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Superradiance-Enhanced Heralded Single Photon Generation in Cold Alkali Ensembles

Travis Hosack

Published Jan 1, 2026• 0 citations
Publisher

Abstract

With the accelerating developments in the field of quantum communication, there will be a need for sources of highly quantum single-photons. In addition to communication applications, the purity, brightness, and fidelity of a given single-photon source are intrinsically linked to the success rate and scalability of a range of photonic technologies in quantum computing, metrology, memory, and cryptography. Further, quantum repeater architectures for maintaining quantum information over long distances will become crucial. In these schemes, end-to-end entanglement is generated by establishing entanglement between the ends of several elementary segments, followed by successive entanglement-swapping operations to increase the entanglement distance until it spans the transmission channel. The most widely studied proposal for the practical implementation of a quantum repeater is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. This consists of off-resonant Raman excitation by a write pulse, which induces a spin-wave excitation in an atomic ensemble and emits a Stokes photon. At a later time, this spin-wave may be read with a high-power resonant pulse, resulting in the emission of an anti-Stokes photon into a well-defined spatial mode. The Stokes photon may be used for entanglement with another ensemble, while the anti-Stokes may be read out for an entanglement-swapping operation. Notably, in the DLCZ protocol, the atomic ensemble serves as both the quantum memory and a heralded single-photon source. Using atomic ensembles for single-photon sources enables the production of highly pure, indistinguishable photons with minimal spectral broadening; however, the generation rate remains a challenge. In the case of DLCZ, the Raman-scattering step is highly probabilistic, reducing the anti-Stokes generation rate considerably. Furthermore, DLCZ requires operation in the low-excitation-probability regime to avoid multiphoton noise contributions. Additionally, the profile of the induced spin wave is suboptimal for readout. To compensate for these drawbacks, in this thesis we report the experimental realization of a variant of DLCZ that leverages collectively enhanced superradiant (SR) emission. Superradiant emission is characterized by intense bursts of photons emitted in a shorter amount of time than independent atomic state decay. By switching to a resonant excitation and optimizing the write profile, the induced spin-wave is primed for readout, thus increasing retrieval efficiency and the level of quantum correlation. We report data for both the DLCZ and SR schemes under similar optical conditions and compare the two. At high write power, the SR protocol exhibits conditional readout probabilities as high as $\sim$1.2\%, compared to $\sim$0.6\% for the DLCZ protocol under the same conditions. We also analyze the temporal distribution of anti-Stokes emission and find a degree of maintenance of the spin-wave profile, even in the anti-Stokes direction.

