Photonic quantum memory in two-level ensembles based on modulating the refractive index in time: equivalence to gradient echo memory

We present a quantum memory protocol that allows to store light in ensembles of two-level atoms, e.g. rare-earth ions doped into a crystal, by modulating the refractive index of the host medium of the atoms linearly in time. We show that under certain conditions the resulting dynamics is equivalent to that underlying the gradient echo memory protocol, which relies on a spatial gradient of the atomic resonance frequencies. We discuss the prospects for an experimental implementation.Comment: 5 pages, 2 figure

Coherent storage and manipulation of broadband photons via dynamically controlled Autler-Townes splitting

The coherent control of light with matter, enabling storage and manipulation of optical signals, was revolutionized by electromagnetically induced transparency (EIT), which is a quantum interference effect. For strong electromagnetic fields that induce a wide transparency band, this quantum interference vanishes, giving rise to the well-known phenomenon of Autler-Townes splitting (ATS). To date, it is an open question whether ATS can be directly leveraged for coherent control as more than just a case of "bad" EIT. Here, we establish a protocol showing that dynamically controlled absorption of light in the ATS regime mediates coherent storage and manipulation that is inherently suitable for efficient broadband quantum memory and processing devices. We experimentally demonstrate this protocol by storing and manipulating nanoseconds-long optical pulses through a collective spin state of laser-cooled Rb atoms for up to a microsecond. Furthermore, we show that our approach substantially relaxes the technical requirements intrinsic to established memory schemes, rendering it suitable for broad range of platforms with applications to quantum information processing, high-precision spectroscopy, and metrology.Comment: 14 pages with 6 figures; 3 pages supplementary info with 2 supplementary figure

Beyond transcoherent states: Field states for effecting optimal coherent rotations on single or multiple qubits

Semiclassically, laser pulses can be used to implement arbitrary transformations on atomic systems; quantum mechanically, residual atom-field entanglement spoils this promise. Transcoherent states are field states that fix this problem in the fully quantized regime by generating perfect coherence in an atom initially in its ground or excited state. We extend this fully quantized paradigm in four directions: First, we introduce field states that transform an atom from its ground or excited state to any point on the Bloch sphere without residual atom-field entanglement. The best strong pulses for carrying out rotations by angle $\theta$ are are squeezed in photon-number variance by a factor of $\rm{sinc}\theta$. Next, we investigate implementing rotation gates, showing that the optimal Gaussian field state for enacting a $\theta$ pulse on an atom in an arbitrary, unknown initial state is number squeezed by less: $\rm{sinc}\tfrac{\theta}{2}$. Third, we extend these investigations to fields interacting with multiple atoms simultaneously, discovering once again that number squeezing by $\tfrac{\pi}{2}$ is optimal for enacting $\tfrac{\pi}{2}$ pulses on all of the atoms simultaneously, with small corrections on the order of the ratio of the number of atoms to the average number of photons. Finally, we find field states that best perform arbitrary rotations by $\theta$ through nonlinear interactions involving $m$-photon absorption, where the same optimal squeezing factor is found to be $\rm{sinc}\theta$. Backaction in a wide variety of atom-field interactions can thus be mitigated by squeezing the control fields by optimal amounts.Comment: Updated formatting following acceptance in Quantu

Measuring the quadrature coherence scale on a cloud quantum computer

Coherence underlies quantum phenomena, yet it is manifest in classical theories; delineating coherence's role is a fickle business. The quadrature coherence scale (QCS) was invented to remove such ambiguity, quantifying quantum features of any single-mode bosonic system without choosing a preferred orientation of phase space. The QCS is defined for any state, reducing to well-known quantities in appropriate limits, including Gaussian and pure states, and perhaps most importantly for a coherence measure, it is highly sensitive to decoherence. Until recently, it was unknown how to measure the QCS; we here report on an initial measurement of the QCS for squeezed light and thermal states of light. This is performed using Xanadu's machine Borealis, accessed through the cloud, which offers the configurable beam splitters and photon-number-resolving detectors essential for measuring the QCS. The data and theory match well, certifying the usefulness of interferometers and photon-counting devices in certifying quantumness.Comment: 11 pages including 4 figures and 1 appendix; close to published versio

Quantum control of Rydberg atoms for mesoscopic-scale quantum state and circuit preparation

Individually trapped Rydberg atoms show significant promise as a platform for scalable quantum simulation and for development of programmable quantum computers. In particular, the Rydberg blockade effect can be used to facilitate both fast qubit-qubit interactions and long coherence times via low-lying electronic states encoding the physical qubits. To bring existing Rydberg-atom-based platforms a step closer to fault-tolerant quantum computation, we demonstrate high-fidelity state and circuit preparation in a system of five atoms. We specifically show that quantum control can be used to reliably generate fully connected cluster states and to simulate the error-correction encoding circuit based on the 'Perfect Quantum Error Correcting Code' by Laflamme et al. [Phys. Rev. Lett. 77, 198 (1996)]. Our results make these ideas and their implementation directly accessible to experiments and demonstrate a promising level of noise tolerance with respect to experimental errors. With this approach, we motivate the application of quantum control in small subsystems in combination with the standard gate-based quantum circuits for direct and high-fidelity implementation of few-qubit modules