A phonon laser

By K. Vahala, ..., & T. Hansch and Th. Udem

Red-detuned laser pumping of an atomic resonance will cool the motion of an ion or atom. The complementary regime of blue-detuned pumping is investigated in this work using a single, trapped Mg⁺ ion interacting with two laser beams, tuned above and below resonance. Widely thought of as a regime of heating, theory and experiment instead show that stimulated emission of centre-of-mass phonons occurs, providing saturable amplification of the motion. A threshold for transition from thermal to coherent oscillating motion has been observed, thus establishing this system as a mechanical analogue to an optical laser—a phonon laser. Such a system has been sought in many different physical contexts.

**Groupmeeting by Xing-xing Xing**

Phase shaping of single-photon wave packets

By H.P. Specht, ..., & D. Rempe

Although the phase of a coherent light field can be precisely known, this is not true for the phase of the individual photons that create the field, considered individually¹. Phase changes within single-photon wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols^{2, 3}. Here we demonstrate arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon's amplitude profile and is verified by means of a two-photon quantum interference measurement^{4, 5}, demonstrating fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon's amplitude^{6, 7, 8, 9, 10}, frequency¹¹, and polarization¹², the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets.

**Groupmeeting by Amir Feizpour**

Laser cooling by collisional redistribution of radiation

By Ulrich Vogl & Martin Weitz

The general idea that optical radiation may cool matter was put forward 80 years ago1. Doppler cooling of dilute atomic gases is an extremely successful application of this concept2,3.More recently, anti-Stokes cooling in multilevel systems has been explored4,5, culminating in the optical refrigeration of solids6–9. Collisional redistribution of radiation has been proposed10 as a different cooling mechanism for atomic two-level systems, although experimental investigations using moderate-density gases have not reached the cooling regime11. Here we experimentally demonstrate laser cooling of an atomic gas based on collisional redistribution of radiation, using rubidiumatoms in argon buffer gas at a pressure of 230 bar. The frequent collisions in the ultradense gas transiently shift a highly red-detuned laser beam(that is, one detuned to amuch lower frequency) into resonance,whereas spontaneous decay occurs close to the unperturbed atomic resonance frequency. During each excitation cycle, kinetic energy of order kBT—that is, the thermal energy (kB, Boltzmann’s constant; T, temperature)—is extracted from the dense atomic sample. In a proof-of-principle experiment with a thermally non-isolated sample, we demonstrate relative cooling by 66 K. The cooled gas has a density more than ten orders of magnitude greater than the typical values used in Doppler-cooling experiments, and the cooling power reaches 87mW. Future applications of the technique may include supercooling beyond the homogeneous nucleation temperature12,13 and optical chillers9.

**Groupmeeting by Dylan Jervis**

Optical entanglement of co-propagating modes

By J. Janousek, .., & H.A. Bachor

Optical entanglement is a key requirement for many quantum communication protocols1. Conventionally, entanglement is formed between two distinct beams, with the quantum corre- lation measurements being performed at separate locations. Such setups can be complicated, requiring the repeated combi- nation of complex resources, a task that becomes increasingly difficult as the number of entangled information channels, or modes, increases. We pave the way towards the realization of optical multimode quantum information systems by showing continuous variable entanglement between two spatial modes within one beam. Our technique is a major advance towards practical systems with minimum complexity. We demonstrate three major experimental achievements. First, only one source is required to produce squeezed light in two orthogonal spatial modes. Second, entanglement is formed through lenses and beam rotation, without the need for a beamsplitter. Finally, quantum correlations are measured directly and simul- taneously using a multipixel quadrant detector.

**Group Meeting By Zachari Medendorp**

Simple pulses for elimination of leakage in weakly nonlinear qubits

By F. Motzoi, ..., F. K. Wilhelm

In realizations of quantum computing, a two-level system (qubit) is often singled out of the many levels of an anharmonic oscillator. In these cases, simple qubit control fails on short time scales because of coupling to leakage levels. We provide an easy to implement analytic formula that inhibits this leakage from any single-control analog or pixelated pulse. It is based on adding a second control that is proportional to the time-derivative of the first. For realistic parameters of superconducting qubits, this strategy reduces the error by an order of magnitude relative to the state of the art, all based on smooth and feasible pulse shapes. These results show that even weak anharmonicity is sufficient and in general not a limiting factor for implementing quantum gates.

**Groupmeeting by Chao Zhuang**