This PDF file contains the front matter associated with SPIE Proceedings Volume 7948, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Ultrasound-modulated optical tomography is a hybrid imaging technique based on detection of the diffused light modulated by a focused ultrasonic wave inside a scattering medium. With the combined advantages of ultrasonic resolution and optical contrast, UOT is ideal for deep tissue imaging. Its growth in popularity and application, however, is hindered by the low efficiency in detecting the modulated diffused photons. Research activities on UOT have therefore been centered on improving its signal detection efficiency by exploring various technical solutions. A prime example is the use of spectral-hole burning (SHB) in UOT. By applying SHB crystal as a spectral filter, one modulation sideband of the diffused light can be efficiently selected while all the other frequency components are strongly suppressed. Immune to both the spatial and temporal incoherence of the signal with a high enough on/off ratio, SHB can boost the UOT imaging ability dramatically and push it towards practical applications. We compare SHB with the other technologies that have been applied to UOT, and identify the unique features that make SHB a preferable tool for UOT. We also discuss the desired improvements from the SHB side, which will help UOT pave the way from research to everyday life.

Thulium-doped crystals are considered for programmable filtering. Such a filter can be achieved by Spectral Hole Burning (SHB). One optically pumps the active ions into a long-lived shelving state, opening a narrow transparency window in the crystal absorption band. We investigate a new shelving scheme in Tm:YAG where the thulium ions are pumped into a ground state nuclear Zeeman sublevel, instead of their natural 10ms-lifetime metastable state. The shelving time is increased to several seconds, reducing the residual population in the transparency window by orders of magnitude. This should enhance the filter's dynamic range, which is essential in demanding filtering applications like ultrasound optical tomography.

The doping of Eu into MgS as well as CaS:Eu thinfilms, produced by Chemically Controlled Pulsed Laser Deposition (CCPLD), offer a lot of potential for the development of ultra-high density; (Terabits per sq. in.) spectral storage devices. These storage capacities are made possible by the use of spectral holeburning technique where many spectral holes can be burned into the inhomogeneously broadened Zero Phonon Line (ZPL) of 4f-5d electronic transition of the Eu ions inside the MgS and CaS lattice. It is shown that one can tailor different optical centers by introducing trace amounts of impurities in the alkaline earth sulfide lattice, especially if these impurities occupy sites close to the europium ion. The effect of doping on MgS and CaS thinfilms with oxygen impurity has been presented. Laser excited fluorescence results have been presented that demonstrate possible atomic tailoring of calcium and magnesium sulfides in creating new oxygen associated europium centers. Such centers are of prime importance in increasing the spectral storage capacity of multilayer thinfilm structures based on these materials.

We demonstrate the optical detection of ultrasound using spectral hole burning in a cryogenic rare earth ion doped crystal. The dispersion due to the hole is used to perform phase to amplitude modulation conversion. This method allows sensitive detection of ultrasonic displacements with the advantage of large étendue. This method is also attractive as it requires only moderate absorption contrast to achieve high sensitivity. We also describe a method for diode laser stabilisation using optical feedback through spectral holes which dramatically reduces the laser phase noise.

We present a detailed theoretical analysis of quantum imaging intended to reveal under what conditions it is superior to imaging with non-entangled photons in order to determine practical bounds on quantum imaging systems. Our analysis includes a description of the propagation and detection of entangled light signals taking into account attenuation, diffraction, and event statistics. Each of these three are significant roadblocks on the path towards practical quantum imaging and we rate how severe each of these is in three imaging regimes (exo-atmospheric, short distance endo-atmospheric, and long distance endo-atmospheric) and three detection regimes (high signal-to-background, low signal-to-background, and saturated). In an attempt to overcome these roadblocks we briefly speculate about the possible role of nonlinear propagation phenomena which may enable entangled light propagation without diffraction, and of X-waves, which may provide for the possibility of overcoming all of the above mentioned roadblocks.

We discuss the prospects of performing Heisenberg-limited quantum sensing and metrology using a Mach-Zehnder interferometer with input states that are superpositions of twin-Fock states and where photon number parity measurements are made on one of the output beams of the interferometer. This study is motivated by the experimental challenge of producing twin-Fock states on opposite sides of a beam splitter. We focus on the use of the so-called pair coherent states for this purpose and discuss a possible mechanism for generating them. We also discuss the prospect of using other superstitions of twin-Fock states, for the purpose of interferometry.

Weak-light all-optical switches would enhance photonics capabilities and enable important optical quantum information processing tasks, but optical switching based on third-order optical nonlinearity is proving to be elusive and controversial. I discuss strategies to generate large cross-phase modulation by combining double electromagnetically induced transparency, competing atomic transitions, electron population engineering for pulse speed control, and using boundary conditions to confine the transverse field beyond the diffraction limit.

