Session H06: Quantum Communication and Networks

Excitation Transport in Dipole-Coupled Photonic Emitter Arrays

In this work, we explore the transport dynamics of ordered and disordered atomic lattices of three-level V-type emitters. Understanding the fundamental mechanisms of excitation transport in quantum systems is pivotal for future generation quantum networks and chiral nanophotonics. Additionally, studying such phenomena can provide insight into the functionality of biologically-inspired light-harversting complexes. While previous studies have demonstrated efficient, unidirectional transport in one-dimensional systems via subradiant guided modes or chiral waveguides, here we examine the properties of two-dimensional arrays coupled to a nanowire extraction site. We characterize the transport efficiency of circularly-polarized chiral excitons as a function of geometry, detuning, extraction rate, dissipation, and static disorder.

    

Demonstration of a compact Franson interferometer for highly non-degenerate time-energy entangled photons

We present results from a time-energy entanglement Bell inequality violation measurement of two highly non-degenerate photons using a compact Franson interferometer. The entangled photons are generated via the spontaneous parametric downconversion of a 532 nm pump photon into time-energy and polarization degrees of freedom hyper-entangled 810 nm and 1550 nm photons. The source is a type II periodically poled potassium titanyl phosphate (KTP) crystal with a fiber-coupled input and free-space output. These wavelengths are of particular interest as they lie at the intersection of free-space quantum communication and terrestrial entanglement distribution due to 810 nm undergoing little atmospheric absorption, making it a candidate for satellite to ground quantum links. Furthermore, 1550 nm is capable of directly interfacing with existing telecommunications infrastructure. We report an initial violation of Bell's inequality by more than 3 standard deviations and describe efforts to improve upon this while maintaining stable operation in a low size, weight, and power (SWaP) package.   

Strongly-interacting photons mediated by Rydberg interactions

Photons usually do not interact with each other. Strong photon-photon interactions have been a long-standing goal in quantum optics, and are crucial to deterministic all-optical quantum logic, single-photon transistors, as well as exploration of optical many-body phenomena. I will show our experimental demonstration of strong interactions between individual photons, by coupling to strongly-interacting Rydberg atoms in free space. I will discuss our on-going efforts to harness these interactions for applications in quantum information processing and quantum networks. Photons propagating through a cloud of ultracold Rb-87, under ladder-scheme electromagnetically induced transparency, will acquire a nonlinear phase if detuned from the intermediate state. We aim to extend such nonlinear phase for non-destructive detection of optical photons. To get around the fundamental dissipation problem associated with these Rydberg-mediated photon-photon interactions, we will explore collective optical responses of sub-wavelength-spaced atomic arrays, for creation of multi-photon entangled states and high-fidelity all-optical devices.   

Photonic chip-based scalable switching of single photons from a trapped ion

Trapped ions are excellent candidates for long-distance quantum networks because of their long qubit coherence times, ability to generate photons entangled with the ion’s qubit states, and high-fidelity single- and two-qubit gates [1,2]. To establish reconfigurable quantum networks, it is advantageous to route single photons from trapped ions using photonic integrated circuits [3]. However, most trapped ions emit photons in the ultra-violet and visible wavelength regime, making them incompatible with present-day photonic foundries. In this work, we show the on-demand routing of single photons emitted from a trapped barium ion, using a foundry-fabricated silicon-nitride photonic integrated circuit [4].  We use quantum frequency conversion to generate C-band telecom single photons from barium ions [5], then couple these photons into a silicon nitride waveguide via an edge coupler. We then route the photons into different output ports of a Mach-Zehnder interferometer using an electrical signal. These results will enable a new generation of compact and reconfigurable integrated photonic devices that can serve as efficient quantum interconnects for quantum computers and sensors.

[1] C. D. Bruzewicz et al.,  Appl. Phys. Rev.6, 21314 (2019).

[2] J. D. Siverns et al., Applied optics 56,  222-230 (2017).

[3] C. Monroe and J. Kim,  Science. 339, 1164–1169 (2013).

[4] U. Saha et al. manuscript in preparation. 

[5] J. Hannegan et al., Appl. Phys. Lett. 119, 084001 (2021).   

