Session D67: Modular Quantum Systems and Networks

Hybrid atom — rare-earth ion quantum interfaces and network nodes

Future global quantum networks will merge heterogeneous quantum systems to simultaneously perform multiple tasks, namely, reliably store, process, and transmit quantum information as well as distribute entanglement. The functionality of such networks relies crucially on coherent interfaces between disparate nodes and telecom photons which connect distant nodes through low-loss optical fibers. We propose a modular hybrid network architecture consisting of a matching pair of telecom nodes with GHz frequency differences to circumvent losses due to quantum frequency conversion processes. On one side, a single atom coupled to a nanophotonic crystal cavity serves as our processing qubit node, generating high-fidelity atom-telecom photon entanglement at fast rates through time-bin photonic qubits. On the other side, a rare-earth ion-doped crystal serves as our memory qubit node, where the inhomogeneous broadening allows broadband storage and spectral multiplexing for multiple time-bin photonic qubits, creating entanglement between the two nodes. In this talk, we will present our results identifying the mode-matching conditions between a rubidium vapor cell and an erbium-doped crystal, where the former serves as a telecom single photon source via phase-matched four-wave mixing. We will also discuss our experimental progress towards establishing the atom-nanophotonic system as the processor node and multimode storage in the memory node.   

Ytterbium ion trap quantum computing: The current state-of-the-art

We present an overview of contemporary quantum computing with ytterbium ion traps, placing the emphasis on industry implementations. We provide brief, concrete descriptions of various key features, such as trap loading, electronic structure, qubit function, gates, error analysis, and benchmarking. We focus on the underlying science and current technologies to provide readers with a holistic picture of available techniques for using ytterbium in contemporary ion trap designs.   

Intra-atomic frequency comb based photonic quantum memory using single-atom-cavity setup

On-demand and efficient storage of photons is an essential element in quantum information processing and long-distance quantum communication. Most of the quantum memory protocols require bulk systems in order to store photons. However, with the advent of integrated photonic chip platforms for quantum information processing, on-chip quantum memories are highly sought after. In this work, we propose a protocol for multi-mode photonic quantum memory using only a single-atom-cavity setup. We show that a single atom containing a frequency comb coupled to an optical cavity can store photons efficiently. Further, this scheme can also be used to store polarization states of light. As examples, we show that the Rubidium and Cesium atoms coupled to nanophotonic waveguide cavities can serve as promising candidates to realize our scheme. This provides a possibility of a robust and efficient on-chip quantum memory to be used in integrated photonic chips.   

Toward development of a quantum transducer to hybridize superconducting and ion trap quantum computers.

Quantum information processing is poised to revolutionize the global information system with dramatically enhanced computational power capable of unlocking solutions to previously unsolvable problems. To date, the two leading platforms are trapped ion- and superconducting circuit-based architectures. Trapped ion qubits exhibit long coherence times and efficient remote entanglement, while superconducting circuits offer scalability and fast quantum gate speeds. In the pursuit of optimal quantum processing, storage, and teleportation, a dedicated hybrid quantum system will perform fast information processing encoded in superconducting circuit qubits and store and teleport the processed information in the form of trapped ion qubits. Due to the significant difference in mass and dimension, direct coupling between the macroscopic superconducting circuits and a microscopic atomic ion is either slow or far off-resonance. To tackle this critical issue, we propose applying circular Rydberg states of strontium (Sr) atoms hypersensitive to a microwave field to interact with the superconducting qubits. Immediately after that, the short-lived Rydberg qubits will be transferred to long-lived spin qubits in Sr⁺ ions by a selective ion-core excitation. Several recently demonstrated state-of-the-art technologies will be implemented on a millimeter-scale chip, including trapping and shuttling neutral atoms along an optical nanofiber and on-chip THz ionization of Rydberg atoms.    