DOI
10.7939/84566

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With the accelerating developments in the field of quantum communication, there will be a need for sources of highly quantum single-photons. In addition to communication applications, the purity, brightness, and fidelity of a given single-photon source are intrinsically linked to the success rate and scalability of a range of photonic technologies in quantum computing, metrology, memory, and cryptography. Further, quantum repeater architectures for maintaining quantum information over long distances will become crucial. In these schemes, end-to-end entanglement is generated by establishing entanglement between the ends of several elementary segments, followed by successive entanglement-swapping operations to increase the entanglement distance until it spans the transmission channel. The most widely studied proposal for the practical implementation of a quantum repeater is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. This consists of off-resonant Raman excitation by a write pulse, which induces a spin-wave excitation in an atomic ensemble and emits a Stokes photon. At a later time, this spin-wave may be read with a high-power resonant pulse, resulting in the emission of an anti-Stokes photon into a well-defined spatial mode. The Stokes photon may be used for entanglement with another ensemble, while the anti-Stokes may be read out for an entanglement-swapping operation. Notably, in the DLCZ protocol, the atomic ensemble serves as both the quantum memory and a heralded single-photon source. Using atomic ensembles for single-photon sources enables the production of highly pure, indistinguishable photons with minimal spectral broadening; however, the generation rate remains a challenge. In the case of DLCZ, the Raman-scattering step is highly probabilistic, reducing the anti-Stokes generation rate considerably. Furthermore, DLCZ requires operation in the low-excitation-probability regime to avoid multiphoton noise contributions. Additionally, the profile of the induced spin wave is suboptimal for readout. To compensate for these drawbacks, in this thesis we report the experimental realization of a variant of DLCZ that leverages collectively enhanced superradiant (SR) emission. Superradiant emission is characterized by intense bursts of photons emitted in a shorter amount of time than independent atomic state decay. By switching to a resonant excitation and optimizing the write profile, the induced spin-wave is primed for readout, thus increasing retrieval efficiency and the level of quantum correlation. We report data for both the DLCZ and SR schemes under similar optical conditions and compare the two. At high write power, the SR protocol exhibits conditional readout probabilities as high as ∼\sim∼1.2%, compared to ∼\sim∼0.6% for the DLCZ protocol under the same conditions. We also analyze the temporal distribution of anti-Stokes emission and find a degree of maintenance of the spin-wave profile, even in the anti-Stokes direction.
The Duan-Lukin-Cirac-Zoller (DLCZ) process in the atomic ensemble is an important means to generate quantum correlation and entanglement between photons and atoms (quantum interface). When a write pulse acts on atoms, the DLCZ quantum memory process will be generated, which has been extensively studied. In the process a spontaneous Raman scattering (SRS) of a Stokes photon is generated, and a spin-wave excitation stored in the atomic ensemble is created at the same time. The higher probability of the generation of Stokes photons will cause more noise and reduce entanglement. On the contrary, the low generation probability of Stokes photons affects the success probability of entanglement distribution on a quantum repeater. How to increase generation probability of Stokes photons without causing more noise is an urgent problem to be resolved. In this work, a 87Rb atomic ensemble is placed in a standing wave cavity which resonates with the Stokes photon. This cavity has a trip length of 0.6 m and a free spectral range (FSR) of 256 MHz. The optical loss of all the optical elements in this cavity is 9%, of which 4% loss originates from the other optical elements and 5% loss from the vacuum chamber of the magneto-optical trap (MOT). The fineness of the cavity with the cold atoms is measured to be ~19.1. By calculating the total probability of Stokes photon emission out of the cavity, we derive the enhancement factor of this standing wave cavity when the cavity loss is l. When this cavity is locked with PDH frequency locking technique, we observe that the production probability of the Stokes photons is 8.7 times higher than that without cavity due to the optical cavity enhancement effect. Under this condition, the relationship between the generation probability of Stokes photons and the power of write beam is studied. The write excitation probability changes linearly with the power of write beam. This work provides an experimental solution to reducing the noise caused by time multimode operation in DLCZ scheme.
Atomic ensembles in the form of rare-earth-doped crystals offer significant potential for implementing a quantum memory due to their long coherence lifetimes, enhanced light-matter interaction, and the potential for integration. We use a double Lambda configuration based on hyperfine levels in praseodymium doped yttrium orthosilicate. In this scheme, a probabilistically generated collective excitation of the internal atomic states in the form of a spin-wave is signaled by emission of a Stokes-shifted herald photon via spontaneous Raman scattering. After a storage delay, a read field addresses the appropriate transition, mapping the spin wave back to a photon, resulting in the emission of an anti-Stokes shifted retrieved photon. Momentum conservation, as dictated by phase-matching conditions, ensure that the herald and retrieved photons are emitted in a well-defined spatial modes relative to the co-propagating write and read optical fields. Inhomogeneous broadening of the absorption profile precludes straightforward implementation of the above scheme, and we resort to spectral hole-burning techniques to prepare a sub-ensemble of atoms with a narrow range of optical transition energy in the ground state. Using a second identical crystal, we also implement an optical spectral filter that suppresses the powerful write and read beams, and spontaneous-emission noise, while transmitting the herald and retrieved photons. We report on the second-order optical correlations between the herald and retrieved photons that are the signature of generation and retrieval of a stored spin-wave excitation. Demonstration of such a quantum memory is an enabling step towards distributing entanglement for a quantum repeater scheme.
Practical realization of quantum repeaters requires quantum memories with high retrieval efficiency, multi-mode storage capacities, and long lifetimes. Here, we report a high-retrieval-efficiency and temporally multiplexed atom-photon entanglement source. A train of 12 write pulses in time is applied to a cold atomic ensemble along different directions, which generates temporally multiplexed pairs of Stokes photons and spin waves via Duan-Lukin-Cirac-Zoller processes. The two arms of a polarization interferometer are used to encode photonic qubits of 12 Stokes temporal modes. The multiplexed spin-wave qubits, each of which is entangled with one Stokes qubit, are stored in a "clock" coherence. A ring cavity that resonates simultaneously with the two arms of the interferometer is used to enhance retrieval from the spin-wave qubits, with the intrinsic retrieval efficiency reaching 70.4%. The multiplexed source gives rise to a ∼12.1-fold increase in atom-photon entanglement-generation probability compared to the single-mode source. The measured Bell parameter for the multiplexed atom-photon entanglement is 2.21(2), along with a memory lifetime of up to ∼125 µs.
Quantum communication aims at generating and distributing qubits between distant sites, in order to implement quantum information protocols, such as quantum key distribution, quantum teleportation and quantum processing. For many quantum protocols, and especially for quantum repeaters, which make quantum communication over long distances much more efficient, storage and/or emission of photonic states is necessary and represents a crucial step towards the realisation of a quantum network. An outstanding challenge of quantum communication is to implement quantum memories for photonic qubits, combining long storage time with high fidelity between the input and output states, and good efficiency. Moreover, to be compatible with quantum network applications, such as quantum repeaters, these memories have to enable heralded storage and on-demand read-out. Cold atomic ensembles hold promise for satisfying both these prerequisites, and are therefore a good candidate as storage medium, offering a compromise between different benchmarks. We are setting up an experiment intended to map polarisation qubits onto an optically pumped cold atomic ensemble of rubidium serving as our quantum memory device. In a first development, our quantum memory device will be operated in the single photon emission regime so as to implement, together with an entangled photon-pair source based on guided-wave optics, the quantum teleportation scheme.
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrated ES between two entangled pairs of spin-wave memories via Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically-correlated spin-wave-photon pairs in 12 spatial modes and then prepare two entangled pairs of spin-wave memories via a multiplexed scheme. Via single-photon Bell measurement on retrieved fields from two memories, we project the two remaining memories never entangled previously into an entangled state with the measured concurrence of C = 0.0124(0.003). The successful probability of ES in our scheme is increased by three times, compared with that in non-multiplexed scheme. Our presented work shows that the generation of entanglement (C>0) between the remaining memory ensembles requires the average cross-correlation function of the spin-wave-photon pairs to be >30 .
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrate ES between different spatial modes for a cold-atom-ensemble quantum memory via the Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically correlated spin-wave-photon pairs in 12 spatial modes. We then prepare two entangled pairs of spin-wave modes through memory multiplexing. Via single-photon Bell-state measurement on retrieved fields from two spin-wave modes, we project the two remaining modes never entangled previously into an entangled state with a measured concurrence of C =0.0124±0.0030. The successful probability of ES in our scheme is increased by three times, compared with that in the scheme without memory multiplexing. Our presented work shows that the generation of entanglement ( C >0) between the remaining spin-wave modes requires the average cross-correlation function of the spin-wave-photon pairs to be ≥30. This presented work represents a significant step toward practical quantum repeaters.