arXiv subjects

Christoph Becher

Publications and source records attributed to Christoph Becher.

At least 55 records · Page 3Linked to original sources

arXiv2017

Coherence properties and quantum control of silicon vacancy color centers in diamond

Atomic-scale impurity spins, also called color centers, in an otherwise spin-free diamond host lattice have proven to be versatile tools for applications in solid-state-based quantum technologies ranging from quantum information processing (QIP) to quantum-enhanced sensing and metrology. Due to its wide band gap, diamond can host hundreds of different color centers. However, their suitability for QIP or sensing applications has only been tested for a handful of these, with the nitrogen vacancy (NV) strongly dominating this field of research. Due to its limited optical properties, the success of the NV for QIP applications however strongly depends on the development of efficient photonic interfaces. In the past years the negatively charged silicon vacancy (SiV) center received significant attention due to its highly favourable spectral properties such as narrow zero phonon line transitions and weak phonon sidebands. We here review recent work investigating the SiV centre's orbital and electron spin coherence properties as well as techniques to coherently control its quantum state using microwave as well as optical fields and we outline potential future experimental directions to improve the SiV's coherence time scale and to develop it into a valuable tool for QIP applications.

quant-ph↗

arXiv2017

All-optical control of the silicon-vacancy spin in diamond at millikelvin temperatures

The silicon-vacancy center in diamond offers attractive opportunities in quantum photonics due to its favorable optical properties and optically addressable electronic spin. Here, we combine both to achieve all-optical coherent control of its spin states. We utilize this method to explore spin dephasing effects in an impurity-rich sample beyond the limit of phonon-induced decoherence: Employing Ramsey and Hahn-echo techniques at 12mK base temperature we identify resonant coupling to a substitutional nitrogen spin bath as the limiting decoherence source for the electron spin.

quant-ph↗

arXiv2017

Coherence and Entanglement Preservation of Frequency-Converted Heralded Single Photons

We report on quantum frequency conversion of near-infrared photons from a wavelength of 854nm to the telecommunication O-band at 1310nm with 8% overall conversion efficiency. Entangled photon pairs at 854nm are generated via type-II spontaneous parametric down conversion. One photon is mixed with a strong pump field in a nonlinear ridge waveguide for its conversion to 1310nm. We demonstrate preservation of first and second order coherence of the photons in the conversion process. Based on this we infer the coherence function of the two-photon state and compare it with the actual measured one. This measurement demonstrates preservation of time-energy entanglement of the pair. With 88% visibility we violate a Bell inequality.

quant-ph↗

arXiv2017

Cavity-enhanced single photon source based on the silicon vacancy center in diamond

Single photon sources are an integral part of various quantum technologies, and solid state quantum emitters at room temperature appear as a promising implementation. We couple the fluorescence of individual silicon vacancy centers in nanodiamonds to a tunable optical microcavity to demonstrate a single photon source with high efficiency, increased emission rate, and improved spectral purity compared to the intrinsic emitter properties. We use a fiber-based microcavity with a mode volume as small as $3.4~λ^3$ and a quality factor of $1.9\times 10^4$ and observe an effective Purcell factor of up to 9.2. We furthermore study modifications of the internal rate dynamics and propose a rate model that closely agrees with the measurements. We observe lifetime changes of up to 31%, limited by the finite quantum efficiency of the emitters studied here. With improved materials, our achieved parameters predict single photon rates beyond 1 GHz.

quant-ph↗

arXiv2017

Coherent control of the silicon-vacancy spin in diamond

Spin impurities in diamond have emerged as a promising building block in a wide range of solid-state-based quantum technologies. The negatively charged silicon-vacancy centre combines the advantages of its high-quality photonic properties with a ground-state electronic spin, which can be read out optically. However, for this spin to be operational as a quantum bit, full quantum control is essential. Here, we report the measurement of optically detected magnetic resonance and the demonstration of coherent control of a single silicon-vacancy centre spin with a microwave field. Using Ramsey interferometry, we directly measure a spin coherence time, T2*, of 115 +/- 9 ns at 3.6 K. The temperature dependence of coherence times indicates that dephasing and decay of the spin arise from single phonon-mediated excitation between orbital branches of the ground state. Our results enable the silicon-vacancy centre spin to become a controllable resource to establish spin-photon quantum interfaces.

quant-ph↗

arXiv2016

Photoluminescence excitation and spectral hole burning spectroscopy of silicon vacancy centers in diamond