Building a Quantum Repeater Using Optomechanical Oscillators as On-Demand Entanglement Sources

The core building block of a long-range quantum network is the repeater, which uses the principle of entanglement swapping to connect two spatially separated nodes. The key requirement is a reliable Einstein-Podolsky-Rosen (EPR) source that can create entanglement between a pair of spatially separated network nodes. A canonical entanglement swapping requires a Bell-basis joint detection of two photons (each is half of the entangled qubit pair in each node). The lack of reliable and deterministic EPR sources has been the major roadblock toward practical quantum repeaters. Here, we propose using an optomechanical oscillator, which produces a photon-phonon entangled pair, as a breakthrough on-demand EPR source. This is enabled by two key characteristics of optomechanical oscillators. First, phonon modes feature an extremely long lifetime. As such, if one oscillator’s phonon mode is populated first, the phonon qubit can be held until the other oscillator’s phonon mode is populated as well. Second, the photon-phonon coupling in an optomechanical oscillator is dynamically tunable. Consequently, once both oscillators’ phonon modes are populated, the coupling in both oscillators can be switched on for long enough such that a maximally entangled qubit pair is generated in each. We derive the entanglement swapping fidelity as a function of optical fiber loss, demonstrating a fidelity of 98% in the limit of negligible fiber loss.

 

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525   

Indistinguishable photons from an artificial atom in silicon photonics

Silicon is the ideal material for building electronic and photonic circuits at scale. Integrated quantum technologies in silicon offer a promising path to scaling by leveraging advanced complementary metal-oxide-semiconductor (CMOS) integration capabilities. However, the lack of deterministic quantum light sources, photon-photon gates, or long-range spin-spin interactions in current approaches poses a major challenge to scalability. In this talk, we will present results on a new type of indistinguishable photon source in silicon photonics based on an artificial atom. We show that a G center in a silicon waveguide can source high-purity telecom-band single photons. We perform high-resolution spectroscopy and time-delayed two-photon interference to demonstrate the indistinguishability of single photons emitted from a G center in a silicon waveguide. Our results show that artificial atoms in silicon photonics can produce highly coherent photons suitable for photonic quantum computation and communication protocols.   

Entangling remote microwave quantum computers with hybrid entanglement swap and variational distillation

Superconducting microwave circuits with Josephson junctions are a major platform for quantum computing. To unleash their full capabilities, the cooperative operation of multiple microwave superconducting circuits is required. Therefore, designing an efficient protocol to distribute microwave entanglement remotely becomes a crucial open problem. Here, we propose a continuous-variable entanglement-swap approach based on optical-microwave entanglement generation, which can boost the ultimate rate by two orders of magnitude at state-of-the-art parameter region, compared with traditional approaches. We further empower the protocol with a hybrid variational entanglement distillation component to provide huge advantage in the infidelity-versus-success-probability trade-off. Our protocol can be realized with near-term device performance, and is robust against non-perfections such as optical loss and noise. Therefore, our work provides a practical method to realize efficient quantum links for superconducting microwave quantum computers.   

Continuous variable entanglement between microwave and optics

Entanglement enables quantum advantage over classical applications. Entanglement between similar systems such as photons, ions, atoms and nuclear spins has already shown this advantage in information processing, communication, cryptography and sensing. Moreover, hybrid entanglement between localized systems and itinerant photons has extended these applications to distributed quantum computing and sensing. Thus far, such hybrid entanglement has remained divided into two paradigms - microwave photons entangled with microwave-based quantum devices such as superconducting qubits, and optical photons entangled with optically-addressable systems such as atoms. Uniting these two paradigms will enable new capabilities in hybrid quantum networks, sensing and meteorology. The required entanglement between itinerant microwave and optical light has not been demonstrated due to the incompatibility of low loss superconductivity and high energy optical photons which prevented the required ultra-low noise conditions. Here, we demonstrate the deterministic preparation of an entangled microwave-optical state in the continuous variable domain that is squeezed 0.7 dB below the vacuum level. We achieve this in a triply resonant, pulsed electro-optic interconnect working in a millikelvin environment.   

