Session B26: Superconducting Circuits: Remote Entanglement and Waveguide QED

Particle production in ultrastrong-coupling waveguide QED

In recent years, the field of light-matter interaction has made a further stride forward with the advent of superconducting qubits ultra-strongly coupled to open waveguides. In this setting, the qubit becomes simultaneously coupled to many different modes of the waveguide, leading to a wealth of non-linear dynamical phenomena.

First, I will show how one can tackle the time-evolution of such a non-trivial system using a novel numerical technique based on an expansion of the full state vector in terms of multi-mode coherent states. Inspired by earlier semi-classical approaches, this numerically exact method provides an important advance compared to the state-of-the-art techniques that have been used so far to study the many-mode ultra-strong coupling regime.

I will then move on to present the central prediction of my work concerning the scattering of low-power coherent signals on a qubit. Most remarkably, I will show that the qubit non-linearity, transferred to the waveguide through the ultra-strong light-matter interaction, is able to split photons from the incoming beam into several lower-energy photons. This splitting leads to the emergence of a low-frequency continuum in the scattered power spectrum that dominates the inelastic signal.   

Waveguide-mediated interaction of artificial atoms in the strong coupling regime, part 1

Embedding of multiple quantum emitters to a common one-dimensional radiation channel gives rise to emergent collective effects: collective emission and long-range exchange interaction. Collective emission leads to super- and sub-radiant states while the signature of exchange interaction, coherent cooperative dynamics, has been obscured by the radiative decay into the one-dimensional channel. In this work, we utilize the sub-radiant state to trap the radiation, acting as an atomic cavity, and strongly couple a probe quantum emitter to the sub-radiant state, effectively creating an atom-cavity system [New J. Phys. 15, 063003 (2012)]. We implement the scheme with transmon qubits coupled to a microwave coplanar waveguide. We discuss the building blocks and design of the qubit-waveguide system, and the characterization of individual waveguide-coupled qubits which highlights the requirements for observing coherent cooperative dynamics. arXiv:1809.09752   

Waveguide-mediated interaction of artificial atoms in the strong coupling regime, part 2

Photon-mediated interactions of quantum emitters in a one-dimensional radiation channel leads to collective emission and long-range exchange interaction. Observation of coherent cooperative dynamics via such interactions, however, has been obscured by radiative decay into the one-dimensional channel. Here, we employ transmon qubits and a microwave coplanar waveguide as artificial atoms coupled to a one-dimensional channel. We circumvent the radiative decay problem by utilizing the entangled dark state of a qubit array arising from collective waveguide emission. The entangled dark state, with a suppressed decay rate, effectively traps radiation as an atomic cavity while exhibiting a large exchange interaction rate with a designated probe qubit. We report the observation of coherent cooperative dynamics in the strong coupling regime and characterize the coherence properties of the collective states involved in the dynamics. In addition, we discuss potential applications of this platform and practical challenges in such systems. arXiv:1809.09752   

Waveguide QED with a giant transmon

In a typical waveguide QED system, a quantum emitter, such as a superconducting transmon qubit, is coupled to a superconducting waveguide in order to study its interaction with the electromagnetic field. In these systems, the emitter is approximated as a point-like object when compared to the wavelength of light. However, recent experiments [1] demonstrated the ability to couple a transmon to surface acoustic waves (SAW) that have a wavelength much smaller than the transmon, thereby creating a “giant” artificial atom. Inspired by these results, there is a recent proposal [2] to couple a transmon at multiple points of a microwave waveguide, with wavelength-scale distances between each coupling point, also making a giant transmon. The proposal predicts the ability to custom design the relaxation rates of different transmon levels and proposes several interesting applications such as single-atom lasing and tunable coupling. We will present preliminary characterization of such a device.   

