# Research

Our research aims at exploring quantum information science at the nanoscale and developing novel quantum photonic devices. Such quantum devices are central for quantum information networks, quantum computation and quantum cryptography systems, and lie at the basis of the forthcoming Quantum Internet.

We focus on quantum control and manipulation of novel nanostructures at the single-photon and single-electron level, using nanowires, quantum dots, and hybrid atomic-solid state systems.

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### Nanowire quantum dots

Quantum dots are among the most promising candidates for various photonic systems in quantum technology. Embedding quantum dots in nanowires boosts these systems with a set of powerful possibilities, such as precise positioning of the emitters, excellent photon extraction efficiency and direct electrical contacting of quantum dots. Notably, nanowire structures can be grown on silicon substrates, allowing for straightforward integration with silicon-based photonic devices. Here we have developed the growth of GaAs quantum dots in AlGaAs nanowires on silicon. We precisely control the dimensions of quantum dots and their position inside nanowires and demonstrate that the emission wavelength can be engineered over the range of at least 30 nm around 765 nm. Our nanowire quantum dots have excellent optical properties, and we now explore this system for advanced photonic devices based on multiple coupled quantum dots in a nanowire.

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Read our recent paper on nanowire quantum dots:

**Nanowire Quantum Dots Tuned to Atomic Resonances**

L. Leandro, C.P. Gunnarsson, R. Reznik, K.D. Jöns, I. Shtrom, A. Khrebtov, T. Kasama, V. Zwiller, G. Cirlin and N. Akopian

**Nano Letters** 18, 7217 (2018)

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### Multi-qubit quantum photonic devices

Semiconductor quantum dots are one of the best on-demand sources of single and entangled photons to date, simultaneously merging the highest brightness and indistinguishability of the emitted photons. They are, therefore, among the strongest candidates for practical single-qubit quantum photonic devices. However, to exploit the full advantage of quantum physics, multi-qubit photonic devices are absolutely necessary. For instance, multi-qubit quantum registers with photonic interfaces are vital in the development of the forthcoming Quantum Internet. Here we aim to realize practical multi-qubit photonic devices for quantum networks based on novel crystal-phase quantum dots in nanowires. This is a recently developed technology uniquely allowing for the fabrication of quantum devices with an ultimate precision of a single atomic layer.

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Read our paper on the discovery of the first crystal-phase quantum dots:

**Crystal Phase Quantum Dots**

N. Akopian, G. Patriarche, L. Liu, J.-C. Harmand and V. Zwiller

**Nano Letters** 10, 1198 (2010)

** With editorial highlights in Nature Nanotechnology and Nature Materials.

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### Epsilon-near-zero grids for on-chip quantum networks

Realization of an on-chip quantum network is a major goal in the field of integrated quantum photonics. Here we aim to develop such networks based on novel epsilon-near-zero (ENZ) materials, exhibiting a close-to-zero relative permittivity. The great advantages of ENZ materials are that electromagnetic waves can propagate in a subwavelength waveguide with acute bends almost without reflection losses and with almost constant phase. We exploit these phenomena to demonstrate numerically that ENZ materials can strongly protect quantum information against decoherence and losses during its propagation in a dense network.

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Read our recent paper on epsilon-near-zero quantum networks:

**Epsilon-Near-Zero Grids for On-chip Quantum Networks**

L. Vertchenko, N. Akopian and A.V. Lavrinenko

**Scientific Reports** 9, 6053 (2019)

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### Hybrid atomic-solid state quantum systems

Hybrid atomic-solid state systems are promising building blocks for quantum photonic technologies because they can combine the advantages of both constituents. Here we explore such hybrid systems by coupling quantum dots and atomic vapors. Quantum dots are engineerable, scalable, tunable and deterministic sources of single and entangled photons. Atomic vapors are widely used as slow-light media and quantum memories. The coupling of single quantum dot emission to atomic vapors opens exciting new avenues where the scalability and functionality of nanostructure devices based on single quantum dots can be combined with the uniformity of atomic systems in hybrid interfaces. We have demonstrated, for instance, frequency-looking of GaAs quantum dot emission to an optical transition in Rb atoms, enabling the use of quantum dots as nodes in quantum networks.

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Read our paper on the realization of the first hybrid atomic-solid state quantum system:

Hybrid semiconductor-atomic interface: slowing down single photons from a quantum dot

N. Akopian, L. Wang, A. Rastelli, O.G. Schmidt and V. Zwiller

**Nature Photonics** 5, 230 (2011)

** With News and Views article and backpage Interview with N. Akopian.

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### Quantum network of artificial atoms

A network is formed from multiple quantum nodes located at different remote places. Quantum information is transferred from node to node by a single photon. An artificial atom (a quantum dot) coupled to natural atoms lies at the core of each node. An artificial atom represents “hardware” for a qubit, that is capable of sending, receiving and storing of quantum information. Coupling to natural atom ensures that all the nodes in the network are independently frequency-locked to the exact same optical transition of an atom. Such frequency-locking of all nodes is important because it ensures that all nodes can “talk” to each other, by sending and receiving photons of the same frequency.

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Read our paper on the first demonstration of frequency-locking of an artificial atom to natural atoms:

**An Artificial Atom Locked to Natural Atoms**

N. Akopian, R. Trotta, E. Zallo, S. Kumar, P. Atkinson, A. Rastelli, O.G. Schmidt and V. Zwiller

**arXiv**:1302.2005 (2013)

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