Observation of Einstein-Podolski-Rosen correlations between microwave and optical photons

Entanglement is 'the characteristic trait’ [1] that distinguishes quantum mechanics from previous theories of physics. Most remarkable, it enables correlations between subsystems that are stronger than what is classically allowed and it is the essential resource behind scaling advantages in emerging quantum technologies. Today, entanglement between similar systems such as photons, ions, atoms, electronic spins or superconducting circuits is routinely generated, detected and used for basic quantum information processing and quantum communication tasks. However, a full-fledged development of such technologies will make it necessary to share this important resource also across very dissimilar physical platforms. Quantum transducers between microwave and optical photons would offer such a capability by coherently interfacing superconducting quantum processors with optical photons for transmitting quantum information over large distances. However, despite significant experimental progress, the ubiquitous trade-off between low conversion efficiencies and added classical noise have so far prevented the observation of genuine non-classical correlations in such devices. Here we report on the deterministic preparation of entanglement between microwave and optical fields in the continuous variable domain. We achieve this in a millikelvin environment using a triply resonant electro-optical transducer that is operated in pulsed mode to minimize added noise [2]. The resulting entangled state is squeezed by 0.7 dB below the vacuum level and violates the EPR bound for classical correlations. This demonstrates the feasibility of transducing genuine quantum correlations across vastly different frequency scales, with wide-ranging implications for quantum technology applications.

[1] E. Schrödinger, Discussion of probability relations between separated systems, Mathematical Proceedings of the Cambridge Philosophical Society, 31, 4 (1935)

[2] Rishabh Sahu, et al., Quantum-enabled operation of a microwave-optical interface, Nature Commun. 13, 1276 (2022)   

Detecting entanglement in two-mode light states using generalized CHSH inequality

Entanglement can be detected between two modes of light by performing a Bell-type experiment involving Mach-Zehnder interferometers (MZIs) [1]. The experiment would involve a Mach-Zehnder interferometer, a strong coherent state and a photodetector forming a unit, each in two distant laboratories. Using two measurement settings per laboratory, controlled by the intensity of the coherent state, the maximal bound of such a CHSH inequality was verified to be $2sqrt{2}$. To improve the detection capabilities of the MZI-based CHSH inequality, we consider $n$ number of measurement settings of the interferometer in each lab and check for the maximum possible violations. We also make a comment on the maximal violations achieved by this generalized CHSH inequality for detecting entanglement in important classes of two-mode light states: entangled coherent states and two-mode squeezed vacuum, which are useful for quantum metrology [2].

[1] M. G. Dastidar, G. Sarbicki, Phys. Rev. A, 105, 062459. (2022)

[2] Polino et al, AVS Quantum Sci., 2(2), 024703. (2020)   

Two-level Quantum Walkers on Directed Graphs: An Application to qRAM

Using a multi-particle continuous-time quantum walk with two internal states [1], we physically implement a quantum random access memory (qRAM). Data with address information are dual-rail encoded into quantum walkers. The walkers pass through perfect binary trees to access the designated memory cells and copy the data stored in the cells. A roundabout gate allocated at each node serves as a router to move the walker from the parent node to one of two child nodes, depending on the internal state of the walker. In this process, the address information is sequentially encoded into the internal states so that the walkers are adequately delivered to the target cells. The present qRAM, which processes 2n m-qubit data, is implemented in a quantum circuit of depth O(n log(n+m)) and requires O(n+m) qubit resources. This is more efficient than the conventional bucket-brigade qRAM that requires O(n2 + nm) steps and O(2n + m) qubit resources for processing. Moreover, since the walkers are not entangled with any device on the binary trees, the cost of maintaining coherence could be reduced. Notably, by simply passing quantum walkers through binary trees, data can be automatically extracted in a quantum superposition state. In other words, any time-dependent control is not required. [1] arXiv:2112.08119 [quant-ph]   

Towards a scalable QRAM architecture based on coupled bosonic modes

Quantum random access memory (QRAM) allows the user to query a database (of classical or quantum data) in superposition. Access to QRAM is required for many quantum algorithms to claim a speedup over their classical counterparts. Such quantum algorithms include Grover's search, quantum chemistry algorithms and quantum machine-learning algorithms, to name a few. In this talk we present a proposal for realizing QRAM based on an architecture of coupled bosonic modes. We describe novel protocols and gate schemes required for scaled-up QRAM devices while leveraging and improving upon previously developed technologies such as the necessary controlled-SWAP (CSWAP) operation. The proposal is believed to be experimentally feasible and within the reach of near-term devices.