Session C6: Spin Measurements in Solids and Gases

Measurement of the Berry Phase in a Single Solid-State Spin Qubit

Geometric phases in quantum mechanics have a long history and may offer some advantages in quantum information processing techniques, e.g. geometric phases are intrinsically robust to fluctuations in control parameters. We demonstrate a controlled way of accumulating geometric phase by Berry's method in a single Nitrogen-Vacancy (NV) center in diamond lattice. We perform state tomography measurement to confirm this Berry phase and we find no evidence for geometric dephasing in our system.   

Optical Spectroscopy and Lifetime Studies of Ytterbium Atoms Trapped in Noble Gas Solids

As a point defect in a lattice, a neutral ytterbium (Yb) atom isolated in a solid noble gas matrix shares many similarities with the celebrated nitrogen-vacancy center in diamond, including optically accessible transitions, their spontaneous emission, and the inter-system crossing. Our studies are undertaken to determine the feasibility of optically polarizing the nuclear spins of the trapped atoms, which has potential applications in testing fundamental symmetries, studying nuclear moments of exotic isotopes, and perhaps quantum memory and quantum computing. The high resolution optical spectra show line splitting owing to the atomic structure of Yb and the symmetry of multiple trapping sites. The Yb transition 6$s^{\mathrm{2}} \quad^{\mathrm{1}}S_{\mathrm{0}}$ -- 6$s$6$p$ $^{\mathrm{3}}P_{\mathrm{1}}$ experiences an enhanced spontaneous emission rate due to the index of refraction of the matrices. The decay of Yb's metastable 6$s$6$p \quad^{\mathrm{3}}P_{\mathrm{0}}$ in both odd and even isotopes are directly observed for the first time and the vacuum hyperfine quenching rate of the odd isotope is extracted.   

Nanoscale Fourier magnetic imaging using nitrogen-vacancy centers in diamond

We present a new technique for extending the spatial dynamic-range of nitrogen-vacancy (NV) center magnetic imaging by use of a Fourier phase encoding technique, in close analogy with conventional magnetic resonance imaging. By applying strong pulsed magnetic field gradients across a broad array of NV centers in diamond, information about the spatial location of each NV center as well as the local magnetic field is mapped onto each NV spin's phase and then read out in Fourier (k-space) by acquiring wide-field images on a camera of all NV centers simultaneously. A Fourier transform then converts this information into a magnetic field image with nanoscale resolution and wide field-of-view. We also discuss how this technique can be extended to nanoscale and wide-field electric field and temperature sensing.   

Interfacing Superconducting Qubits and Telecom Photons via a Rare-Earth Doped Crystal

Superconducting qubits (SCQ) are promising candidates for scalable quantum computation. However, they are essentially stationary, which makes them less suitable for quantum information transport. Interfacing short telecom photons with SCQ's would enable the combination of SCQ with low loss optical fiber networks and a fast, reliable quantum network could be realized. To this end, we propose and theoretically analyze a scheme for coupling optical photons to a SCQ, using a rare earth doped crystal (REDC) coupled to the microwave cavity as an interface. The idea is first to store an optical photon by mapping it to a spin excitation in a REDC and then transfer this excitation to a SCQ via a microwave cavity. Due to intrinsic and engineered inhomogeneous broadening of the optical and spin transitions employed in REDC for the storage of short optical photon pulses, we suggest and optimize a special transfer protocol using staggered $\pi$-pulses.   

L\'evy flights in laser cooling of nuclear spins

Interaction between an electronic spin and its surrounding nuclear spin environment is a major source of decoherence in most solid-state spin qubits. We develop a phenomenological model for nuclear spin diffusion in the presence of electronic dark states. As an quantitative example, we study the diffusion in $^{13}$C nuclear spin bath of an NV$^-$ impurity in diamond. We use this model to predict that the nuclear diffusion time-scales exhibit L\'evy statistics--- enabling nuclear spins to remain trapped in certain configurations for long times. We comment on observing such statistics by measuring photon scattering rates that are dependent on nuclear diffusion rates, leading to quantitative measurements of the non-equilibrium bath dynamics in such central-spin systems.   