Silicon-vacancy (SiV) centers in diamond are promising systems for quantum information applications due to their bright single photon emission and optically accessible spin states. Furthermore, SiV centers in low-strain diamond are insensitive to pertubations of the dielectric environment, i.e. they show very weak spectral diffusion. This property renders ensembles of SiV centers interesting for sensing applications. We here report on photoluminescence excitation (PLE) spectroscopy on an SiV ensemble in a low strain, CVD-grown high quality diamond layer, where we measure the fine structure with high resolution and obtain the linewidths and splittings of the SiV centers. We investigate the temperature dependence of the width and position of the fine structure peaks. Our measurements reveal linewidths of about 10 GHz as compared to a lifetime limited width on the order of 0.1 GHz. This difference arises from the inhomogeneous broadening of the transitions caused by residual strain. To overcome inhomogeneous broadening we use spectral hole burning spectroscopy which enables us to measure a nearly lifetime limited homogeneous linewidth of 279 MHz. Furthermore, we demonstrate evidence of coherent interaction in the system by driving a $Λ$-scheme. Additional measurements on single emitters created by ion implantation confirm the homogeneous linewidths seen in the spectral hole burning experiments and relate the ground state splitting to the decoherence rate.

quant-ph↗

arXiv2016

Ultrafast, all-optical coherent control of single silicon vacancy colour centres in diamond

Recently, the negatively charged silicon vacancy centre (SiV) in diamond has emerged as a novel promising system for quantum information processing (QIP) due to its superior spectral properties and advantageous electronic structure: The SiV offers an optically accessible Λ-type level structure with a large orbital level splitting even without the need of an external magnetic field and previous studies determined the ground state coherence time of the centre to be on the order of 35-45ns. While these studies already indicate the feasibility of an ultrafast all-optical coherent control of the SiV, no experimental realization has been presented so far. Here, we report on optical Rabi oscillations and Ramsey interference between the ground and the excited state of the SiV using 12 ps long, resonant rotation pulses. Moreover, utilizing a Λ-scheme, we demonstrate Raman-based coherent Rabi rotations and Ramsey interference within the ground state manifold using a single, 1ps long off-resonant pulse. These measurements prove the accessibility of the complete set of single-qubit operations relying solely on optical fields and pave the way for high-speed QIP applications using SiV centres.

quant-ph↗

arXiv2016

Site selective growth of heteroepitaxial diamond nanoislands containing single SiV centers

We demonstrate the controlled preparation of heteroepitaxial diamond nano- and microstructures on silicon wafer based iridium films as hosts for single color centers. Our approach uses electron beam lithography followed by reactive ion etching to pattern the carbon layer formed by bias enhanced nucleation on the iridium surface. In the subsequent chemical vapor deposition process, the patterned areas evolve into regular arrays of (001) oriented diamond nano-islands with diameters of <500nm and a height of approx. 60 nm. In the islands, we identify single SiV color centers with narrow zero phonon lines down to 1 nm at room temperature.

cond-mat.mtrl-sci↗

arXiv2015

Quantum and classical telecommunication channel multiplexing

We demonstrate the multiplexing of a classical coherent and a quantum state of light in a single telecommunciation fiber. For this purpose we make use of spontaneous parametric down conversion and quantum frequency conversion to generate photon pairs at 854 nm and the telecom O-band. The herald photon triggers a telecom C-band laser pulse. The telecom single photon and the laser pulse are combined and coupled to a standard telecom fiber. Low background time correlation of the classical and quantum signal behind the fiber shows successful telecommunication channel multiplexing.

quant-ph↗

arXiv2015

Telecom-heralded single photon absorption by a single atom

We present, characterize, and apply a photonic quantum interface between the near infrared and telecom spectral regions. A singly resonant optical parametric oscillator (OPO) operated below threshold, in combination with external filters, generates high-rate ($>2.5\cdot10^6~{\rm s}^{-1}$) narrowband photon pairs ($\sim 7$ MHz bandwidth); the signal photons are tuned to resonance with an atomic transition in Ca$^+$, while the idler photons are at telecom wavelength. Quantum interface operation is demonstrated through high-rate absorption of single photons by a single trapped ion ($\sim 670~{\rm s}^{-1}$), heralded by coincident telecom photons.

quant-ph↗

arXiv2015

Nanoimplantation and Purcell enhancement of single NV centers in photonic crystal cavities in diamond

We present the controlled creation of single nitrogen-vacancy (NV) centers via ion implantation at the center of a photonic crystal cavity which is fabricated in an ultrapure, single crystal diamond membrane. High-resolution placement of NV centers is achieved using collimation of a 5keV-nitrogen ion beam through a pierced tip of an atomic force microscope (AFM). We demonstrate coupling of the implanted NV centers' broad band fluorescence to a cavity mode and observe Purcell enhancement of the spontaneous emission. The results are in good agreement with a master equation model for the cavity coupling.

