arXiv subjects

J. Borregaard

Publications and source records attributed to J. Borregaard.

10 recordsLinked to original sources

arXiv2025

Entanglement Assisted Non-local Optical Interferometry in a Quantum Network

The sensitivity of non-local optical measurements at low light intensities, such as those involved in long baseline telescope arrays, can be improved by using remote entanglement. Here, we demonstrate the use of entangled quantum memories in a quantum network of Silicon-vacancy centers in diamond nano-cavities to experimentally perform such non-local phase measurements. Specifically, we combine the generation of event-ready remote quantum entanglement, photon mode erasure that hides the "which-path" information of temporally and spatially separated incoming optical modes, and non-local, non-destructive photon heralding enabled by remote entanglement to perform a proof-of-concept entanglement-assisted differential phase measurement of weak incident light between two spatially separate stations. Demonstrating successful operation of the remote phase sensing protocol with a fiber link baseline up to 1.55 km, our results open the door for a new class of quantum enhanced imaging applications.

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arXiv2025

Satellite-assisted Entanglement Distribution with High-Dimensional Photonic Encoding

Satellite-assisted entanglement distribution is a promising approach for realizing long-range quantum networking. However, the limited coherence time of existing quantum memories makes it challenging to obtain multiple event-ready entangled pairs between ground stations since one pair decoheres before the successful distribution of another. We demonstrate how this can be circumvented by pairing existing satellite-compatible spontaneous parametric down conversion (SPDC) sources with qudit-compatible quantum memories on ground. By operating the SPDC source as a source of time-bin encoded photonic qudits, simultaneous distribution of multiple entangled pairs between the ground stations can be achieved at a significantly higher rate than if the SPDC sources was operated as a source of photonic qubits. We find that for achievable coherence times of several seconds and demonstrated satellite performances from the Micius satellite, the qudit operation leads to several orders of magnitude faster distribution rates than the qubit-based operation when more than one event-ready high-quality (Bell pair fidelity $\geq0.95$) entangled pair is desired. To ensure high-quality entanglement distribution, we consider multiplexed quantum memory operation storage and, in the qubit case, we also consider storage cutoff times.

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arXiv2024

Satellite-assisted quantum communication with single photon sources and atomic memories

Satellite-based quantum repeaters are a promising means to reach global distances in quantum networking due to the polynomial decrease of optical transmission with distance in free space, in contrast to the exponential decrease in optical fibers. We propose a satellite-based quantum repeater architecture with trapped individual atomic qubits, which can serve both as quantum memories and true single photon sources. This hardware allows for nearly deterministic Bell measurements and exhibits long coherence times without the need for costly cryogenic technology in space. We develop a detailed analytical model of the repeater, which includes the main imperfections of the quantum hardware and the optical link, allowing us to estimate that high-rate and high-fidelity entanglement distribution can be achieved over inter-continental distances. In particular, we find that high fidelity entanglement distribution over thousands of kilometres at a rate of 100 Hz can be achieved with orders of magnitude fewer memory modes than conventional architectures based on optical Bell state measurements.

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arXiv2023

High-Precision Observable Estimation with Single Qubit Quantum Memory

The estimation of multi-qubit observables is a key task in quantum information science. The standard approach is to decompose a multi-qubit observable into a weighted sum of Pauli strings. The observable can then be estimated from projective single qubit measurements according to the Pauli strings followed by a classical summation. As the number of Pauli strings in the decomposition increases, shot-noise drastically builds up, and the accuracy of such estimation can be considerably compromised. Access to a single qubit quantum memory, where measurement data may be stored and accumulated can circumvent the build-up of shot noise. Here, we describe a many-qubit observable estimation approach to achieve this with a much lower number of interactions between the multi-qubit device and the single qubit memory compared to previous approaches. Our algorithm offers a reduction in the required number of measurements for a given target variance that scales $N^{\frac{2}{3}}$ with the number of Pauli strings $N$ in the observable decomposition. The low number of interactions between the multi-qubit device and the memory is desirable for noisy intermediate-scale quantum devices.

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arXiv2022

Entangling remote qubits using the single-photon protocol: an in-depth theoretical and experimental study

The generation of entanglement between remote matter qubits has developed into a key capability for fundamental investigations as well as for emerging quantum technologies. In the single-photon, protocol entanglement is heralded by generation of qubit-photon entangled states and subsequent detection of a single photon behind a beam splitter. In this work we perform a detailed theoretical and experimental investigation of this protocol and its various sources of infidelity. We develop an extensive theoretical model and subsequently tailor it to our experimental setting, based on nitrogen-vacancy centers in diamond. Experimentally, we verify the model by generating remote states for varying phase and amplitudes of the initial qubit superposition states and varying optical phase difference of the photons arriving at the beam splitter. We show that a static frequency offset between the optical transitions of the qubits leads to an entangled state phase that depends on the photon detection time. We find that the implementation of a Charge-Resonance check on the nitrogen-vacancy center yields transform-limited linewidths. Moreover, we measure the probability of double optical excitation, a significant source of infidelity, as a function of the power of the excitation pulse. Finally, we find that imperfect optical excitation can lead to a detection-arm-dependent entangled state fidelity and rate. The conclusion presented here are not specific to the nitrogen-vacancy centers used to carry out the experiments, and are therefore readily applicable to other qubit platforms.

