Session P30: Semiconductor Qubits - Communication and Hybridization

Circuit Quantum Electrodynamics with Semiconductor Quantum Dots

Research on semiconductor quantum dots has tremendously contributed to the understanding of the physics of individual charges and spins in a solid state environment. Typically, quantum dots are investigated by direct current transport measurements or using quantum point contacts for charge sensing. Instead, we have realized a novel device in which a semiconductor double quantum dot is dipole coupled to a GHz-frequency high-quality tranmission line resonator. This approach allows us to characterize the properties of the double dot by measuring both its dispersive and dissipative interaction with the resonator [1]. In addition to providing a new readout mechanism, this architecture has the potential to isolate the dots from the environment and to provide long distance coupling between spatially separated dots. These features are expected to improve the potential for realizing a quantum information processor with quantum dots as previously demonstrated for superconducting circuits making use of circuit quantum electrodynamics. \newline [1] T. Frey et al., arXiv:1108.5378v1 (2011)   

Dispersive microwave readout of a silicon double quantum dot

Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in several different architectures in solid state and atomic physics. The electronic states of a semiconductor quantum dot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of a semiconductor qubit as realized in silicon by coupling to a superconducting resonant circuit. We report progress on a novel design of a lateral double quantum dot with a unique accumulation gate that allows for control of the spatial location of the 2DEG on the device, allowing the lossy 2DEG to be decoupled from the resonator.   

Coupling a single charge in a nanowire double quantum dot to a high-Q superconducting microwave resonator

In the field of circuit quantum electrodynamics (cQED), superconducting microwave cavities have provided an important tool with which to probe single superconducting qubits and mediate interactions between distant qubits [1,2]. In view of applying the cQED architecture to semiconductor qubits [3,4], we have integrated a tunable InAs nanowire double quantum dot (DQD) into a high-Q ($>2000$), high frequency ($\sim6$ GHz) niobium microwave resonator. Our design enables a dipole coupling of $\sim40$ MHz between the DQD and cavity. We will present experimental results demonstrating how the microwave cavity can be used to explore coherence in nanowire DQDs and discuss prospects for achieving the strong coupling regime. \\[4pt] [1] A. Wallraff et al., Nature 431, 162 (2004)\\[0pt] [2] L. DiCarlo et al., Nature 467, 574 (2010)\\[0pt] [3] J.R. Petta et al., Science 309, 2180 (2005)\\[0pt] [4] K.D. Petersson et al., Phys. Rev. Lett. 105, 246804 (2010)   

Communication between spin qubits with microwave photons

We consider possibilities of quantum state transfer between spin qubits (e.g. electrons in quantum dots) utilizing a microwave transmission line and a tunable coupler. We outline the possibility for optimal quantum transfer between qubit nodes depending on qubit-resonator coupling strengths and tunability, and in the presence of imperfections. Implications of such systems to practical quantum computing in silicon and/or GaAs quantum dots are considered.   

Spin echo measurements for hybrid qubit systems

We have performed electron spin echo measurements on donor spins in silicon samples using micro resonator structures fabricated in thin superconducting niobium films on sapphire substrates. The ability to implement refocusing pulses is an important constituent towards the realization of spin memories for hybrid superconducting/spin qubit applications. The resonators consist of quarter wavelength sections of coplanar waveguide with one end shorted and the other end capacitively coupled to a common feed line. These devices are suitable for pulsed ESR experiments on a commercial spectrometer at microwave frequencies between 9 and 10GHz, and in-plane magnetic fields of about 0.35T, which is well below the critical field of the thin film superconductor. Samples are flip-chip mounted on the resonator, and the magnetic component of the microwave radiation that extends into the sample is used for ESR excitation and detection. The high filling factor due to the small resonator size and the high quality factors that can be obtained with superconductors lead to high sensitivity to small numbers of spins, making our devices an attractive alternative to conventional resonators for certain ESR applications. The low microwave power requirements are appealing for low temperature measurements.   

Long-distance spin-spin coupling via floating gates

The electron spin is a natural two level system that allows a qubit to be encoded. When localized in a gate defined quantum dot, the electron spin provides a promising platform for a future functional quantum computer. The essential ingredient of any quantum computer is entanglement---between electron spin qubits---commonly achieved via the exchange interaction. Nevertheless, there is an immense challenge as to how to scale the system up to include many qubits. Here we propose a novel architecture of a large scale quantum computer based on a realization of long-distance quantum gates between electron spins localized in quantum dots. The crucial ingredients of such a long-distance coupling are floating metallic gates that mediate electrostatic coupling over large distances. We show, both analytically and numerically, that distant electron spins in an array of quantum dots can be coupled selectively, with coupling strengths that are larger than the electron spin decay and with switching times on the order of nanoseconds.   

Quantum gates between non-neighboring spin qubits

In quantum circuits involving many qubits, we usually need to perform gates such as CNOT between qubits that are not proximal. For spin qubits, this requires intermediate gate operations because the exchange interaction is very short ranged. Here, we consider three quantum dots in a linear array. We explore the effective coupling between the two outer spins mediated by the central spin. By using the central spin as a ``bus", we show how to efficiently perform gates such as CNOT between the outer spins. We find that arbitrary two-qubit gates can be achieved by applying the bus operations repetitively, with additional single-qubit rotations.   

Resonant Adiabatic Passages of Spin Qubits

We study adiabatic quantum teleportation through a spin chain with XX and Heisenberg couplings. We show that an adiabatic quantum teleportation protocol on a three-spin chain can be mapped exactly onto two parallel and coherent adiabatic passage channels, one for each spin orientation. When the time evolution is non-adiabatic, the information transfer displays a series of resonances where perfect transmission fidelity is achieved. This resonant operation is both fast and robust, indicating a possible new route for implementing robust quantum gates between spin qubits.   