Nanoscale NMR Spectroscopy and Imaging of Multiple Nuclear Species

We utilize nitrogen-vacancy (NV) color centers located a few nanometers from the surface of a diamond chip to perform optically-detected NMR spectroscopy and imaging of multiple nuclear spin species (1H, 19F, 31P) on sub-micron length scales. The strong dipolar interaction between nuclear spins in a sample at the diamond surface and the electronic spin of a shallow NV center can be detected optically as a change in NV fluorescence. We interrogate single NV centers to perform NMR spectroscopy on nanoscale sample volumes containing a few hundred polarized nuclear spins. We also employ a wide-field imaging apparatus, which uses a diamond chip containing a high-density NV layer near its surface, to demonstrate optical NMR imaging of samples containing multiple nuclear species with sub-micron spatial resolution. This work provides a new modality for NMR spectroscopy and imaging in previously unachievable regimes.   

Spin cooling via incoherent feedback in an ensemble of cold $^{87}$Rb atoms

We report an experimental study of a new technique for spin cooling an ensemble of ultracold atoms via quantum non-demolition (QND) measurement and incoherent feedback [Phys. Rev. Lett., 111,103601 (2013)]. We observe 12dB of spin noise reduction, or a factor- of-63 reduction in phase-space volume, after two rounds of cooling. This technique has direct application in generating highly entangled macroscopic singlet states via measurement-induced spin squeezing [New J. Phys. 12 053007(2010)]. We report the generation of such macroscopic singlet states with up to 3dB of squeezing relative to the standard quantum limit, and a violation of the generalised spin squeezing inequality [Phys. Rev. Lett. 99, 250405 (2007)] an entanglement witness for the collective spin state, by more than 3 standard deviations.   

Optimization of a Quantum Memory with Telecom-Wavelength Conversion

Quantum networks provide conduits capable of transmitting quantum information that connect to nodes at remote locations where the quantum information can be stored or processed. Fiber-based transmission of quantum information over long distances may be achieved using quantum memory elements and quantum repeater protocols. We report on progress towards a quantum memory based on the generation of off-axis, spontaneously emitted single photons by a 795 nm write-laser beam interacting with a cold $^{87}$Rb ensemble. The detection of a single photon heralds the creation of a spin wave in the atomic cloud. Single photons associated with undesirable optical transitions are filtered out by an $^{85}$Rb vapor cell filled with a buffer gas whose optical density is augmented with light induced atom desorption (LIAD) and heating. The photons are converted into the telecom band by difference frequency generation in a PPLN crystal and sent down a long optical fiber. The atomic state is read out via the interaction of a read-pulse with the quantum memory. With such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.   

Coherent Quantum-Noise Cancellation by a four-wave mixer in hot Rubidium Vapor

The study of quantum noise plays a significant role for achieving quantum precision measurement and developing quantum information protocol by properly generating desired quantum states. Here we present our experimental results of coherent quantum noise cancellation of the quantum correlated twin beams by four-wave mixing in a hot rubidium vapor. The twin beams are prepared from the first four-wave mixer, and each beam has a quantum amplified noise which is well above the corresponding shot noise limit. By simultaneously seeding the twin beams into a second four-wave mixer and using two sets of balanced homodyne detection systems, we found that the noise of the new output beams can be largely suppressed and can be close to their corresponding shot noise limits by coherent quantum noise cancellation due to quantum destructive interference. We also studied the loss effect on the efficiency of quantum noise cancellation. Based on these results, we also experimentally realized a quantum nonlinear SU(1,1) interferometer which achieved about 4dB signal to noise ratio enhancement compared to the traditional Mach-Zender interferometer.