quant-ph↗

arXiv2014

All-optical formation of coherent dark states of silicon-vacancy spins in diamond

Spin impurities in diamond can be versatile tools for a wide range of solid-state-based quantum technologies, but finding spin impurities which offer sufficient quality in both photonic and spin properties remains a challenge for this pursuit. The silicon-vacancy center has recently attracted a lot of interest due to its spin-accessible optical transitions and the quality of its optical spectrum. Complementing these properties, spin coherence is essential for the suitability of this center as a spin-photon quantum interface. Here, we report all-optical generation of coherent superpositions of spin states in the ground state of a negatively charged silicon-vacancy center using coherent population trapping. Our measurements reveal a characteristic spin coherence time, T2*, exceeding 250 nanoseconds at 4 K. We further investigate the role of phonon-mediated coupling between orbital states as a source of irreversible decoherence. Our results indicate the feasibility of all-optical coherent control of silicon-vacancy spins using ultrafast laser pulses.

quant-ph↗

arXiv2014

Deterministic coupling of a single silicon-vacancy color center to a photonic crystal cavity in diamond

Deterministic coupling of single solid-state emitters to nanocavities is the key for integrated quantum information devices. We here fabricate a photonic crystal cavity around a preselected single silicon-vacancy color center in diamond and demonstrate modification of the emitters internal population dynamics and radiative quantum efficiency. The controlled, room-temperature cavity coupling gives rise to a resonant Purcell enhancement of the zero-phonon transition by a factor of 19, coming along with a 2.5-fold reduction of the emitter's lifetime.

quant-ph↗

arXiv2014

Narrow-band single photon emission at room temperature based on a single Nitrogen-vacancy center coupled to an all-fiber-cavity

We report the realization of a device based on a single Nitrogen-vacancy (NV) center in diamond coupled to a fiber-cavity for use as single photon source (SPS). The device consists of two concave mirrors each directly fabricated on the facets of two optical fibers and a preselected nanodiamond containing a single NV center deposited onto one of these mirrors. Both, cavity in- and output are directly fiber-coupled and the emission wavelength is easily tunable by variation of the separation of the two mirrors with a piezo-electric crystal. By coupling to the cavity we achieve an increase of the spectral photon rate density by two orders of magnitude compared to free-space emission of the NV center. With this work we establish a simple all-fiber based SPS with promising prospects for the integration into photonic quantum networks.

physics.optics↗

arXiv2013

Optical signatures of silicon-vacancy spins in diamond

Colour centres in diamond have emerged as versatile tools for solid-state quantum technologies ranging from quantum information to metrology, where the nitrogen-vacancy centre is the most studied to-date. Recently, this toolbox has expanded to include different materials for their nanofabrication opportunities, and novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with ultrabright single photon emission predominantly into the desirable zero-phonon line. The challenge for utilizing this centre is to realise the hitherto elusive optical access to its electronic spin. Here, we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. In low-strain bulk diamond spin-selective excitation under finite magnetic field reveals a spin-state purity approaching unity in the excited state. We also investigate the effect of strain on the centres in nanodiamonds and discuss how spin selectivity in the excited state remains accessible in this regime.

cond-mat.mes-hall↗

arXiv2013

The electronic structure of the silicon vacancy color center in diamond

The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar response of single centers and a SiV ensemble in a low strain reference sample proves our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects.

cond-mat.mtrl-sci↗

arXiv2013

Evaluation of nitrogen- and silicon-vacancy defect centres as single photon sources in quantum key distribution

We demonstrate a quantum key distribution (QKD) testbed for room temperature single photon sources based on defect centres in diamond. A BB84 protocol over a short free-space transmission line is implemented. The performance of nitrogen-vacancy (NV) as well as silicon-vacancy defect (SiV) centres is evaluated and an extrapolation for next-generation sources with enhanced efficiency is discussed.

quant-ph↗

arXiv2013

Single-photon frequency conversion in nonlinear crystals

Frequency conversion of single photons in a nonlinear crystal is theoretically discussed. Losses and noise are included within a Heisenberg-Langevin formalism for the propagating photon field. We calculate the first- and second-order correlation functions of the frequency-converted light when the input is a train of single-photon pulses. This model allows one to identify the requirements on the nonlinear device so that it can be integrated in a quantum network.

quant-ph↗