arXiv subjects

Gerhard Rempe

Publications and source records attributed to Gerhard Rempe.

At least 19 recordsLinked to original sources

arXiv2026

Metropolitan entanglement distribution between an atom and a near-visible photon

Entanglement distribution is the overarching purpose of quantum networks. While communication over long distances can use deployed fiber infrastructure, it requires photons in the telecom band. However, advanced quantum network nodes do not operate at such wavelengths. Here we overcome this limitation with two tailor-made low-noise quantum-frequency converters to distribute entanglement between a single atom and a resonant photon over 14km line-of-sight via 24km of a deployed commercial fiber. The photon at wavelength 780nm is first entangled with the atom, then converted to the telecom S-band, and finally back-converted after propagation through the fiber. This link enables a photon transfer efficiency of 1.7% while affecting the atom-photon entanglement fidelity by less than 1%. This brings integration of atomic quantum nodes with existing long-distance fiber networks into reach, enabling novel applications in quantum information processing.

quant-ph↗

arXiv2026

Efficient entanglement of three remote single-atom quantum-network nodes

Entanglement distributed over a set of individually addressable qubit nodes is the enabling resource for a plethora of applications ranging from tests of quantum physics to secure and modular quantum information networks. Entanglement between two memory qubits has been realized on various platforms, but extension to more nodes remains rare and formidably challenging. The principal bottleneck is the efficiency of the light-matter interfaces connecting the qubit nodes to their communication channels. Here, we efficiently generate, distribute and store a three-qubit entangled state across three independent laboratories containing single atoms coupled to optical resonators. We sequentially entangle the atoms pairwise, two by heralded photonic entanglement swapping and two by heralded state transfer. We reach a three-qubit entanglement fidelity of 77(1)% and an entanglement lifetime above 200us. The observed qubit correlations violate Mermin's inequality while closing the detection loophole. Our three-qubit entanglement-generation efficiency is 0.16%. This unprecedented efficiency of our scheme establishes a clear route towards multi-node quantum networks.

quant-ph↗

arXiv2025

A source of heralded atom-photon entanglement for quantum networking

Communication in quantum networks suffers notoriously from photon loss. Resulting errors can be mitigated with a suitable measurement herald at the receiving node. However, waiting for a herald and communicating the measurement result back to the sender in a repeat-until-success strategy makes the protocol slow and prone to errors from false heralds such as detector dark counts. Here we implement an entanglement herald at the sending node by employing a cascaded two-photon emission of a single atom into two optical fiber cavities: The polarization of one photon is entangled with the spin of the atom, and the second photon heralds entanglement generation. We show that heralding improves the atom-photon entanglement in-fiber efficiency and fidelity to 68(3)% and 87(2)%, respectively. We highlight the potential of our source for noise-limited long-distance quantum communication by extending the range for constant fidelity or, alternatively, increasing the fidelity for a given distance.

quant-ph↗

arXiv2025

Optimizing decoherence in the generation of optical Schr\"odinger cat states

We propose to use cavity Rydberg electromagnetically induced transparency to generate Schr\"odinger cat states of optical photons. We predict that this should make it possible to generate states with relatively large mean photon numbers. With existing technology, mean photon numbers around 30 seem feasible. The main limitation is photon loss during the process, which generates the state. The ability to tune the strength of the photon loss caused by atomic spontaneous emission makes it possible to have little decoherence despite significant photon loss during the generation of the state.

quant-ph↗

arXiv2025

Tweezer-assisted subwavelength positioning of atomic arrays in an optical cavity

We present an experimental technique that enables the preparation of defect-free arrays of 87Rb atoms within a microscopic high-finesse optical standing-wave cavity. By employing optical tweezers, we demonstrate atom positioning with a precision well below the cavity wavelength, a crucial requirement for cavity-QED experiments in which maximum atom-cavity coupling strength is required. We leverage our control capabilities to assemble an array of up to seven atoms with an efficiency that exceeds previous probabilistic methods by 4 orders of magnitude. The atoms are subsequently transferred from the tweezer array to a two-dimensional intracavity optical lattice that offers enhanced coherence for spin qubits while maintaining strong atom confinement. Our system overcomes the efficiency limitations of previous probabilistic loading techniques of cavity-coupled atom arrays and opens the path to multiqubit quantum networks with atoms strongly coupled to optical cavities.

quant-ph↗

arXiv2025

Two-cavity-mediated photon-pair emission by one atom

Photon-pair sources are widely used in quantum optics and quantum information experiments. Despite their broad deployment, there has not yet been an on-demand implementation with efficient into-fiber photon generation and high single-photon purity. Here we report on such a source based on a single atom with three energy levels in ladder configuration and coupled to two optical fiber cavities. We efficiently generate photon pairs with in-fiber emission efficiency of $\eta_{\mathrm{pair}}=16(1)\%$ and study their temporal correlation properties. We simulate theoretically a regime with strong atom-cavity coupling and find that photons are directly emitted from the ground state, i.e. without atomic population in any intermediate state. We propose a scenario to observe such a double-vacuum-stimulated effect experimentally.

