Single electron spin in a semiconductor quantum dot (QD) and single photon wavepacket propagating in an optical waveguide are investigated as carriers of quantum bit (qubit) for information processing. Cavity quantum electrodynamics of the coupled system composed of charged QD, microcavity and waveguide provides a quantum interface for the interplay of stationary spin qubits and flying photon qubits via cavity assisted optical control. This interface forms the basis for a wide range of essential functions of a quantum network, including transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single photon wavepacket with arbitrarily specified shape. The cavity assisted optical process also made possible ultrafast initialization and QND readout of the spin qubit in QD. In addition, the strong optical nonlinearity of dot-cavity-waveguide coupled system enables phase gate and entanglement operation for flying single photon qubits in waveguides. The coherence of the electron spin is the wellspring of these quantum applications being investigated. At low temperature and strong magnetic field, the dominant cause of electron spin decoherence is the coupling with the interacting lattice nuclear spins. We present a quantum solution to the coupled dynamics of the electron with the nuclear spin bath. The decoherence is treated in terms of quantum entanglement of the electron with the nuclear pair-flip excitations driven by the various nuclear interactions. A novel nuclear interaction, mediated by virtue spin-flips of the single electron, plays an important role in single spin free-induction decay (FID). The spin echo not only refocuses the dephasing by inhomogeneous broadening in ensemble dynamics but also eliminates the decoherence by electron-mediated nuclear interaction. Thus, the decoherence times for single spin FID and ensemble spin echo are significantly different. The quantum theory of decoherence also leads to a method of coherence recovery of the electron by disentanglement, realized through maneuvering the nuclear bath evolution by control of the electron spin-flip. The studies form the basis to outline the construction of a solid-state quantum network for scalable and distributed processing of quantum information
Single electron spin in a semiconductor quantum dot (QD) and single photon wavepacket propagating in an optical waveguide are investigated as carriers of quantum bit (qubit) for information processing. Cavity quantum electrodynamics of the coupled system composed of charged QD, microcavity and waveguide provides a quantum interface for the interplay of stationary spin qubits and flying photon qubits via cavity assisted optical control. This interface forms the basis for a wide range of essential functions of a quantum network, including transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single photon wavepacket with arbitrarily specified shape. The cavity assisted optical process also made possible ultrafast initialization and QND readout of the spin qubit in QD. In addition, the strong optical nonlinearity of dot-cavity-waveguide coupled system enables phase gate and entanglement operation for flying single photon qubits in waveguides. The coherence of the electron spin is the wellspring of these quantum applications being investigated. At low temperature and strong magnetic field, the dominant cause of electron spin decoherence is the coupling with the interacting lattice nuclear spins. We present a quantum solution to the coupled dynamics of the electron with the nuclear spin bath. The decoherence is treated in terms of quantum entanglement of the electron with the nuclear pair-flip excitations driven by the various nuclear interactions. A novel nuclear interaction, mediated by virtue spin-flips of the single electron, plays an important role in single spin free-induction decay (FID). The spin echo not only refocuses the dephasing by inhomogeneous broadening in ensemble dynamics but also eliminates the decoherence by electron-mediated nuclear interaction. Thus, the decoherence times for single spin FID and ensemble spin echo are significantly different. The quantum theory of decoherence also leads to a method of coherence recovery of the electron by disentanglement, realized through maneuvering the nuclear bath evolution by control of the electron spin-flip. The studies form the basis to outline the construction of a solid-state quantum network for scalable and distributed processing of quantum information
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.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states. In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system. These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.
A cavity coupling, a charged nanodot, and a fiber can act as a quantum interface, through which a stationary spin qubit and a flying photon qubit can be interconverted via a cavity-assisted Raman process. This Raman process can be made to generate or annihilate an arbitrarily shaped single-photon wave packet by pulse shaping the controlling laser field. This quantum interface forms the basis for many essential functions of a quantum network, including sending, receiving, transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single-photon wave packet with arbitrarily specified shape and average photon number. Numerical study of errors from noise and system parameters on the operations shows high fidelity and robust tolerance.
