Session Y31: Quantum Networks II: Distributed Quantum Computing

Live

Modular networks are a promising paradigm for realizing increasingly complex quantum devices, but rapid and faithful communication between modules remains a challenge. In this work, the low-loss standing wave mode of a demountable, all-superconducting coaxial cable is used as a quantum bus in a simple quantum network consisting of two 3D cavity bosonic qubits housed in separate modules. We demonstrate several key networking requirements such as state transfer and entanglement generation with high fidelity, due to the all-superconducting nature of the bus and the absence of lossy directional elements (circulators). Crucially, we also implement bosonic quantum error correction techniques in the communication protocols to further mitigate photon loss. In particular, we achieve breakeven for the state transfer of multi-photon qubits when compared to the single-photon encoding and Bell state entanglement generation via two-photon interference with half the infidelity as the single-photon case.   

Live

Most quantum communication implementations rely on exchanging photonic states across a network and suffer from photon loss in the links coupling the modules. Surprisingly, if the communication channel can be treated as a single lossy stationary mode, reservoir engineering can be used for entanglement generation. We report on an experiment that demonstrates this approach by symmetrically coupling two modules to a coaxial cable acting as a shared reservoir. This allows us to engineer an effective non-local dissipation on two bosonic qubits housed in separable 3D cavity modules that can be used for entanglement generation. Combining this with additional local stabilization of the qubit manifolds enables us to deterministically generate a Bell pair.   

Live

We describe a new quantum walk formalism to bound the growth of quantum operators. We then apply these new mathematical tools to prove a linear light cone bound for non-interacting particles on a d-dimensional cubic lattice with power-law hopping. In particular, the dynamics is constrained within a linear light cone if α>d+1. We present an efficient protocol to perform quantum state transfer using Hamiltonian dynamics with long-range interactions. The time to transfer n qubits a sufficiently large distance is proportional to the square root of n. Even without error correction, the fidelity of this multi-qubit state transfer process remains finite for arbitrarily well-separated qubits in the presence of uncorrelated random errors in coupling constants.

Y. Hong and A. Lucas, "Fast high-fidelity multi-qubit state transfer with long-range interactions", arXiv:2009.06587 [quant-ph]   

Live

Deterministic chip-to-chip quantum protocols between distant quantum processors are a promising route for scaling up quantum computers. Through the emission and absorption of single microwave photons these protocols [1,2,3,4] allow for quantum communication between superconducting circuits located on separate chips, also at meter-scale distances [5]. In order to achieve high fidelities in these algorithms, the photon emission and absorption must be fast and reliable.
At the same time, fast high-fidelity readout of the qubits is key for applications ranging from quantum computing algorithms to tests of fundamental physics. Both features require a careful selection of the device parameters. In this talk, we present the main challenges and a first implementation of a superconducting circuit fulfilling the aforementioned needs.

[1] C. Axline et al., Nature Physics 14, 705 (2018)
[2] P. Campagne-Ibarcq et al., PRL 120, 200501 (2018)
[3] P. Kurpiers et al., Nature 558, 264 (2018)
[4] N. Leung et al., npj Quantum Information 5, 18 (2019)
[5] P. Magnard et al., arXiv:2008.01642 (2020)   

Live

A promising route for scaling up quantum computers based on superconducting circuit technology is to connect multiple processors to each other using chip-to-chip deterministic quantum protocols [1,2,3,4,5]. The protocols are used either between chips housed in a single cryogenic system or spatially separated ones connected by a cryogenic microwave quantum network.
We have realized such a modular cryogenic quantum network connecting two dilution refrigerators separated by 5 meters [6] and plan to extend it to 30 meters. In this talk, we present the design and concept of the cryogenic link we have developed. We analyse its thermal properties and review its electrical characteristics. We further discuss the modularity and distance scales attainable with our system.

[1] C. Dickel et al., PRB 97, 064508 (2018)
[2] C. Axline et al., Nature Physics 14, 705 (2018)
[3] P. Campagne-Ibarcq et al., PRL 120, 200501 (2018)
[4] P. Kurpiers et al., Nature 558, 264 (2018)
[5] N. Leung et al., npj Quantum Information 5, 18 (2019)
[6] P. Magnard et al., arXiv:2008.01642 (2020)   

Live

The general desire to break the symmetry of reciprocity in engineered photonic structures has garnered an immense amount of recent interest, as nonreciprocity is fundamental for the design of optical devices which allow for unidirectional routing of photonic signals. Specifically, nonreciprocal microwave-frequency devices are crucial to efforts at quantum-information processing with superconducting circuits. However, although nonreciprocal concepts are realizable in quantum architectures, nonreciprocity itself holds up to the classical level and is not inherently quantum; the fundamental aspects of nonreciprocity in the quantum regime have yet to be fully investigated. A first question one might ask is if a nonreciprocal, and hence dissipative, system can generate entanglement? In this talk we will address this question and discuss under which conditions a bipartite system can generate entanglement.   

Live

Photons are ideal carriers of quantum information to communicate over large distances in quantum networks. Many applications, such as quantum routers and quantum repeaters, require interactions between two or more photonic qubits. Such photon-photon gates are typically realized by interactions with a non-linear medium. Probabilistic and deterministic photon-photon gates, based on reflection off or overlap with a non-linear medium, have previously been demonstrated in the optical regime. In this talk, we present absorption and re-emission-based, deterministic, photon-photon gates. We emit shaped itinerant photons in the microwave regime from a source chip, containing two transmon qubits, and send them to a second similar chip which implements a universal gate set by combining controlled absorption and re-emission with single qubit gates and qubit-photon C-PHASE gates. We evaluate the performance of the gate set by quantum process tomography and find internal process fidelities of 90 % for arbitrary single photon gates and photon-photon C-PHASE gates limited mainly by decoherence of the gate-assisting transmon qubits.   