quant-ph↗

arXiv2024

Coherence of Symmetry-Protected Rotational Qubits in Cold Polyatomic Molecules

Polar polyatomic molecules provide an ideal but largely unexplored platform to encode qubits in rotational states. Here, we trap cold (100-600 mK) formaldehyde (H$_2$CO) inside an electric box and perform a Ramsey-type experiment to observe long-lived (~100 $\mu$s) coherences between symmetry-protected molecular states with opposite rotation but identical orientation, representing a quasi-hidden molecular degree of freedom. As a result, the observed qubit is insensitive to the magnitude of an external electric field, and depends only weakly on magnetic fields. Our findings provide a basis for future quantum and precision experiments with trapped cold molecules.

physics.atom-ph↗

arXiv2024

A nondestructive Bell-state measurement on two distant atomic qubits

One of the most fascinating aspects of quantum networks is their capability to distribute entanglement as a nonlocal communication resource. In a first step, this requires network-ready devices that can generate and store entangled states. Another crucial step, however, is to develop measurement techniques that allow for entanglement detection. Demonstrations for different platforms suffer from being either not complete, or destructive, or local. Here we demonstrate a complete and nondestructive measurement scheme that always projects any initial state of two spatially separated network nodes onto a maximally entangled state. Each node consists of an atom trapped inside an optical resonator from which two photons are successively reflected. Polarisation measurements on the photons discriminate between the four maximally entangled states. Remarkably, such states are not destroyed by our measurement. In the future, our technique might serve to probe the decay of entanglement and to stabilise it against dephasing via repeated measurements.

quant-ph↗

arXiv2024

A quantum-network register assembled with optical tweezers in an optical cavity

Quantum computation and quantum communication are expected to provide users with capabilities inaccessible by classical physics. However, scalability to larger systems with many qubits is challenging. One solution is to develop a quantum network consisting of small-scale quantum registers containing computation qubits that are reversibly interfaced to communication qubits. Here we report on a register that uses both optical tweezers and optical lattices to deterministically assemble a two-dimensional array of atoms in an optical cavity. Harnessing a single-atom addressing beam, we stimulate the emission of a photon from each atom and demonstrate multiplexed atom-photon entanglement with a generation-to-detection efficiency approaching 90$\%$. Combined with cavity-mediated quantum logic, our approach provides a possible route to distributed quantum information processing.

quant-ph↗

arXiv2024

Fusion of deterministically generated photonic graph states

Entanglement has evolved from an enigmatic concept of quantum physics to a key ingredient of quantum technology. It explains correlations between measurement outcomes that contradict classical physics, and has been widely explored with small sets of individual qubits. Multi-partite entangled states build up in gate-based quantum-computing protocols, and $\unicode{x2013}$ from a broader perspective $\unicode{x2013}$ were proposed as the main resource for measurement-based quantum-information processing. The latter requires the ex-ante generation of a multi-qubit entangled state described by a graph. Small graph states such as Bell or linear cluster states have been produced with photons, but the proposed quantum computing and quantum networking applications require fusion of such states into larger and more powerful states in a programmable fashion. Here we achieve this goal by employing an optical resonator containing two individually addressable atoms. Ring and tree graph states with up to eight qubits, with the names reflecting the entanglement topology, are efficiently fused from the photonic states emitted by the individual atoms. The fusion process itself employs a cavity-assisted gate between the two atoms. Our technique is in principle scalable to even larger numbers of qubits, and is the decisive step towards, for instance, a memory-less quantum repeater in a future quantum internet.

quant-ph↗

arXiv2022

Efficient generation of entangled multi-photon graph states from a single atom

Entanglement is a powerful concept with an enormous potential for scientific and technological advances. A central focus in modern research is to extend the generation and control of entangled states from few to many qubits, and protect them against decoherence. Optical photons play a prominent role as these qubit carriers are naturally robust and easy to manipulate. However, the most successful technique to date for creating photonic entanglement is inherently probabilistic and therefore subject to severe scalability limitations. Here we avoid these by implementing a deterministic protocol with a single memory atom in a cavity. We interleave controlled single-photon emissions with tailored atomic qubit rotations to efficiently grow Greenberger-Horne-Zeilinger states of up to 14 photons and linear cluster states of up to 12 photons with a fidelity lower bounded by 76(6)% and 56(4)%, respectively. Thanks to a source-to-detection efficiency of 43.18(7)% per photon we measure these large states about once every minute, orders of magnitude faster than in any previous experiment. In the future, this rate could be increased even further, the scheme could be extended to two atoms in a cavity, or several sources could be quantum mechanically coupled, to generate higher-dimensional cluster states. Overcoming the limitations encountered by probabilistic schemes for photonic entanglement generation, our results may offer a way towards scalable measurement-based quantum computation and communication.

quant-ph↗

arXiv2021

Bright and Dark States of Light: The quantum Origin of Classical Interference

Classical theory asserts that several electromagnetic waves cannot interact with matter if they interfere destructively to zero, whereas quantum mechanics predicts a nontrivial light-matter dynamics even when the average electric field vanishes. Here we show that in quantum optics classical interference emerges from collective bright and dark states of light, \textit{i.e.}, entangled superpositions of multi-mode photon-number states. This makes it possible to explain wave interference using the particle description of light and the superposition principle for linear systems.

