Quantum Imaging - UMBC Objective Study the physics of multi-photon imaging for entangled state, coherent state and chaotic thermal state: distinguish their quantum and classical nature, in particular, the necessary and/or unnecessary role of quantum entangle- ment in quantum imaging and lithography. Study the “magic mirror” for “ghost” imaging. Muti-photon sources and measurement devices. ApproachAccomplishments Using entangled two-photon and three-photon states created via optical nonlinear interaction in spontaneous and stimulated modes for multi- photon spatial correlation study and imaging; Using chaotic light source, coherent light source for two-photon spatial correlation study and ghost imaging; Using photon counting and classical current- current correlation circuit to explore the nature of two-photon correlation. *The experiment: R. Meyers, K.S. Deacon, and Y.H. Shih, “ A new type of ghost imaging experi- ment ”, submitted to Phys. Rev. Lett., (2007). *The physics: Y.H. Shih, “ Quantum Imaging ”, IEEE Journal of Selected Topics in Quantum Electronics, 13, 1016 (2007). * The detector: High uniformity, stability, and reliability large-format InGaAs APD arrays. *The photon source: J.M. Wen, P. Xu, M.H. Rubin, Y.H. Shih, “Transverse correlations in triphoton entanglement: geometrical and physical optics, Phys. Rev. A76, (2007). New type of ghost imaging experiment: a successful collaboration with ARL.

Objective: “Study the physics of multi-photon imaging, distinguish their quantum and classical nature, in parti- cular, the necessary and/or unnecessary role of quantum entanglement in quantum imaging.” Part I: The Physics of Quantum Imaging

Quantum imaging has demonstrated two peculiar features: (1)Enhancing the spatial resolution beyond diffraction limit; (2) Reproducing “ghost” images in a “nonlocal” manner. Either the nonlocal behavior observed in ghost imaging or the apparent violation of the uncertainty principle explored in the quantum lithography are due to the coherent superposition of two-photon amplitudes, a nonclassical entity corresponding to different yet indistinguishable alternative ways of triggering a joint-detection event.

Classical Imaging - the concepts

Idealized Classical Imaging Point-point relationship between the object-plane and the image-plane is the result of constructive interference of the fields. Gaussian thin lens equation

Diffraction-limited Classical Imaging Point-“spot” relationship between the object plane and the image plane is the result of constructive superposition: a diffraction pattern. Gaussian thin lens equation somb(x) = 2J 1 (x) / x

Biphoton Ghost Imaging

“Ghost” Image: an EPR Experiment in momentum and position correlation. SoSo SiSi Biphoton “ghost” Imaging PRA, 52, R3429 (1995); PRL, 74, 3600 (1995).

“Can quantum mechanical physical reality be considered complete?” Einstein, Poldosky, Rosen, Phys. Rev. 47, 777 (1935). Proposed the entangled two-particle state according to the principle of quantum superposition: What is so special about entangled two-photon state? Although:

What is so special about entangled two-photon states? In EPR’s language, the signal and the idler may come out from any point of the object plane; however, if the signal (idler) is found in a certain position, the idler (signal) must be found in the same position, with 100% certainty. A typical EPR state:

Photon #1 stop at a point on object plane Photon #2 stop at a unique point on image plane Result of a constructive superposition of two-photon amplitudes, a nonclassical entity corresponding to different yet indistinguishable alternative ways of triggering a joint-detection event. Biphoton “ghost” Imaging &

Biphoton Ghost Imaging

Chaotic light Ghost Imaging

S.C X2X2 X2X2 D2D2 D1D1 Lens-less ghost Imaging Thermal light (Near-field) X2X2 X2X2 C.C

Thermal light source Ghost Image with chaotic light Correlator Photon Counting Correlation Measurement Near-field

Recent Experiment Ghost image of an Army soldier A photon counting detector, D 1, is used to collect and to count all the photons that are randomly scattered-reflected from the soldier. A CCD array (2D) was facing the light source instead of the object. An image of the soldier was observed in the joint-detection of D 1 and the CCD. In collaboration with ARL

Chaotic light Ghost Imaging Image Constant Background

Chaotic light ghost Imaging

D1D1 D2D2 Source Superposition of two-photon amplitudes

Superposition of classical fields? Does it make any sense in Maxwell theory of light?

HBT and Ghost Imaging k-k Momentum-Momentum Correlation x-x Position-Position Correlation Imaging

HBT Thermal light ghost imaging Far-field 1956 Near-field 2006 We cannot but stop to ask: What has been preventing this simple move for 50 years ( )? Something must be terribly misleading to give us such misled confidence not to even try the near-field measurement in half a century. 50 years

Statistical correlation of intensity fluctuation ? ???

Identical modes: Different modes: The HBT experiment was successfully interpreted as statistical correlation of intensity fluctuations. In HBT, the measurement is in far-field (Fourier transform plane). The measured two intensities have the same fluctuations while the two photodetectors receive the same mode and thus yield maximum correlation. When the two photodetector receive different modes, however, the intensities have different fluctuations, and thus no correlation is observable. Mode 1 Mode 2 Far-field (Fourier transform Plane) k1k1 k2k2

Statistical correlation of intensity fluctuation ??? It does not work for near field !!! Identical modes: Different modes: Near-field

The physics of chaotic light ghost imaging? “Can Two-photon Correlation of Chaotic Light Be Considered as Correlation of Intensity Fluctuations?” PRL, 96, (2006) (G. Scarcelli,V. Berardi, and Y.H. Shih). “A New Type of Ghost Imaging Experiment”, submitted to Phys. Rev. Lett., (2007) (R. Meyers, K.S. Deacon, and Y.H. Shih). “Quantum Imaging”, IEEE J. of Selected Topic in Quantum Electronics, 13, 1016 (2007) (Y.H. Shih).

