We propose in this White Paper a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra-light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms, and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity.KCL-PH-TH/2019-65, CERN-TH-2019-126

We control the electronic structure of the silicon-vacancy (SiV) color-center in diamond by changing its static strain environment with a nano-electro-mechanical system. This allows deterministic and local tuning of SiV optical and spin transition frequencies over a wide range, an essential step towards multi-qubit networks. In the process, we infer the strain Hamiltonian of the SiV revealing large strain susceptibilities of order 1 PHz/strain for the electronic orbital states. We identify regimes where the spin-orbit interaction results in a large strain suseptibility of order 100 THz/strain for spin transitions, and propose an experiment where the SiV spin is strongly coupled to a nanomechanical resonator.arXiv:1801.09833v2 [quant-ph]

We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a Pr 3þ ∶Y 2 SiO 5 crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio > 10. Narrow-band spectral filtering based on spectral hole burning in a second Pr 3þ ∶Y 2 SiO 5 crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of ð2.0 AE 0.3Þ × 10 −3 photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks. DOI: 10.1103/PhysRevLett.114.230501 PACS numbers: 03.67.Hk, 42.50.Ex, 42.50.Gy, 78.55.Qr Photonic quantum memories are essential in quantum information science (QIS) where they are used as quantum interfaces between flying and stationary qubits. They enable the synchronization of probabilistic quantum processes, e.g., in quantum communication [1,2] and computing [3]. The implementation of quantum memories (QMs) for light requires strong interactions between individual photons and matter. This can be achieved by placing individual quantum systems (e.g., single atoms) in high finesse cavities [4] or by using ensembles of atoms, where the photons are mapped onto collective atomic excitations. Atomic systems are natural candidates as QMs [5][6][7][8][9][10][11][12][13][14], but solid state systems offer interesting perspectives for scalability and integration into existing technology [15][16][17][18][19][20][21].Rare-earth ion doped solids are promising candidates for high performance solid state QMs since they have excellent coherence properties at cryogenic temperatures [22]. They also exhibit large static inhomogeneous broadening of the optical transitions, which can be tailored and used as a resource for various storage protocols, e.g., enabling temporally [23] and spectrally [24] multiplexed quantum memories. Recent experimental progress includes qubit storage [15,[24][25][26][27], highly efficient quantum storage of weak coherent states [16], storage of entangled and single photons [17,18,28], entanglement between two crystals [29], and quantum teleportation [30].Yet, nonclassical states have so far only been stored as collective optical atomic excitations with fixed storage times [17,18,28]. While this may provide a useful resource if combined with massive multiplexing and deterministic quantum light sources [24], the ability to read-out the stored state on demand is essential for applications where the quantum memory is used as a synchronizing device. On-demand read-out can be achieved by actively controlling the optical collective exc...

Solid-state quantum emitters that couple coherent optical transitions to long-lived spin states are essential for quantum networks. Here we report on the spin and optical properties of single tin-vacancy (SnV) centers in diamond nanostructures. Through magneto-optical spectroscopy at 4 K, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, and measure electron spin lifetimes. We find that the optical transitions are consistent with the radiative lifetime limit and that the spin lifetimes are longer than for other inversion-symmetric color centers under similar conditions. These properties indicate that the SnV is a promising candidate for quantum optics and quantum networking applications.A central goal of quantum information processing is the development of quantum networks consisting of stationary, long-lived matter qubits coupled to flying photonic qubits [1,2], with applications in quantum computing, provably secure cryptography, and quantumenhanced metrology [3]. Among matter qubits, quantum emitters in wide-bandgap semiconductors [4,5] have emerged as leading systems as their coherent, spinselective optical transitions act as an interface between quantum information stored in their spin degrees of freedom and emitted photons. While most work has so far focused on the nitrogen-vacancy (NV) center in diamond [6-8], its relatively poor optical properties, including a low percentage of emission into the coherent zero-phonon-line (ZPL) [9] and large spectral diffusion when located near surfaces [10,11], have fueled the investigation of alternative emitters. These include the group-IV color centers in diamond [12], comprising the silicon-vacancy (SiV) [13][14][15][16], germanium-vacancy (GeV) [17,18], and the recently observed lead-vacancy (PbV) [19] centers. These centers have a large fraction of emission into the ZPL and a crystallographic inversion symmetry that limits spectral diffusion and inhomogeneous broadening [20,21]. Unlike the NV center, however, the electronic spin coherence of SiV and GeV centers is limited by phonon scattering to an upperlying ground-state orbital [22,23], requiring operation at dilution-refrigerator temperatures (∼ 100 mK) [24,25], or controllably induced strain [26] to achieve long coherence times.The tin-vacancy (SnV) center in diamond [27, 28] is a group-IV color center that promises favorable optical properties and long spin coherence time at readily achievable temperatures (liquid helium, ∼ 4 K). DFT calculations predict that the SnV has the same symmetry as the SiV and GeV[9], while experimental measurement of a large ground-state orbital splitting indicates that single-phonon scattering, the dominant spin dephasing mechanism of SiV and GeV centers at liquid helium temperatures, should be suppressed significantly [27]. In this work, we report spectroscopic measurements that are consistent with the conjectured electronic structure of the SnV, demonstrate that its optical...

The uncontrolled interaction of a quantum system with its environment is detrimental for quantum coherence. For quantum bits in the solid state, decoherence from thermal vibrations of the surrounding lattice can typically only be suppressed by lowering the temperature of operation. Here, we use a nano-electro-mechanical system to mitigate the effect of thermal phonons on a spin qubit – the silicon-vacancy colour centre in diamond – without changing the system temperature. By controlling the strain environment of the colour centre, we tune its electronic levels to probe, control, and eventually suppress the interaction of its spin with the thermal bath. Strain control provides both large tunability of the optical transitions and significantly improved spin coherence. Finally, our findings indicate the possibility to achieve strong coupling between the silicon-vacancy spin and single phonons, which can lead to the realisation of phonon-mediated quantum gates and nonlinear quantum phononics.