Control of electron spin decoherence in contact with a mesoscopic bath of many interacting nuclear spins in an InAs quantum dot is studied by solving the coupled quantum dynamics. The nuclear spin bath, because of its bifurcated evolution predicated on the electron spin up or down state, measures the which-state information of the electron spin and hence diminishes its coherence. The many-body dynamics of the nuclear spin bath is solved with a pair-correlation approximation. In the relevant timescale, nuclear pair-wise flip–flops, as elementary excitations in the mesoscopic bath, can be mapped into the precession of non-interacting pseudo-spins. Such mapping provides a geometrical picture for understanding the decoherence and for devising control schemes. A close examination of nuclear bath dynamics reveals a wealth of phenomena and new possibilities of controlling the electron spin decoherence. For example, when the electron spin is flipped by a π-pulse at τ, its coherence will partially recover at as a consequence of quantum disentanglement from the mesoscopic bath. In contrast to the re-focusing of inhomogeneously broadened phases by conventional spin-echoes, the disentanglement is realized through shepherding quantum evolution of the bath state via control of the quantum object. A concatenated construction of pulse sequences can eliminate the decoherence with arbitrary accuracy, with the nuclear–nuclear spin interaction strength acting as the controlling small parameter.
The ultimate goal of quantum information science is to build quantum networks for distributed quantum computing or for the secure sharing of information between spatially remote parties [1]. Quantum networks utilize static (matter) quantum bits (qubits) to store and process quantum information at local nodes, and photons as flying qubits for long-distance quantum state transmission between different nodes. To realize a quantum network, it is crucial to achieve light-matter entanglement and reversible quantum-state transfer between light and matter, i.e., the light-matter quantum interface, and the quantum repeater for large-scale quantum communications [2]. Recent experiments have shown that both electrons and holes confined in semiconductor quantum dots (QDs) have long spin relaxation time (T1e, T1h ~ ms) and long spin coherence time (T2e ~ s, T2h > 100 ns). Moreover, fast spin cooling and ultra-fast spin manipulation as well as spin echoes to preserve the spin coherence have also been demonstrated. Undoubtedly these rapid progresses imply that the QD spin is a good candidate for matter qubit in quantum information processing. Furthermore, QD-based single photon sources have been also developed. Therefore, semiconductor QDs offer a good platform for solid-state quantum networking. Here we present two types of conditional quantum gates, i.e., the photon-spin entangling gates [3-4] using a single QD spin in a single-sided or double-sided optical microcavity. Both gates are universal and deterministic (if they are optimized). The spin-selective coherent photon-spin interaction enhanced by the cavity QED lead to giant circular birefringence, which allows us to build these gates. We will show some of the key applications [3-5], including: (i) single-shot quantum non-demolition measurement of spin; (ii) single-photon based spin manipulations and spin echoes to preserve the spin coherence; (iii) photon-spin, spin-spin and photon-photon entanglement generation with high fidelity and high efficiency; (iv) deterministic photon-spin interface and perfect spin memory; (v) complete and loss-resistant Bell-state analyzer, which could increase the distance of state-of-the-art quantum communications by one order of magnitude (from current 100 km to over 1000 km) [5]; (vi) the full quantum repeater combining quantum memory, entanglement swapping and purifications for arbitrary long distance communications [5]; (vii) spin-controlled single photon sources. All these schemes are feasible with current semiconductor technology [6], and we have recently seen conditional phase shifts in uncharged QD-cavity systems [7]. The versatile spin-cavity systems can be applied in all aspects of quantum information science and technology, not only for large-scale quantum communication networks, but also for scalable quantum computing with either photons or spins as qubits. References [1] H.J. Kimble, Nature (London) 453, 1023 (2008). [2] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998). [3] C.Y. Hu et al., Phys. Rev. B 78, 085307 (2008); ibid. 78, 125318 (2008). [4] C.Y. Hu et al., Phys. Rev. B 80, 205326 (2009). [5] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [6] S. Reitzenstein et al., Appl. Phys. Lett. 90, 251109 (2007). [7] A.B. Young et al., ArXiv: quant-ph 1011.0384 (2010).
We propose a dot-cavity-waveguide coupled system as a quantum interface for stationary spin qubits and flying photon qubits. The exact solution to the dynamics of the interface is given which allows the generation or reception of an arbitrarily designated wave-packet of a single photon. Such control can be used for realizing operations such as quantum state transfer, entanglement swapping or deterministic entanglement creation in a quantum network. The interface can also be used as a deterministic source and an efficient detector of a single photon wavepacket with an arbitrarily specified shape and average photon number.