Studying collective effects in 3D waveguide QED with frequency and time-domain resolved spectroscopy

Collective behavior of coupled quantum emitters has been studied extensively theoretically starting from seminal Dicke’s paper. Experimentally however many of the theoretical predictions have never been checked since it requires sophisticated experimental techniques. The recently appeared platform of 3D cQED offers unique opportunities in controlling independent qubits and their respective couplings control as well as long coherence times. We experimentally realized a system, which allows to investigate frequency chirping in a Dicke physics model, as well as to study cooperative behavior against collective noise and parameters spread. We study the system dynamics in frequency, as well as time-domain and compare it to theoretical predictions.
  

Real-time detection of an itinerant microwave photon using dressed-state engineering

Several schemes for single microwave photon detection have been proposed and demonstrated lately in circuit quantum electrodynamics. However, all experimental demonstration to date are performed in the time-gated mode. In this presentation, we demonstrate a real-time detection of itinerant microwave photons. In our setup, a superconducting flux qubit is coupled to two resonators, which have substantial difference in the dispersive shifts. Under an adequate choice of the frequency and the power of the qubit drive, one resonator is used to form an impedance-matched Λ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. We observe quantum jump produced by an itinerant microwave photon and attain a single-photon-detection efficiency of ~0.35. The detection efficiency of this detector is limited by the relatively short qubit relaxation time.   

Quantum non demolition parity measurements of itinerant microwave fields

Capitalizing on recent demonstrations of quantum non demolition (QND) detection of individual itinerant microwave photons [1,2] we demonstrate parity measurements applied to input fields with photon number up to ten.

Defining a mode to be characterized by the temporal shape of a displacement field added onto it, we recover its Wigner function without making use of reconstruction or maximum likelihood methods. We present results of this phase space tomography method for propagating fields for various classical and quantum input fields.

The non demolition character of the measurement also allows for heralding highly non-classical states of light, such as multi-photon cat states, which are relevant for quantum information processing.

[1] Kono et al., Nat. Phys 14, 546-549 (2018)
[2] Besse et al., PRX 8, 021003 (2018)   

Violating Bell's Inequality with Remotely-Connected Superconducting Qubits

Quantum communication relies on the efficient generation of entanglement between remote quantum nodes, due to entanglement's key role in achieving and verifying secure communications. Remote entanglement has been realized using a number of different probabilistic schemes, but deterministic remote entanglement has only recently been demonstrated, using a variety of superconducting circuit approaches. However, the deterministic violation of a Bell inequality, a strong measure of quantum correlation, has not to date been demonstrated in a superconducting quantum communication architecture, in part because achieving sufficiently strong correlation requires fast and accurate control of the emission and capture of the entangling photons. Here we present a simple and robust architecture for achieving this benchmark result in a superconducting system.   

Microwave remote state preparation vs. quantum cryptography

Quantum communication protocols employ nonclassical correlations as a resource for an efficient transfer of quantum states [R. Di Candia et al., EPJ Quantum Technol. 2, 25 (2015)]. As a fundamental protocol, remote state preparation (RSP) aims at the preparation of a known quantum state at a remote location using classical communication and quantum entanglement. In our experiment, we use flux-driven Josephson parametric amplifiers and linear circuit elements to generate propagating two-mode squeezed (TMS) microwave states acting as quantum resource [K. G. Fedorov et al., Phys. Rev. Lett. 117, 020502 (2016); K. G. Fedorov et al., Sci. Rep. 8, 6416 (2018)]. Combined with a classical feedforward, we use these TMS states to remotely prepare single-mode squeezed states. Furthermore, we analyze the consumption of quantum discord in our experiment and interpret our results in the framework of a quantum cryptographic protocol analogous to the Vernam cipher.   

Generating Non-Classical and Spatially-Correlated Photons in a Waveguide QED Architecture

In waveguide quantum electrodynamics (wQED), atoms interact with a guided continuum of photonic modes. These systems can serve as a platform to study exotic radiation phenomena. The electrical distance between atoms along the waveguide is controllable by tuning the frequency, and thus the wavelength, of light emitted from the atoms. This enables the exploration of a wide variety of radiation phenomena. In this work, we experimentally study devices with multiple superconducting transmon qubits that are strongly coupled to a one-dimensional co-planar waveguide. We present experimental results demonstrating the generation of non-classical and spatially correlated photons in a wQED setting.   