quant-ph↗

arXiv2021

Quantum-Logic Gate between Two Optical Photons with an Average Efficiency above 40%

Optical qubits uniquely combine information transfer in optical fibers with a good processing capability and are therefore attractive tools for quantum technologies. A large challenge, however, is to overcome the low efficiency of two-qubit logic gates. The experimentally achieved efficiency in an optical controlled NOT (CNOT) gate reached approximately 11% in 2003 and has seen no increase since. Here we report on a new platform that was designed to surpass this long-standing record. The new scheme avoids inherently probabilistic protocols and, instead, combines aspects of two established quantum nonlinear systems: atom-cavity systems and Rydberg electromagnetically induced transparency. We demonstrate a CNOT gate between two optical photons with an average efficiency of 41.7(5)% at a postselected process fidelity of 81(2)%. Moreover, we extend the scheme to a CNOT gate with multiple target qubits and produce entangled states of presently up to five photons. All these achievements are promising and have the potential to advance optical quantum information processing in which almost all advanced protocols would profit from high-efficiency logic gates.

quant-ph↗

arXiv2021

High-resolution 'magic'-field spectroscopy on trapped polyatomic molecules

Rapid progress in cooling and trapping of molecules has enabled first experiments on high resolution spectroscopy of trapped diatomic molecules, promising unprecedented precision. Extending this work to polyatomic molecules provides unique opportunities due to more complex geometries and additional internal degrees of freedom. Here, this is achieved by combining a homogeneous-field microstructured electric trap, rotational transitions with minimal Stark broadening at a 'magic' offset electric field, and optoelectrical Sisyphus cooling of molecules to the low millikelvin temperature regime. We thereby reduce Stark broadening on the $J=5\leftarrow4$ ($K=3$) transition of formaldehyde at $364\,$GHz to well below $1\,$kHz, observe Doppler-limited linewidths down to $3.8\,$kHz, and determine the 'magic'-field line position with an uncertainty below $100\,$Hz. Our approach opens a multitude of possibilities for investigating diverse polyatomic molecule species.

physics.atom-ph↗

arXiv2021

Detecting an Itinerant Optical Photon Twice without Destroying It

Nondestructive quantum measurements are central for quantum physics applications ranging from quantum sensing to quantum computing and quantum communication. Employing the toolbox of cavity quantum electrodynamics, we here concatenate two identical nondestructive photon detectors to repeatedly detect and track a single photon propagating through a $60\,\mathrm{m}$ long optical fiber. By demonstrating that the combined signal-to-noise ratio of the two detectors surpasses each single one by about two orders of magnitude, we experimentally verify a key practical benefit of cascaded non-demolition detectors compared to conventional absorbing devices.

quant-ph↗

arXiv2021

Quantum Teleportation between Remote Qubit Memories with Only a Single Photon as a Resource

Quantum teleportation enables the deterministic exchange of qubits via lossy channels. While it is commonly believed that unconditional teleportation requires a preshared entangled qubit pair, here we demonstrate a protocol that is in principle unconditional and requires only a single photon as an ex-ante prepared resource. The photon successively interacts, first, with the receiver and then with the sender qubit memory. Its detection, followed by classical communication, heralds a successful teleportation. We teleport six mutually unbiased qubit states with average fidelity $\overline{\text{F}}=(88.3\pm1.3)\%$ at a rate of $6\,\mathrm{Hz}$ over $60\,\mathrm{m}$.

quant-ph↗

arXiv2021

Nondestructive detection of photonic qubits

One of the biggest challenges in experimental quantum information is to keep the fragile superposition state of a qubit alive. Long lifetimes can be achieved for material qubit carriers as memories, at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering. The loss problem can be mitigated with a nondestructive photonic qubit detector that heralds the photon without destroying the encoded qubit. Such detector is envisioned to facilitate protocols where distributed tasks depend on the successful dissemination of photonic qubits, to improve loss-sensitive qubit measurements, and to enable certain quantum key distribution attacks. Here we demonstrate such a detector based on a single atom in two crossed fibre-based optical resonators, one for qubit-insensitive atom-photon coupling, the other for atomic-state detection. We achieve a nondestructive detection efficiency upon qubit survival of $(79\pm3)\,\%$, a photon survival probability of $(31\pm1)\,\%$, and preserve the qubit information with a fidelity of $(96.2\pm0.3)\,\%$. To illustrate the potential of our detector we show that it can provide, already with current parameters, an advantage for long-distance entanglement and quantum-state distribution, resource optimization via qubit amplification, and detection-loophole-free Bell tests.

quant-ph↗

arXiv2021

A Quantum-Logic Gate between Distant Quantum-Network Modules

The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60m. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection which triggers a final qubit rotation. We use the gate for remote entanglement creation of all four Bell states. Our non-local quantum-logic gate could be extended both to multiple qubits and many modules for a tailor-made multi-qubit computing register.

quant-ph↗