Karim Murr

Electromagnetically induced transparency with single atoms in a cavity

Optical nonlinearities offer unique possibilities for the control of light with light. A prominent example is electromagnetically induced transparency (EIT), where the transmission of a probe beam through an optically dense medium is manipulated by means of a control beam. Scaling such experiments into the quantum domain with one (or just a few) particles of light and matter will allow for the implementation of quantum computing protocols with atoms and photons, or the realization of strongly interacting photon gases exhibiting quantum phase transitions of light. Reaching these aims is challenging and requires an enhanced matter–light interaction, as provided by cavity quantum electrodynamics. Here we demonstrate EIT with a single atom quasi-permanently trapped inside a high-finesse optical cavity. The atom acts as a quantum-optical transistor with the ability to coherently control the transmission of light through the cavity. We investigate the scaling of EIT when the atom number is increased one-by-one. The measured spectra are in excellent agreement with a theoretical model. Merging EIT with cavity quantum electrodynamics and single quanta of matter is likely to become the cornerstone for novel applications, such as dynamic control of the photon statistics of propagating light fields or the engineering of Fock state superpositions of flying light pulses.

Nonlinear spectroscopy of photons bound to one atom

Nonlinear optics traditionally involves macroscopic atomic ensembles or solid-state crystals. The observation of a nonlinear two-photon resonance in a system consisting of one single atom trapped inside an optical cavity demonstrates nonlinear optics at the level of individual quanta.

Two-photon gateway in one-atom cavity quantum electrodynamics

Atoms absorb and emit resonant light photon by photon. However, an atom strongly coupled to the light field of a high-finesse optical resonator forms a new system with new characteristics, enabling the emission and absorption of photon pairs. The energy eigenstates of the coupled system form an anharmonic ladder of doublets, the so-called Jaynes-Cummings ladder. The first doublet, consisting of the two so-called normal modes, has been observed in various cavity QED systems. It can be explained classically using Maxwell equations as arising from the coupling of a classical dipole to a classical electromagnetic field. In contrast, the higher-order doublet has so far no classical analogue. It requires both the quantization of the atomic internal state as well as the quantization of light.

Trapping and observing single atoms in a blue-detuned intracavity dipole trap

An experimental sample trace of a trapping event is presented 85Rb atoms have been launched from an atomic fountain along the y-direction. Arming the trigger at 210 ms after launch selects atoms with velocities <0.1 m/s. Because the probe field is resonant with the empty cavity, single atoms cause a steep decrease in the transmission (3). The storage times (A-B) are of the order of a few 10 ms. The Stark shift as measured by normal-mode spectroscopy is much smaller than the trap height. In conclusion, we have realized a blue-detuned intracavity dipole trap, now allowing measurements in cavity QED while preserving the free-space properties of the atom.A single atom strongly coupled to a cavity mode is stored by three-dimensional confinement in blue-detuned cavity modes of different longitudinal and transverse order. The vanishing light intensity at the trap center reduces the light shift of all atomic energy levels. This is exploited to detect a single atom by means of a dispersive measurement with 95% confidence in 10 micros, limited by the photon-detection efficiency. As the atom switches resonant cavity transmission into cavity reflection, the atom can be detected while scattering about one photon.

Photon-by-photon feedback control of a single-atom trajectory

Feedback is one of the most powerful techniques for the control of classical systems. An extension into the quantum domain is desirable as it could allow the production of non-trivial quantum states and protection against decoherence. The difficulties associated with quantum, as opposed to classical, feedback arise from the quantum measurement process—in particular the quantum projection noise and the limited measurement rate—as well as from quantum fluctuations perturbing the evolution in a driven open system. Here we demonstrate real-time feedback control of the motion of a single atom trapped in an optical cavity. Individual probe photons carrying information about the atomic position activate a dipole laser that steers the atom on timescales 70 times shorter than the atom’s oscillation period in the trap. Depending on the specific implementation, the trapping time is increased by a factor of more than four owing to feedback cooling, which can remove almost all the kinetic energy of the atom in a quarter of an oscillation period. Our results show that the detected photon flux reflects the atomic motion, and thus mark a step towards the exploration of the quantum trajectory of a single atom at the standard quantum limit.

Observation of squeezed light from one atom excited with two photons

Single quantum emitters such as atoms are well known as non-classical light sources with reduced noise in the intensity, capable of producing photons one by one at given times. However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example is the predicted ability of a single atom to produce quadrature-squeezed light, which has fluctuations of amplitude or phase that are below the shot-noise level. However, such squeezing is much more difficult to observe than the emission of single photons. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms, but despite experimental efforts, single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity, which is several orders of magnitude larger than in typical macroscopic media. This produces observable quadrature squeezing, with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons, the squeezed light stems from the quantum coherence of photon pairs emitted from the system. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

Three-Photon Correlations in a Strongly Driven Atom-Cavity System

The quantum dynamics of a strongly driven, strongly coupled single-atom-cavity system is studied by evaluating time-dependent second- and third-order correlations of the emitted photons. The coherent energy exchange, first, between the atom and the cavity mode, and second, between the atom-cavity system and the driving laser, is observed. Three-photon detections show an asymmetry in time, a consequence of the breakdown of detailed balance. The results are in good agreement with theory and are a first step towards the control of a quantum trajectory at larger driving strength.

Large Suppression of Quantum Fluctuations of Light from a Single Emitter by an Optical Nanostructure

We investigate the reduction of the electromagnetic field fluctuations in resonance fluorescence from a single emitter coupled to an optical nanostructure. We find that such hybrid systems can lead to the creation of squeezed states of light, with quantum fluctuations significantly below the shot-noise level. Moreover, the physical conditions for achieving squeezing are strongly relaxed with respect to an emitter in free space. A high degree of control over squeezed light is feasible both in the far and near fields, opening the pathway to its manipulation and applications on the nanoscale with state-of-the-art setups.

Electromagnetically Induced Transparency with Single Atoms in a Cavity

We report on the observation of Electromagnetically Induced Transparency (EIT) with a single atom quasi‐permanently trapped inside a high‐finesse optical cavity. In the experiment, the atom acts as a quantum‐optical transistor with the ability to coherently control the transmission of light through the cavity.

Light force fluctuations in a strongly coupled atom–cavity system

Between mirrors, the density of electromagnetic modes differs from the one in free space. This changes the radiation properties of an atom as well as the light forces acting on an atom. It has profound consequences in the strong-coupling regime of cavity quantum electrodynamics. For a single atom trapped inside the cavity, we investigate the atom–cavity system by scanning the frequency of a probe laser for various atom–cavity detunings. The avoided crossing between atom and cavity resonance is visible in the transmission of the cavity. It is also visible in the loss rate of the atom from the intracavity dipole trap. On the normal-mode resonances, the dominant contribution to the loss rate originates from dipole-force fluctuations which are dramatically enhanced in the cavity. This conclusion is supported by Monte Carlo simulations.