The universal transpose of quantum states is an anti-unitary transformation that is not allowed in quantum theory. In this work, we investigate approximating the universal transpose of quantum states of two-level systems (qubits) using the method known as structural physical approximation. We also report its experimental implementation in linear optics. The scheme is optimal in that the maximal fidelity is attained, and also practical as measurement and preparation of quantum states that are experimentally feasible within current technologies are solely applied.

We describe a procedure to construct a free-space quantum key distribution system that can carry many bits of information per photon. We also describe the current status of our laboratory implementation of these plans.

We demonstrate an efficient photon number detector for visible wavelengths using a fast-gated silicon avalanche photodiode. Using sub-nanosecond voltage gates with a self-differencing circuit, the device is able to resolve up to four photons in an incident optical pulse, with a detection probability of up to 91.1 % at 1 GHz. With this performance and close to room temperature operation, fast-gated silicon avalanche photodiodes are ideal for optical quantum information processing that requires single-shot photon number detection.

In the perspective of a future all-optical communication network optical shift register will play an important role especially for what concerns several binary functions, such as serial to parallel conversion and cyclic operations, that are involved in techniques allowing error detection and correction as parity check, or cyclic redundancy check. During the last decades, several attempts of realizing circulating memories or shift register in the optical domain were made, with some limits in terms of functionality, number of bit to be stored (under three), scalability or photonic integrability. In this paper, we present a new approach to realize a circulating optical shift register consisting on an SOA-based optical buffer (OB) and a bit selecting circuit (BSC). The OB is potentially integrable and is able to store a finite number of bit at high bit rate. The BSC returns consecutive bits at a lower clock rate, achieving proper shift register function. The bit selection is realized by means of four wave mixing (FWM) in a Kerr medium, and the sequence cancellation is allowed to enable new sequence storing. Experimental validation of the scheme for f_B=59MHz and f_B=236MHz shows optical signal to noise ratio per bit penalty of 5.6dB at BER=10^-9.

The superactivation of zero-capacity optical quantum channels makes it possible to use two zero-capacity optical quantum channels with a positive joint capacity at the output. Currently, we have no theoretical results for describing all possible combinations of superactive zero-capacity channels, hence there should be many other possible combinations. This paper shows a fundamentally new method of finding the conditions for the superactivation of the asymptotic Lloyd-Shor-Devetak (LSD) channel capacity of zero-capacity optical quantum channels. We introduce a new geometrical representation, called the quantum informational superball representation. Our method provides a fundamentally new and efficient algorithmic solution for discovering all possible superactive channels and it can be applied to analyze the capacity of these channels.

One of the challenges in quantum key distribution is to ensure adequate security against eavesdropping. The B92 protocol achieves this through the transmission of a strong and weak pulse, with key bits coded onto the weak pulse and channel monitoring done using the bit error rate of the strong pulse. However, the protocol also assumes synchronization between transmitter and receiver using a shared or transmitted clock signal. We propose a scheme that uses a mode locked laser for the strong pulse train, and a nonlinear four-wave mixing processes to generate the weak pulse train. The detector relies on dispersion effects in the transmission channel to delay the weak pulse relative to the strong pulse, and then use the strong pulse to gate the detection of the weak pulse. The scheme is thus self-synchronized and offers the option of additional security using four-wave-mixing processes.

The combination of the long electron state spin coherence time and the optical coupling of the ground electronic states to an excited state manifold makes the nitrogen-vacancy (NV) center in diamond an attractive candidate for quantum information processing. To date the best spin and optical properties have been found in centers deep within the diamond crystal. For useful devices it will be necessary to engineer NVs with similar properties close to the diamond surface. We report on properties including charge state control and preferential orientation for near surface NVs formed either in CVD growth or through implantation and annealing.

Electron spins in quantum dots under coherent control exhibit a number of novel feedback processes. Here, we present experimental and theoretical evidence of a feedback process between nuclear spins and a single electron spin in a single charged InAs quantum dot, controlled by the coherently modified probability of exciting a trion state. We present a mathematical model describing competition between optical nuclear pumping and nuclear spin-diffusion inside the quantum dot. The model correctly postdicts the observation of a hysteretic sawtooth pattern in the free-induction-decay of the single electron spin, hysteresis while scanning a narrow-band laser through the quantum dot's optical resonance frequency, and non-sinusoidal fringes in the spin echo. Both the coherent electron-spin rotations, implemented with off-resonant ultrafast laser pulses, and the resonant narrowband optical pumping for spin initialization interspersed between ultrafast pulses, play a role in the observed behavior. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse timing, potentially allowing more complex coherent control operations.

We demonstrate magnetometry by detection of the spin state of high-density nitrogen-vacancy (NV) ensembles in diamond using optical absorption at 1042 nm. With this technique, measurement contrast and collection efficiency can approach unity, leading to an increase in magnetic sensitivity compared to the more common method of collecting red fluorescence. Working at 75 K with a sensor with effective volume 50x50x300 μm³, we project photon shot-noise limited sensitivity of 5 pT in one second of acquisition and bandwidth from DC to a few MHz. Operation in a gradiometer configuration yields a noise floor of 7 nT_rms at ~110 Hz in one second of acquisition. We also present measurements of the zero-field splitting parameters as a function of temperature, a calibration which is essential for ultra-sensitive magnetometry at low frequencies.