Quantum phase transitions in a spin bus system

A spin chain can be used as a quantum data bus to enable long distance interactions and to create multi-qubit entanglement between spin qubits. The operation of the spin bus strongly depends on its ground state properties. When the ground state changes abruptly, quantum phase transitions (QPTs) occur and affect the bus operation. Here, we describe the theory of QPTs induced by an external magnetic field in a Heisenberg spin chain which acts as a spin bus. We study the non-analytic behavior of the entanglement between qubits connected to the spin bus and its scaling properties near a quantum critical point. In some cases, we find the entangling properties actually \emph{grow} with the length of the chain. We also analyze the magnetically induced anisotropy and disorder effects on the effective interactions between qubits.   

Coupling semiconductor qubits to phonons via a nanomechanical phoniton system

We explore the possibility of strongly coupling semiconductor qubit states to nanomechanical resonators (phonons) in silicon. These systems may be relevant to qubit transduction schemes, as supporting technology for quantum information processing, for qubit characterization, and for quantum-enabled devices. Specifically, we consider systems where cavity phonons can interact with suitable qubit states in the 1-10 GHz (and higher) regime (tunable using strain, electric and/or magnetic fields). These results may be useful for several solid-state devices as well as being of interest to the optomechanics community.   

Phonon Mediated Off-resonant Quantum Dot-Cavity Interaction

Optically controlled quantum dot (QD) spins coupled to semiconductor microcavities constitute a promising platform for robust and scalable quantum information processing devices. In recent experiments on coupled QD optical cavity systems a pronounced interaction between the dot and the cavity has been observed even for detunings of many cavity linewidths. This interaction has been attributed to an incoherent cavity enhanced phonon-mediated scattering process and is absent in atomic systems. We demonstrate that despite its incoherent nature, this process preserves the signatures of coherent interaction between a QD and a strong driving laser, which may be observed via the optical emission from the off-resonant cavity. Under bichromatic driving of the QD, the cavity emission exhibits spectral features consistent with optical dressing of the QD transition, namely Rabi side-bands. These cavity emission measurements are more akin to absorption measurements of a strongly driven QD rather than resonance fluorescence measurements. In addition to revealing new aspects of the off-resonant QD-cavity interaction, this result provides a new, simpler means of coherently probing QDs and opens the possibility of employing off-resonant cavities to optically interface QD-nodes in quantum networks.   

Rabi-Vibronic resonance at large number of vibrational quanta

Rabi oscillations of a resonantly driven two-level system (qubit) which is linearly coupled to a cavity (vibrational mode) with frequency, $\omega_0$, much smaller than the driving frequency, are studied theoretically. We show that, for small coupling constant $\lambda \ll 1$, Rabi oscillations are strongly modified in the vicinity of the {\em Rabi-vibronic resonance} $\Omega_R=\omega_0$, where $\Omega_R$ is the Rabi frequency proportional to the amplitude of the driving field. The width of the resonance is shown to be $(\Omega_R-\omega_0) \sim \lambda^{4/3}\omega_0$, and is much larger than the polaronic frequency shift, $\lambda^2\omega_0$. We show that within the resonant domain of $\Omega_R$ the actual frequency of the Rabi oscillations exhibits bistable behavior as a function of $\Omega_R$. Most importantly, within the resonant domain, the oscillator is highly excited, which allows one to treat it classically. Decay of the Rabi oscillations due to losses in the cavity and spontaneous emission of two-level system are also studied.   

Photon heralded entanglement between radiatively mismatched matter qubits

Photon heralded entanglement usually requires well matched radiative characteristics of two matter-based qubits. In practice, we may want to entangle two solid-state or atomic qubits with different radiative lifetime, or to entangle a solid-state qubit with an atomic qubit that has significant mismatch in lifetime and transition frequency. We propose a protocol that can effectively shape the emitted photon pulses of two matter qubits to a common analytically known function by simply tuning the classical laser fields applied to each qubit. Entanglement fidelity and success rates are found to be promising under current experimental conditions.   

Fast optically-controlled two-qubit operation for cavity-coupled semiconductor quantum dots

Electron spin qubit systems based on charged InAs/GaAs quantum dots have demonstrated long coherence times and the capability of ultra fast single- and two-qubit operations in a configuration when two dots are tunnel-coupled. Designing fast two- and multi-qubit gates for spatially-separated quantum dots is currently an important challenge. We propose fast optically controlled design where a two-qubit gate is mediated by a photonic crystal cavity mode. The design addresses the challenge of scalability and does not require quantum dots to have the same energies. The proposed gate scheme is also compatible with available optically induced single qubit rotations.   

H1 Photonic Crystal Microcavities for Quantum Information

Semiconductor quantum dots coupled to photonic crystal microcavities show promise for quantum information processing in solid-state systems. For many applications, the cavity mode needs to have high extraction efficiency and unpolarized emission. Here we describe a possible implementation using the two orthogonally-polarized, spectrally-degenerate dipole modes of the H1 photonic crystal microcavity. By modifying the shape of the far-field profile, high collection efficiency from the H1 cavity modes can be achieved while maintaining a high cavity quality factor. We optimize and experimentally measure the far-field profiles of our cavities, which show good agreement with simulations. We also implement techniques to minimize the energy splitting of the two dipole modes due to fabrication imperfections, which are compatible with the far-field optimization.