Quantum Electromagnetics

Chapter 15 derived the fundamental theory and eigenstates for the quantized radiation field and then showed how the quantum vacuum gives rise to spontaneous emission. This chapter now goes more deeply into the meaning and implications of the quantized field. The polarization of a photon can be used as a qubit and the photonic qubit is called a flying qubit. It enables transmission of information from one node to another. Spontaneous emission is shown to enable creation of an entangled state between a photonic qubit and the spin of an electron. Spontaneous emission can also degrade the performance of some device designs and in other devices it can enhance performance such as for a single photon emitter. In this we show how to engineer the vacuum to control spontaneous emission.

Nondestructive detection of photonic qubits

One of the biggest challenges in experimental quantum information is to sustain the fragile superposition state of a qubit1. Long lifetimes can be achieved for material qubit carriers as memories2, at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering3. The loss problem can be mitigated with a nondestructive photonic qubit detector that heralds the photon without destroying the encoded qubit. Such a detector is envisioned to facilitate protocols in which distributed tasks depend on the successful dissemination of photonic qubits4,5, improve loss-sensitive qubit measurements6,7 and enable certain quantum key distribution attacks8. 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 and the other for atomic-state detection9. We achieve a nondestructive detection efficiency upon qubit survival of 79 ± 3 per cent and a photon survival probability of 31 ± 1 per cent, and we preserve the qubit information with a fidelity of 96.2 ± 0.3 per cent. To illustrate the potential of our detector, we show that it can, with the current parameters, improve the rate and fidelity of long-distance entanglement and quantum state distribution compared to previous methods, provide resource optimization via qubit amplification and enable detection-loophole-free Bell tests.