Progress on Multispecies Ion Trap Quantum Network Node

Trapped atomic ions are one of the leading platforms for quantum computing systems and quantum networks. We plan to utilize a modular architecture consisting of three separate traps, each containing a 171Yb+ memory qubit and a 138Ba+ communication qubit. We will collect single 493 nm photons from the middle node of the network using two in-vacuo high numerical aperture (NA=0.8) aspheric lenses. These lenses replace the NA=0.6 multi-component objectives we use to collect single photons from the end node traps. We present preliminary fiber coupling results and heating rate measurements from this system. We also discuss how to generate GHZ states among the three traps using both atomic species.   

Effects of Cavity Birefringence on Polarisation Encoded Remote Entanglement Generation

The remote entanglement of ions in distinct traps via single photons distributed over a photonic network is a promising method for scaling ion trap quantum computers. Collecting the single network photons using optical cavities can greatly enhance the entanglement rate.  

 

We present results of a study into the fidelity loss due to cavity birefringence when using polarisation-encoding of the photonic qubit. We then consider three techniques for restoring the fidelity: windowing of the photon detection time; electro-optic rotation of the cavity output photon polarisation; and local rotation of the ion qubit conditioned on the photon arrival times. We demonstrate that these methods can partially restore the lost fidelity at the cost of substantially increased technical complexity. For current standards of mirror fabrication, fidelity loss is likely to be significant even post-correction. 

 

We conclude that cavity birefringence is a crucial consideration for polarisation-encoded entanglement of remote atomic systems, and unless significant improvements are made to micro-mirror fabrication techniques, alternative remote entanglement schemes, such as time-bin encoding, may be superior.    

Multiplexed quantum repeaters based on dual-species trapped-ion systems

Trapped ions form an advanced technology platform for quantum information processing with long qubit coherence times, high-fidelity quantum logic gates, optically active qubits, and a potential to scale up in size while preserving a high level of connectivity between qubits. These traits make them attractive not only for quantum computing but also for quantum networking. Dedicated, special-purpose trapped-ion processors in conjunction with suitable interconnecting hardware can be used to form quantum repeaters that enable high-rate quantum communications between distant trapped-ion quantum computers in a network. In this regard, hybrid traps with two distinct species of ions, where one ion species can generate ion-photon entanglement that is useful for optically interfacing with the network and the other has long memory lifetimes, useful for qubit storage, have been proposed for entanglement distribution. We consider an architecture for a repeater based on such dual-species trapped-ion systems. We propose and analyze a repeater protocol based on spatial and temporal mode multiplexing for entanglement distribution across a line network of such repeaters. Our protocol offers enhanced rates compared to rates previously reported for such repeaters.   

Demonstration of Optimal Non-Projective Measurement of Binary Coherent States

Quantum state discrimination is a central problem in quantum measurement theory, with applications spanning from quantum communication to computation. Quantum mechanics allows for the realization of optimized measurements based on photon counting for the discrimination of nonorthogonal coherent states able to surpass the conventional limits of detection, such as the homodyne and heterodyne limits. Such measurements have a large potential for increasing sensitivities and information transfer in communications and for information processing. In this talk I will describe our current work in the problem of generalized measurements for coherent state discrimination.  We implement an optimal inconclusive measurement for binary coherent states [1], a non-projective measurement that allows for achieving the lowest probability of error for a given rate of inconclusive results. This measurement encompasses standard measurement paradigms for state discrimination, specifically minimum error and unambiguous discrimination, and allows to transition between them in an optimal way.

 

[1] K. Nakahira and T. S. Usuda, Phys.Rev.A 86, 052323 (2012).

    

Entangling Circularly Polarized Light with the Quantum Zeno Effect

The quantum Zeno effect reveals that a continuously observed quantum system exhibits a natural suppression of its time-evolution.  And, as a consequence, the system experiences a  measurement-dependent restriction of accessible quantum states.  Therefore, one should be able to force a group of particles into an entangled state with the quantum Zeno effect.  Here, we present a scheme, supported by numerical simulations, where an unentangled photon pair enters each side of a coupled waveguide and evolves into a polarization entangled state.  Additionally, we find that the same technique can be used in the generation of W-states.  Our findings present a robust avenue for photonic-based quantum applications without the need for distribution protocols.