A source of heralded atom-photon entanglement for quantum networking

TL;DR

Photon loss challenges quantum networking; heralding at the sender can mitigate errors and timing uncertainties. The authors realize heralded atom-photon entanglement by cascaded two-photon emission from a single atom into two crossed fiber cavities, entangling a photon's polarization with the atomic spin while using a second photon as a herald. They achieve in-fiber qubit, herald, and heralded-qubit efficiencies of eta_q = 43(3)%, eta_h = 34(2)%, and eta_qh = 68(3)%, with entangled-state fidelity to a Bell state up to 0.87(2). The work demonstrates how heralding enables timing-gated measurements and improved resilience to detector noise, boosting prospects for noise-limited long-distance quantum networks and enabling telecom-wavelength operation via alternative atomic transitions. This heralded-source approach fits naturally into quantum repeater architectures and can be augmented with multiplexing and telecom-compatible transitions to extend reach.

Abstract

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.

Figures (7)

• Figure 1: (a) A single atom coupled to two crossed fiber cavities emits a photon entangled with the atom and another photon that heralds the entangled state generation. Photon polarization and atomic qubit measurements are subsequently performed. (b) After optically pumping the atom into state $\left|5^2S_{1/2}, F=1,m_F=0\right>$, it is optically excited to state $\left|5^2D_{5/2}, F=3, m_F=0\right>$. A polarization qubit is generated in the qubit cavity, and the cavity-enhanced emission of a $\pi$ polarized photon in the herald cavity brings the atom into a superposition of two well-defined spin states.
• Figure 2: (a) Time histogram showing the photons detected in the qubit (upper plot) and herald (lower plot) cavities during the entanglement generation and measurement periods. The fluorescence counts are magnified by a factor 20. The grey area represents the microwave (MW) pulses used for the atomic qubit measurement. (b) Photon number histogram measured during the cavity-enhanced fluorescence atomic state detection when the atom is in state $\left|5^2S_{1/2}, F=1\right>$ (black bars) and in state $\left|5^2S_{1/2}, F=2\right>$ (pink bars). (c) Fidelity of the atomic state readout as a function of the measurement duration.
• Figure 3: Real part of the atom-photon density matrix corresponding to the situation where the herald cavity is not resonant to the lower transition (a), the herald cavity is resonant to the lower transition (b), the herald cavity is resonant to the lower transition and only events with a herald photon detection are considered (c). The shaded areas represent the ideal state in Eq. \ref{['eq:bellstate']}. (d) Entangled state fidelity compared to an ideal Bell state as a function of the atomic qubit measurement-delay time. The dashed line represents the fidelity lower bound for an entangled state.(e) In-fiber efficiency to generate a qubit photon ($\eta_{q}$), a herald photon ($\eta_{h}$) and a heralded qubit photon ($\eta_{q|h}$).
• Figure 4: (a) Qubit and herald photon detection time histogram. (b) Qubit-herald photon coincidence counts as a function of the photon detection time delay. (c) Entangled state fidelity as a function of the qubit measurement noise rate. For the green data points, the same photon qubit measurement gate is used for every heralded entanglement event (with gate duration $t_g=400ns$). The blue data points indicate the situation in which the herald photon detection time information is used in order to gate the qubit photon measurement (with $t_g=40ns$). The inset shows the heralded qubit photon efficiency as a function of the measurement gate duration ($t_g$). The blue data uses the herald photon arrival time in order to define the gate center and the green data uses a sequence trigger.
• Figure 5: A single atom initially in state $\left|i\right>$ interacts with microwave radiation that couples this state with state $\left| \uparrow\right>$.