Live

We demonstrate the successful realization of unconditional quantum teleportation in the microwave regime over the distance of 42 cm by exploiting two-mode squeezing and analog feedforward. We generate squeezed and feedforward signals in the GHz regime by using superconducting Josephson parametric amplifiers. We realize quantum teleportation of coherent states with fidelities exceeding the no-cloning limit, thus, proving the unconditional security of the protocol. Furthermore, our experiments reveal the influence of the feedforward gain and entanglement strength on the teleportation fidelity in the presence of finite noise and losses. In the end, we demonstrate that quantum microwave communication is feasible over macroscopic distances in the cryogenic environment. Our results enable future implementations of microwave quantum local area networks and distributed quantum computing with superconducting circuits.   

Live

A network of distributed, superconductor-based, quantum processors may overcome the challenge of adding qubits without promoting errors and cross-talk. Quantum links can also couple distant qubits in one array to decrease circuit depth requirements. Quantum computing will further expand to complex, heterogeneous networks. Key is a high-level link model that reflects important physics while abstracting details. Questions remain as to which constraints dominate among noise, latency, throughput, loss and range. Answers vary between media.

We explore a distributed quantum system interconnect model and compare architectures in terms of technology, topology and scale. Based on experimental literature, we model short microwave links between superconducting chips. High speed and efficiency suggest near-term potential to accelerate paths to quantum advantages. We contrast with long-term potential of optical links. Qualitatively distinct media fit varied applications, timelines and physical analogies.   

Live

The problem of distilling Bell pairs in an optimal manner was solved early in the development of the quantum information field, as long as one can assume the availability of perfect local quantum hardware. Much work has since ensued, searching for purification circuits that can approximate this optimum even on realistic noisy hardware, especially in the context of the NISQ era. Multipartite entanglement purification has followed a similar trajectory, including efficient protocols for GHZ and Cluster state purification between distant parties. However, the availability of practical multiparty protocols running on realistic noisy hardware is limited. Here we present a heterogenous approach to entanglement distilation, in which simple small entangled states are directly used as the sacrificial distillation resource in the purification of larger much more expensive entangled resources. Such protocols significantly reduce the network requirements for a NISQ era entanglement distilation.   

Live

We propose a protocol of quantum teleportation based on shared qubits. The plaintext is divided into blocks with same length, and represented by the eigenstates under Z-measurements. The ciphertext is encryted by a special circuit, which is determined by the former block of palintext. We demonstrate details of the method in a scenario where Alice and Bob attempts to communicate via several shared qubits under this protocol. Meanwhile, an eavesdropping is introduced to test its safety against wiretapping. Performance of the method is estimated based on theoretical analysis and numerical simulation.   

Live

Efficiently distributing and manipulating entanglement is essential in the construction of large scale quantum networks. In this project, we focus on tripartite entanglement structure of stabilizer state and its transformation through local operations. Suppose the tripartite entangled state ρ^ABC is hold by Alice, Bob and Charlie, respectively. An interesting question to ask is whether it is possible to transfer all the A|BC entanglement to system A and B by Charlie's local operation and classical communication to Alice and Bob, namely entanglement shaping. We call the possibility of disentangling C from AB as "the bigger man principle" when the entanglement E(A|B) equals E(A|BC) upon the operation. This study contributes to the understanding of stabilizer state entanglement and can serve as a guiding principle in quantum network.   

Live

Unlike bipartite states, multipartite states have no known simple method to fully characterize entanglement, and itis an interesting problem if such characterization is possible. We shed light on this problem by presenting a methodto decompose tripartite graph states into Bell pairs and GHZ states through a series of Clifford operations in eachsubsystem, and hence showing that entanglement of tripartite graph states can be fully characterized with 4 numbers;the number of Bell pairs between each pair of subsystems and the number of GHZ states shared by all subsystems.   

Not Participating

The tremendous success in scaling superconducting quantum information processors (SQIPs) has triggered the need for networking architectures between such devices. While, conceptually, microwave-to-optics-to-microwave approaches seem attractive, they, despite considerable efforts, still suffer from low conversion efficiencies in practice. On the other hand, SQIP systems naturally come with a millikelvin cryostat and microwave connectivity. As a viable alternative for quantum local area networks, we have set up a 6.6m long cryogenic link between one homemade and one Oxford Instruments Triton500 dilution refrigerator. This link also offers the possibility of routing signals between more than two refrigerators via an intermediate cold stage ("cold network node"). Here, we demonstrate the successful transfer of a continuous-variable quantum state over this cryogenic link with negligible losses. In the future, we plan to implement quantum microwave teleportation as a proof-of-principle communication protocol for all-microwave quantum local area networks.   

Live

The deterministic entanglement of two remote qubits has recently been demonstrated with microwave photons, optical photons and surface acoustic wave phonons. However, the deterministic generation and transmission of multi-qubit entanglement has not been demonstrated, primarily due to limited state transfer fidelities. Here, we report a quantum network comprising two separate superconducting quantum nodes connected by a 1 meter-long coaxial cable, where each node includes three qubits. By directly connecting the coaxial cable to one qubit in each node, we can transfer quantum states between the nodes with a process fidelity of 0.911. Using the high-fidelity communication link, we can prepare a three-qubit Greenberger-Horne-Zeilinger (GHZ) state in one node and deterministically transfer this state to the other node, with a transferred state fidelity of 0.656. We further deterministically generate a six-qubit GHZ state, globally distributed within the network, with a state fidelity of 0.722. The GHZ state fidelities are clearly above the threshold of 1/2 for genuine multipartite entanglement, and show that this architecture can be used to coherently link together multiple superconducting quantum processors, providing a modular approach for building large-scale quantum computers.