Quantum Communication with Microwave Photons

Sharing information coherently between physically separated chips in a netwrok of quantum computers could be an essential element for realizing a viable quantum information processing system. A direct, deterministic quantum channel may be advantageous both for larger scale fault-tolerant or non-error-corrected quantum processors realizing universal quantum computation or solving noisy intermediate-scale quantum (NISQ) problems. We implement a deterministic state transfer and entanglement protocol between individually packaged chips connected by coaxial lines [1]. Individual chips may serve as universal nodes capable of sending, receiving, storing, and processing quantum information. Our protocol is based on an all-microwave process, which entangles or transfers the state of a superconducting qubit with a time-symmetric itinerant single photon. We transfer qubit states at rates of 50 kHz, absorb photons at the receiving node with a probability of 98 %, achieve a transfer process fidelity of 80 %, prepare on demand remote entanglement with a fidelity of 79 % and show that time bin encoding can be used to further improve these metrics.

[1] P. Kurpiers et al., Nature 558, 264-267 (2018)   

Raman Transitions between two Superconducting Cavity Modules via Parametric Conversion: Part I

We investigate a simple quantum network capable of state transfer and entanglement between superconducting 3D cavities in two spatially separated modules. Bidirectional communication between the cavity modes is established by coupling each cavity to the same standing-wave mode of a superconducting coaxial cable via parametric conversion. The system can be made robust to loss in the cable by using off-resonant conversion to engineer a virtual Raman transition between the three modes (cavity-cable-cavity), which suppresses the population of the lossy cable mode. Preliminary experimental results regarding state transfer will be discussed.   

Raman Transitions between two Superconducting Cavity Modules via Parametric Conversion: Part II

Entanglement generation between modules in a quantum network allows for gates between qubits in separate modules. A partial swap of population between two modes can be used to generate entanglement. We engineer such an entangling operation with a virtual Raman transition via a standing-wave mode of a coaxial cable. Preliminary results towards generating on-demand entanglement between two modules will be shown. Schemes for improving the entanglement by making use of multiphoton quantum states will be discussed.   

Generation of a microwave time-bin qubit with a superconducting qubit

Quantum information can be encoded in a propagating photonic qubit by constructing a set of computational basis states with one or more modes of light. This encoding scheme defines the characteristic properties of the qubit such as its decoherence properties and how it retains phase information. By encoding the qubit in a basis constructed from two orthogonal temporal modes, as a time-bin qubit, it is possible to detect and correct photon decay during information transfer with a parity measurement.
We experimentally demonstrate a protocol for deterministic on-demand generation of a time-bin qubit in the microwave regime through microwave-driven coherent control of a transmon qubit placed in a three-dimensional cavity. To perform quantum state tomography on a prepared time-bin qubit state, we apply iterative maximum likelihood estimation on time-bin encoded single-photon signal squeezed in different quadratures. We also discuss different factors affecting the time-bin qubit state preparation fidelity. Our protocol can be used as a means of realizing robust information transfer in quantum networks.   

Time-Bin Entanglement Between Remote Superconducting Cavities

Generation of entanglement between qubits connected by a lossy channel is an important primitive for large scale quantum information processing. Time-bin entanglement allows one to counter this imperfection through detection of photon loss errors in the channel. We present an experiment for time-bin entanglement between remote superconducting cavities. Stimulating a particular three-body Raman transition performs an entangling gate between a flying photon, target system, and an ancillary mode used as an entanglement witness. This ancillary mode, equivalent to a photon detector, is used to herald success of the protocol. The success rate of this protocol is expected to reach the transmission of the channel. Through local measurement of the ancillary modes, we can detect photon loss errors in the channel and herald the creation of an entangled Fock state between the remote cavities. We discuss experimental progress towards the implementation of this protocol.