Quantum information processing (QIP) relies on delicate superposition states that are sensitive to interactions with environment. The quantum gates are imperfect and the use of quantum error correction coding (QECC) is essential to enable the fault-tolerant computing and to deal with quantum errors. The most critical gate, CNOT-gate, has been implemented as a probabilistic device by using integrated optics. CNOT-gates from linear optics provide only probabilistic outcomes and as such are not suitable for large-scale computation. In this paper, we show that arbitrary set of universal quantum gates and gates from Clifford group, needed in QECC, can be implemented based on cavity quantum electrodynamics (CQED). We further show that encoders/decoders for quantum LDPC codes can be implemented based on Hadamard and CNOT gates using CQED. Finally, we perform simulations and evaluate performance of several classes of quantum LDPC codes suitable for implementation in CQED technology.

Nitrogen-vacancy centers in diamond are widely studied both as a testbed for solid state quantum optics and for their applications in quantum information processing and magnetometry. Here we demonstrate coupling of the nitrogen-vacancy centers to gap plasmons in metal nano-slits. We use diamond samples where nitrogen-vacancy centers are implanted tens of nanometers under the surface. Silver nano-slits are patterned on the sample such that diamond ridges tens of nanometers wide fill the slit gap. We measure enhancement of the spontaneous emission rate of the zero photon line by a factor of 3 at a temperature of 8K.

We demonstrate interference between discrete photons emitted by two different semiconductor quantum dots and quantify their degree of indistinguishability. The quantum dot emission energies are tuned into resonance by straining the samples. Upon interference on a beamsplitter, the photons are shown to be 18.1% indistinguishable, resulting in a coincidence detection rate below the classical limit. Post-selecting only those detections occurring within a short time of each other increases the measured indistinguishability to 47%. The photons are partially distinguishable due to dephasing of the exciton states, and post-selection is also affected by the detector response time.

Spin is a fundamental property of electrons and plays an important role in information storage. For spin-based quantum information technology, preparation and read-out of the electron spin state must be spin coherent, but both the traditional preparation and read-out of the spin state are projective to up/down spin states, which do not have spin coherence. We have recently demonstrated that the polarization coherence of light can be coherently transferred to the spin coherence of electrons in a semiconductor. We have also developed a new scheme named tomographic Kerr rotation (TKR) by generalizing the traditional KR to directly read out the spin coherence of optically prepared electrons without the need for the spin dynamics, which allows the spin projection measurement in an arbitrary set of basis states. These demonstrations were performed using g-factor-controlled semiconductor quantum wells with precessing and non-precessing electrons. The developed scheme offers a tool for performing basis-independent preparation and read-out of a spin quantum state in a solid. These results encourage us to make a quantum media converter between flying photon qubits and stationary electron spin qubits in semiconductors.

One of the criteria for the physical implementation of quantum computing is that it must be possible to manipulate individual qubits to prepare them in the same well-defined state. In schemes involving linear optics and single photons from self-assembled quantum dots, this requirement can be intractable as quantum dots form with a distribution of emission energies. We have designed a structure based on InAs quantum dots embedded at the center of an AlGaAs/GaAs/AlGaAs quantum well which facilitates carrier confinement under application of electric fields up to |F| = 500 kVcm ^-1. This is an order of magnitude greater than previous reports. Through the quantum-confined Stark effect, individual dots can be tuned by up to 25meV, three orders of magnitude greater than their linewidth. This unprecedented tuning range allows many quantum dots to be tuned to the same energy with ease, enabling us to carry out two-photon interference measurements using remote quantum dots. Furthermore, we demonstrate scalability by performing the interference experiment with three pairs of quantum dots. By post-selecting events where two photons arrive simultaneously at a beamsplitter, we find that the interference visibility is limited only by the timing resolution of our system. We also performed measurements as a function of energy detuning which showed that maximum visibility can only be achieved when the dots are degenerate in energy. This opens up the possibility of transferring quantum information between remote solid-state systems.

We describe a frequency-coded scheme to implement the BB84 quantum key distribution protocol using spin wave (SW)-optical interactions. The interaction of SWs with optical coherent states allows TE↔TM mode conversion with simultaneous change of frequency and polarization while introducing a phase difference between the two modes. To implement the BB84 protocol, key bits are encoded as relative phases between the TE and TM modes. The proposed scheme offers a higher key rate, due to a modulation frequency as high as 25 GHz, which also relaxes the specifications on the optical filter at the receiver. In addition, SW-optical interactions yield the added security of truly single-sideband modulation.