Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y31: Quantum Networks II: Distributed Quantum ComputingLive

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Sponsoring Units: DQI Chair: Andy C. Y. Li 
Friday, March 19, 2021 11:30AM  11:42AM Live 
Y31.00001: Communication with a demountable quantum bus part 1 James Teoh, Luke D Burkhart, Yaxing Zhang, Christopher J Axline, Luigi Frunzio, Michel Devoret, Liang Jiang, Steven Girvin, Robert J Schoelkopf 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 lowloss standing wave mode of a demountable, allsuperconducting 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 allsuperconducting 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 multiphoton qubits when compared to the singlephoton encoding and Bell state entanglement generation via twophoton interference with half the infidelity as the singlephoton case. 
Friday, March 19, 2021 11:42AM  11:54AM Live 
Y31.00002: Communication with a demountable quantum bus part 2: Entanglement via reservoir engineering Nathanael Cottet, James Teoh, Luke D Burkhart, Luigi Frunzio, Michel Devoret, Robert J Schoelkopf 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 nonlocal 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. 
Friday, March 19, 2021 11:54AM  12:06PM Live 
Y31.00003: Fast and highfidelity multiqubit state transfer Yifan Hong, Andrew Lucas 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 noninteracting particles on a ddimensional cubic lattice with powerlaw 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 longrange 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 multiqubit state transfer process remains finite for arbitrarily wellseparated qubits in the presence of uncorrelated random errors in coupling constants. 
Friday, March 19, 2021 12:06PM  12:18PM Live 
Y31.00004: Quantum Communication Between Distant Superconducting Circuits Combined With Fast, HighFidelity Readout Simon Storz, Paul Magnard, Josua Schär, Philipp Kurpiers, JeanClaude Besse, Kevin Reuer, Mihai Gabureac, Abdulkadir Akin, Baptiste Royer, Alexandre Blais, Andreas Wallraff Deterministic chiptochip 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 meterscale distances [5]. In order to achieve high fidelities in these algorithms, the photon emission and absorption must be fast and reliable. 
Friday, March 19, 2021 12:18PM  12:30PM Live 
Y31.00005: Extending Cryogenic Microwave Quantum Networks Josua Schär, Simon Storz, Paul Magnard, Philipp Kurpiers, Janis Luetolf, Fabian Marxer, Melvin Frey, Niti Kohli, Reto Schlatter, Alain Fauquex, Andreas Wallraff A promising route for scaling up quantum computers based on superconducting circuit technology is to connect multiple processors to each other using chiptochip 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. 
Friday, March 19, 2021 12:30PM  12:42PM Live 
Y31.00006: Entanglement Generation in Nonreciprocal Systems Lindsay Orr, Saeed Khan, C. W. Sandbo Chang, Nils Buchholz, Christopher Wilson, Anja Metelmann 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 microwavefrequency devices are crucial to efforts at quantuminformation 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. 
Friday, March 19, 2021 12:42PM  12:54PM Live 
Y31.00007: A Universal Quantum Gate Set Operating on Itinerant Microwave Photons Kevin Reuer, JeanClaude Besse, Lucien Wernli, Paul Magnard, Philipp Kurpiers, Andreas Wallraff, Christopher Eichler 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 photonphoton gates are typically realized by interactions with a nonlinear medium. Probabilistic and deterministic photonphoton gates, based on reflection off or overlap with a nonlinear medium, have previously been demonstrated in the optical regime. In this talk, we present absorption and reemissionbased, deterministic, photonphoton 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 reemission with single qubit gates and qubitphoton CPHASE 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 photonphoton CPHASE gates limited mainly by decoherence of the gateassisting transmon qubits. 
Friday, March 19, 2021 12:54PM  1:06PM Live 
Y31.00008: Microwave quantum teleportation Kirill Fedorov, Stefan Pogorzalek, Michael Renger, Qiming Chen, Yuki Nojiri, Matti Partanen, Achim Marx, Rudolf Gross, Frank Deppe We demonstrate the successful realization of unconditional quantum teleportation in the microwave regime over the distance of 42 cm by exploiting twomode 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 nocloning 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. 
Friday, March 19, 2021 1:06PM  1:18PM Live 
Y31.00009: Modeling Networks of Superconducting Qubit Arrays from the Physical to Processing Layers Nicholas LaRacuente, Kaitlin N. Smith, Poolad Imany, Kevin L. Silverman, Fred Chong A network of distributed, superconductorbased, quantum processors may overcome the challenge of adding qubits without promoting errors and crosstalk. 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 highlevel 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. 
Friday, March 19, 2021 1:18PM  1:30PM Live 
Y31.00010: Heterogeneous Multipartite Entanglement Purification for SizeConstrained Quantum Devices Stefan Krastanov, Alexander Sanchez de la Cerda, Prineha Narang 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. 
Friday, March 19, 2021 1:30PM  1:42PM Live 
Y31.00011: Protocol of Quantum Communication Based on Shared Qubits Junxu Li, Sabre Kais 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 Zmeasurements. 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. 
Friday, March 19, 2021 1:42PM  1:54PM Live 
Y31.00012: Tripartite entanglement shaping in stabilizer formalism Changchun Zhong, Yat Wong, Liang Jiang 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 ABC 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(AB) equals E(ABC) upon the operation. This study contributes to the understanding of stabilizer state entanglement and can serve as a guiding principle in quantum network. 
Friday, March 19, 2021 1:54PM  2:06PM Live 
Y31.00013: Decomposition of tripartite graph states Yat Wong, Changchun Zhong, Liang Jiang 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. 
Friday, March 19, 2021 2:06PM  2:18PM Not Participating 
Y31.00014: Continuousvariable quantum microwave state transfer between two dilution refrigerators Frank Deppe, Kirill Fedorov, Matti Partanen, Michael Renger, Stefan Pogorzalek, Qiming Chen, Yuki Nojiri, Achim Marx, Rudolf Gross, Harriet van der Vliet, Anthony J. Matthews, Ziad Melhem The tremendous success in scaling superconducting quantum information processors (SQIPs) has triggered the need for networking architectures between such devices. While, conceptually, microwavetoopticstomicrowave 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 continuousvariable quantum state over this cryogenic link with negligible losses. In the future, we plan to implement quantum microwave teleportation as a proofofprinciple communication protocol for allmicrowave quantum local area networks. 
Friday, March 19, 2021 2:18PM  2:30PM Live 
Y31.00015: Deterministic multiqubit entanglement in a quantum network Youpeng Zhong, HungShen Chang, Audrey Bienfait, Etienne Dumur, MingHan Chou, Christopher R Conner, Joel Grebel, Rhys G Povey, Haoxiong Yan, David I Schuster, Andrew N Cleland 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 multiqubit 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 meterlong 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 highfidelity communication link, we can prepare a threequbit GreenbergerHorneZeilinger (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 sixqubit 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 largescale quantum computers. 
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