Shruti Puri

Resonance Fluorescence from an Artificial Atom in Squeezed Vacuum

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.

Engineering the quantum states of light in a Kerr-nonlinear resonator by two-photon driving

Photonic cat states stored in high-Q resonators show great promise for hardware efficient universal quantum computing. We propose an approach to efficiently prepare such cat states in a Kerr-nonlinear resonator by the use of a two-photon drive. Significantly, we show that this preparation is robust against single-photon loss. An outcome of this observation is that a two-photon drive can eliminate undesirable phase evolution induced by a Kerr nonlinearity. By exploiting the concept of transitionless quantum driving, we moreover demonstrate how non-adiabatic initialization of cat states is possible. Finally, we present a universal set of quantum logical gates that can be performed on the engineered eigenspace of such a two-photon driven resonator and discuss a possible realization using superconducting circuits. The robustness of the engineered subspace to higher-order circuit nonlinearities makes this implementation favorable for scalable quantum computation.Quantum Computing: An engineered quantum box for Schrodinger’s catEfficient preparation of the so called photonic Schrodinger’s cat states, is possible by engineering the energy levels in a quantum box. We propose an approach for fast, high-fidelity preparation and manipulation of cat states in a nonlinear cavity by the use of a parametric drive. This preparation is robust against single-photon loss from the cavity and as we show, can be easily realized using superconducting circuits. The robustness of the engineered subspace to higher-order circuit nonlinearities makes this implementation favorable and of immediate practical importance for realization of a scalable, hardware efficient platform for universal quantum computation. Our scheme offers significant improvements over existing techniques for preparation of cat states which become increasingly burdensome because of noise and for large sized cats.

Two-qubit geometric phase gate for quantum dot spins using cavity polariton resonance

We describe a design to implement a two-qubit geometric phase gate, by which a pair of electrons confined in adjacent quantum dots are entangled. The entanglement is a result of the Coulomb exchange interaction between the optically excited exciton-polaritons and the localized spins. This optical coupling, resembling the electron-electron Ruderman-Kittel-Kasuya-Yosida (RKKY) inter- actions, offers high speed, high fidelity two-qubit gate operation with moderate cavity quality factor Q. The errors due to the finite lifetime of the polaritons can be minimized by optimizing the optical pulse parameters (duration and energy). The proposed design, using electrostatic quantum dots, maximizes entanglement and ensures scalability.

Quantum annealing with all-to-all connected nonlinear oscillators

Quantum annealing aims to solve combinatorial optimization problems mapped on to Ising interactions between quantum spins. A critical factor that limits the success of a quantum annealer is its sensitivity to noise, and intensive research is consequently focussed towards developing noise-resilient annealers. Here we propose a new paradigm for quantum annealing with a scalable network of allto-all connected, two-photon driven Kerr-nonlinear resonators. Each of these resonators encode an Ising spin in a robust degenerate subspace formed by two coherent states of opposite phases. The fully-connected optimization problem is mapped onto local fields driving the resonators, which are themselves connected by local four-body interactions. We describe an adiabatic annealing protocol in this system and analyze its performance in the presence of photon loss. Numerical simulations indicate substantial resilience to this noise channel, making it a promising platform for implementing a large scale quantum Ising machine. Finally, we propose a realistic implementation of this scheme in circuit QED.Quantum annealing aims at solving combinatorial optimization problems mapped to Ising interactions between quantum spins. Here, with the objective of developing a noise-resilient annealer, we propose a paradigm for quantum annealing with a scalable network of two-photon-driven Kerr-nonlinear resonators. Each resonator encodes an Ising spin in a robust degenerate subspace formed by two coherent states of opposite phases. A fully connected optimization problem is mapped to local fields driving the resonators, which are connected with only local four-body interactions. We describe an adiabatic annealing protocol in this system and analyse its performance in the presence of photon loss. Numerical simulations indicate substantial resilience to this noise channel, leading to a high success probability for quantum annealing. Finally, we propose a realistic circuit QED implementation of this promising platform for implementing a large-scale quantum Ising machine.

Initialization of a spin qubit in a site-controlled nanowire quantum dot

A fault-tolerant quantum repeater or quantum computer using solid-state spin-based quantum bits will likely require a physical implementation with many spins arranged in a grid. Quantum dots embedded in nanowires have been demonstrated to be able to store single charges, and be positioned in regular arrays. In this paper we show that it is possible to optically pump a spin trapped in a site-controlled nanowire quantum dot, resulting in high-fidelity spin-qubit initialization. This represents the first step towards establishing spins in nanowire quantum dots as quantum memories suitable for use in a large-scale, fault-tolerant quantum computer or repeater based on all-optical control of the spin qubits.

Single-shot quantum nondemolition measurement of a quantum-dot electron spin using cavity exciton-polaritons

We propose a scheme to perform single-shot quantum non-demolition (QND) readout of the spin of an electron trapped in a semiconductor quantum dot (QD). Our proposal relies on the interaction of the QD electron spin with optically excited, quantum well (QW) microcavity exciton-polaritons. The spin-dependent Coulomb exchange interaction between the QD electron and cavity polaritons causes the phase and intensity response of left circularly polarized light to be different to that of the right circularly polarized light, in such a way that the QD electrons spin can be inferred from the response to a linearly polarized probe. We show that, by careful design of a sample with coupled QD and QW, it is possible to eliminate spin-flip Raman transitions. Thus, a QND measurement of the QD electron spin can be performed within a few 10s of nanoseconds with fidelity

Universal logic gates for quantum-dot electron-spin qubits using trapped quantum-well exciton polaritons

\sim 99.9

High-fidelity resonator-induced phase gate with single-mode squeezing

%. This improves upon current optical QD spin readout techniques across multiple metrics, including speed and scalability.

Quantum annealing with a network of all-to-all connected, two-photon driven Kerr nonlinear oscillators

In this paper, we develop and analyze a hardware platform for a scalable quantum computer based on semiconductor quantum dot (QD) electron spin qubits and their Coulomb exchange interaction with microcavity quantum well (QW) exciton-polaritons. This approach is based on the framework of the previously proposed QuDOS architecture (Quantum Dots with Optically Controlled Spins) for surface code quantum computation. Despite the developments in techniques for implementing quantum non-demolition (QND) measurement and nearest-neighbor gates in the QD electron spin system, achieving the resource requirements for fault-tolerance still remains a challenge. In order to overcome this, our scheme relies on the indirect optical control of QD spins, resulting from the long-ranged Coulomb exchange interaction between the spin qubits and optically excited, spin-polarized, QW exciton-polaritons. We develop schemes for implementing a fast, high-fidelity, single qubit gate, a two-qubit geometric phase gate and single-shot QND measurement. The control mechanism, namely, the exchange interaction, enables the optical manipulation of single electron spin in Faraday geometry. Furthermore, we investigate various decoherence mechanisms critical for the robustness of the gate operations. This paper, introduces the first plausible polariton-based on-chip hardware platform that could support universal, fault-tolerant operations for implementing large-scale quantum algorithms.