ultrafast, all-optical spin echo technique to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors18 and repeater networks1,2. An ultrafast, all-optical spin echo technique is used to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors and repeater networks.

‘In this chapter, we introduced and analyze techniques that allow truly secure secret key sharing over long distances, using public, open channels, where the laws of quantum mechanics ensure the security of the long distance key sharing – an idea generally referred to as the essence of a quantum repeater. We describe several proof-of-principle experiments where technology based on self-assembled quantum dots is used as the backbone of a future quantum repeater.’

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.

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.

Entanglement between stationary quantum memories and photonic qubits is crucial for future quantum communication networks. Although high-fidelity spin-photon entanglement was demonstrated in well-isolated atomic and ionic systems, in the solid-state, where massively parallel, scalable networks are most realistically conceivable, entanglement fidelities are typically limited due to intrinsic environmental interactions. Distilling high-fidelity entangled pairs from lower-fidelity precursors can act as a remedy, but the required overhead scales unfavourably with the initial entanglement fidelity. With spin-photon entanglement as a crucial building block for entangling quantum network nodes, obtaining high-fidelity entangled pairs becomes imperative for practical realization of such networks. Here we report the first results of complete state tomography of a solid-state spin-photon-polarization-entangled qubit pair, using a single electron-charged indium arsenide quantum dot. We demonstrate record-high fidelity in the solid-state of well over 90%, and the first (99.9%-confidence) achievement of a fidelity that will unambiguously allow for entanglement distribution in solid-state quantum repeater networks.

Individual electron and hole quantum dot spin qubits can be coherently manipulated using picosecond modelocked laser pulses; an all-optical spin-echo was implemented that decouples slow environmental changes. While dephasing and decoherence mechanisms for electrons and holes are intrinsically different, similar qualitative results are obtained, except for dynamic nuclear polarization effects that affect the controllability of electrons. In addition, we demonstrate spin-photon entanglement in a charged InAs quantum dot, using an ultrafast downconversion technique that converts a single, spontaneously emitted photon at 900 nm into a 1560 nm photon with picosecond timing resolution. This ultrafast conversion technique allows quantum erasure of which-path frequency information in the spontaneous emission process.

In this chapter, we shall derive how ultrafast optical control can be used in combination with Larmor precession for full SU(2) control of a single quantum dot electron spin. Such ultrafast electron spin control was first reported in Refs. [1–3]. In combination with accurate timing control over the optical fields used to realize this SU(2) control, arbitrary pulse (control) patterns can be applied to the spin [3]. As we will show in Sect. 3.3.2, such pulse sequences can be used to overcome the effects of slowly varying Larmor precession due to variations in the spin’s solid state environment. As a particular example, we will study the effects of the hyperfine interaction between an electron spin and the nuclear spins inside the quantum dot [4–7] – the latter will be discussed in detail in Sect. 3.3.1.

While fast, complete SU(2) control of a long-lived qubit is an essential ingredient of any quantum information system, our focus on quantum communication systems, and quantum repeaters in particular, requires a means of interfacing the stationary, matter and memory qubits, with flying, photonic qubits. As we derived in Sects. 1.1.3.1 and 1.4, entanglement between a spin-qubit and a photonic qubit [1–5] provides exactly the type of interface required in order to generate remotely-entangled spin-pairs and implement the quantum teleportation and entanglement swapping schemes that are at the heart of quantum repeaters [6–11].

The strong contact hyperfine interaction between a single electron and the nuclear spins in a quantum dot [1–5] represents a severe limitation on its use as a qubit, as we discussed in Sect. 3.3.1. In particular, it results in non-linear, non-Markovian dynamics of the electron-nuclear system [6–9], making the use of the Larmor-precession as a single qubit gate cumbersome. While dynamical decoupling, in essence an extension of the simple spin echo presented in Sect. 3.3.2 to multiple refocusing pulses, could be used to decouple the effects of the nuclear spin bath [10–14], avoiding the contact hyperfine interaction altogether would be a much simpler solution. For some materials, this could be achieved by careful isotopic engineering: examples for group-IV and II–VI materials can be found in Refs. [15–19]. However, In, Ga and As do not possess any spin-0 isotopes, making this approach impossible for the self-assembled quantum dots used in this work.