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arXiv2021

Qubit teleportation between non-neighboring nodes in a quantum network

Future quantum internet applications will derive their power from the ability to share quantum information across the network. Quantum teleportation allows for the reliable transfer of quantum information between distant nodes, even in the presence of highly lossy network connections. While many experimental demonstrations have been performed on different quantum network platforms, moving beyond directly connected nodes has so far been hindered by the demanding requirements on the pre-shared remote entanglement, joint qubit readout and coherence times. Here we realize quantum teleportation between remote, non-neighboring nodes in a quantum network. The network employs three optically connected nodes based on solid-state spin qubits. The teleporter is prepared by establishing remote entanglement on the two links, followed by entanglement swapping on the middle node and storage in a memory qubit. We demonstrate that once successful preparation of the teleporter is heralded, arbitrary qubit states can be teleported with fidelity above the classical bound, even with unit efficiency. These results are enabled by key innovations in the qubit readout procedure, active memory qubit protection during entanglement generation and tailored heralding that reduces remote entanglement infidelities. Our work demonstrates a prime building block for future quantum networks and opens the door to exploring teleportation-based multi-node protocols and applications.

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arXiv2019

Quantum Networks with Deterministic Spin-Photon Interfaces

We consider how recent experimental progress on deterministic solid state spin-photon interfaces enable the construction of a number of key elements of quantum networks. After reviewing some of the recent experimental achievements, we discuss their integration into Bell state analyzers, quantum non-demolition detection, and photonic cluster state generation. Finally, we outline how these elements can be used for long-distance entanglement generation and quantum key distribution in a quantum network.

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arXiv2016

Scalable photonic network architecture based on motional averaging in room temperature gas

Quantum interfaces between photons and ensembles of atoms have emerged as powerful tools for quantum technologies. A major objective for such interfaces is high fidelity storage and retrieval of a photon in a collective quantum state of many atoms. This requires long-lived collective superposition states, which is typically achieved with immobilized atoms. Thermal atomic vapors, which present a simple and scalable resource, have, so far, only been used for continuous variable processing or for discrete variable processing on short time scales where atomic motion is negligible. We develop a theory based on the concept of motional averaging to enable room temperature discrete variable quantum memories and coherent single photon sources. We show that by choosing the interaction time so that atoms kept under spin protecting conditions can cross the light beam several times during the interaction combined with suitable spectral filtering, we erase the "which atom" information and obtain an efficient and homogenous coupling between all atoms and the light. Heralded single excitations can thus be created and stored as collective spinwaves, which can later be read out to produce coherent single photons in a scalable fashion. We demonstrate the feasibility of this approach to scalable quantum memories with a proof-of-principle experiment with room temperature atoms contained in microcells with spin protecting coating, placed inside an optical cavity. The experiment is performed at conditions corresponding to a few photons per pulse and clearly demonstrates a long coherence time of the forward scattered photons, which is the essential feature of the motional averaging.

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arXiv2015

Heralded quantum gates with integrated error detection in optical cavities

We propose and analyze heralded quantum gates between qubits in optical cavities. They employ an auxiliary qubit to report if a successful gate occurred. In this manner, the errors, which would have corrupted a deterministic gate, are converted into a non-unity probability of success: once successful the gate has a much higher fidelity than a similar deterministic gate. Specifically, we describe that a heralded , near-deterministic controlled phase gate (CZ-gate) with the conditional error arbitrarily close to zero and the success probability that approaches unity as the cooperativity of the system, C, becomes large. Furthermore, we describe an extension to near-deterministic N- qubit Toffoli gate with a favorable error scaling. These gates can be directly employed in quantum repeater networks to facilitate near-ideal entanglement swapping, thus greatly speeding up the entanglement distribution.

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arXiv2012

Hybrid Quantum Repeater Protocol With Fast Local Processing

We propose a hybrid quantum repeater protocol combining the advantages of continuous and discrete variables. The repeater is based on the previous work of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways of improving this protocol. In the previous protocol entangled single-photon states are produced and grown into superpositions of coherent states, known as two-mode cat states. The entanglement is then distributed using homodyne detection. To improve the protocol, we replace the time-consuming non-local growth of cat states with local growth of single-mode cat states, eliminating the need for classical communication during growth. Entanglement is generated in subsequent connection processes. Furthermore the growth procedure is optimized. We review the main elements of the original protocol and present the two modifications. Finally the two protocols are compared and the modified protocol is shown to perform significantly better than the original protocol.

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