In the previous chapters, detuned optical pulses were used to generate qubit rotations. While deriving the effect of the laser pulses on the spin, we generally ignored overall, global phases. However, in this chapter, we shall report on experiments were the global (geometric) phases of a (detuned) 2-level interaction can be visualized in Ramsey interferometry, by beating the phase against the Larmor precession. In the context of 2-qubit interactions, geometric phases play a crucial role for the realization of entangling gates [1, 2]; the realization of a fast, pulsed global phase for single quantum dot electron spins is essential for this task.

The quantum bits used in the remainder of this work, are individual electron (Chaps. 3–5 and 7) or hole spins (Chap. 6) in self-assembled quantum dots [1, 2]. Spins, either as direct spin-1/2 particles or as pseudospin-submanifolds of larger systems, are generally considered as good candidate-qubits due to their relatively limited interaction with the environment [3]. In addition, and as we shall show below, the confinement of the spin to quantum dots provides an additional protection of the spin degree of freedom.

Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. We implement a downconversion quantum interface, where the single photons emitted from a semiconductor quantum dot at 910 nm are downconverted to 1560 nm using a fiber-coupled periodically poled lithium niobate waveguide and a 2.2-$\mu$m pulsed pump laser. The single-photon character of the quantum dot emission is preserved during the downconversion process: we measure a cross-correlation $g^{(2)}(\tau = 0) = 0.17$ using resonant excitation of the quantum dot. We show that the downconversion interface is fully compatible with coherent optical control of the quantum dot electron spin through the observation of Rabi oscillations in the downconverted photon counts. These results represent a critical step towards a long-distance hybrid quantum network in which subsystems operating at different wavelengths are connected through quantum frequency conversion devices and 1.5-$\mu$m quantum channels.

We present a proposal for a quantum repeater which uses fault-tolerant quantum error correction to overcome decoherence limitations to creating long-distance entanglement between quantum dot spin qubits.

Electron spins in quantum dots under coherent control exhibit a number of novel feedback processes. Here, we present experimental and theoretical evidence of a feedback process between nuclear spins and a single electron spin in a single charged InAs quantum dot, controlled by the coherently modified probability of exciting a trion state. We present a mathematical model describing competition between optical nuclear pumping and nuclear spin-diffusion inside the quantum dot. The model correctly postdicts the observation of a hysteretic sawtooth pattern in the free-induction-decay of the single electron spin, hysteresis while scanning a narrowband laser through the quantum dot's optical resonance frequency, and non-sinusoidal fringes in the spin echo. Both the coherent electron-spin rotations, implemented with off-resonant ultrafast laser pulses, and the resonant narrow-band optical pumping for spin initialization interspersed between ultrafast pulses, play a role in the observed behavior. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse timing, potentially allowing more complex coherent control operations., Comment: 15 pages, 7 figures. Corrected and expanded discussion. Now includes analysis of spin-echo and optical pumping experiments, in addition to FID

We present results of angle-dependent magnetoresistance measurements and direct optical images of underdoped $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ which reveal partial detwinning by action of a 14 T magnetic field. The relative change in the twin domain population for the given field is modest, of order $5--15\text{ }\mathrm{%}$. The associated field-induced hysteretic changes in the resistivity are up to 5% for intermediate Co concentrations, implying a large in-plane resistivity anisotropy in the broken-symmetry state. Based on the generic anisotropic susceptibility of collinear antiferromagnets, we infer a smaller resistivity along the antiferromagnetic ordering direction. The observation of field-induced motion of twin boundaries indicates a substantial magnetoelastic coupling in this material.

The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate time exceeds 10 5 , suggesting strong promise for future quantum information processors.

We report the observation of a feedback process between the nuclear spins in a single charged quantum dot and its trion transition, driven by a periodic sequence of optical pulses. The pulse sequence intersperses off-resonant ultrafast pulses for coherent electron-spin rotation and resonant narrow-band optical pumping. The feedback manifests as a hysteretic triangle-like pattern in the free-induction-decay of the single spin. We present a simple, quasi-analytic numerical model to describe this observation, indicating that the feedback process results from the countering effects of optical nuclear pumping and nuclear spin-diffusion inside the quantum dot. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse timing, potentially allowing more complex coherent control operations., 4 pages, 3 figures

We report an ultrafast downconversion quantum interface, where 910-nm single photons from a quantum dot are downconverted to the 1.5-μm telecom band with sub-10 picosecond pulses at 2.2-μm, enabling the demonstration of quantum-dot spin-photon entanglement.