The Physics * * The nonlocal behavior observed in biphoton ghost imaging is due to the superposition of two-photon amplitudes, a nonclassical entity corresponding to different yet indistinguish-able alternative ways of triggering a joint-detection event. The lens-less ghost imaging of thermal light is an interference phenomenon involving the superposition indistinguishable two- photon alternatives, rather than statistical correlation of intensity fluctuation. Ghost imaging: a quantum phenomenon of light. *

Part II: The Theory

Lensless imaging with a classical statistical source Klyshko Picture

The current-current correlation is given by where This are derived using

In the paraxial approximation

Point spread function

All the rays from the point at  A coherently add at in the region  centered at  B. One can say the same thing in terms of the wave vectors.

The correlation function is then given by

If we change the detection scheme, we get a different result. In the paraxial approximation: Now each point of the detector A acts like the source of a spherical wave that illuminates the entire object; consequently,

We see that although the two results are the same for completely incoherent sources, they differ in the more realistic case. When looked at in the Klyshko picture, allowing for the phase conjugate nature of the source, the last case looks like coherent imaging, while the previous case looks like incoherent imaging. for d A =d B

Part III: The Detector

Range Finder, Stand Off Detection, and 3-D Lidar (APD Arrays) Applications IR LASER TRANSMITTER AND RECEIVER DIfferential SCattering/DIfferential Absorption Lidar (DISC/DIAL)

Space Optical Communications Quantum Communication & Image Applications High Performance Photon Counting Detectors and Arrays

Mesa vs. Guard-Ring Potential issues with mesa APDS for space applications: –Short lifetime from early breakdown (reliability) –Dark current increases over time (stability) Mesa Guard-Ring

Reliability of Guard-Ring APDS Aging test condition: 200 o C/I=100  A Testing method: measure dark current at M~10 periodically * S. Tanaka et al on OFC 2003

Key Issues for Photon Counting (PC) 1. Low Dark Counts: Dark current is caused by surface leakage, tunneling, defects assisted tunneling. Can be reduced by decrease the electrical field in the active (absorption) region. 2. High Gain and High Differential Gain High gain can be obtained with high bias voltage. However, with high bias, a high dark current will also be produced. High differential gain relies on high rising slope of APD (dG/dV). An ideal PC APD will have a straight angle I-V curve, which can be achieved with better device designs. 3. Designing and Fabricating Materials with Reduced After- Pulse Dark Current (AFDC) Amplitude and Duration AFDC comes from traps in the avalanche regions and trapped carriers in the hetero-interface. Interstitial Zn atoms created during the diffusion processes are source of traps and can be activated and converted to substitutional dopants by appropriate annealing procedures. More steps of InGaAsP quaternary layers (1.1Q, 1.2Q, 1.3Q, 1.5Q,..etc.) can added to the InP/InGaAs interface to reduce hole trapping.

Geiger Mode Operations-I 1.Passive Quenching – Biased above breakdown voltage ( V op -V b < ~2V), rely on a series resistance to reduce voltage drop on the APD when avalanche is taking place. Advantages: simple circuits, disadvantages: long recovery time. +V op V out V op I V VBVB I op

Geiger Mode Operations-II 2. Active Quenching – Normally biased above breakdown voltage ( V op - V b < ~2V). Once sensed that an avalanche process is taking place, immediately reduce the bias below V b. Advantages: relatively short recovery time (still limited by afterpulse dark counts). Disadvantages: more complex circuits. V op I V VBVB I op +V op V out Avalanche Sensing And trigger circuits Control Voltage supplier

Geiger Mode Operations-III 3. Gated Operations – Normally biased below or around V b. A short electrical pulse is applied to the APD terminal to raise the bias voltage above V b and gain when photons are coming. Advantages : simpler ckt and very high gain. Disadvantages: can only work with periodic signals and synchronization is an issue. +V op V out V dc I V VBVB I dc V ac

Optimize Design To Achieve High Differential Gain and Low Dark Current VpVp VBVB Reducing the distance between the punch through voltage and the breakdown voltage will help to reduce the voltage drop falling on the small bandgap absorption region

Dark Current I-V Characteristics Changing with Temperature * The dark current is reduced The gain is increased A sharp rising gain with the bias voltage will help to choose good operating points.

16x16 Arrays and V br Color Map

64x64 Arrays and V br

Breakdown Voltage Distribution

Innovative Current Bias Scheme Current bias mode resolves several common difficult problems from conventional voltage biasing mode in arrays. No need to know the breakdown voltage Less sensitive to the temperature fluctuation Much easier to control the bias point. Ideal for APD arrays operations Partially solved the after-pulsing problem.

Photon Counting Testing Setup

Operating temperature Dependence II = 1k Hz, FWHM=20 ns

Dark Count Probability Versus V ac at Different Temperatures

Dark Count Probability Versus the Combination of I DC and V ac at Fixed Temperatures

Detection Efficiency ~25% Achieved Under Gated Mode Operations V B -V P =22V V B -V P =14V

After Pulsing Problems 1.NASA lidar group did studies on one of the best reported commercial InGaAs photon counting APD product and found that within the claimed 10% DE, the portion of total counts caused by after-pulsing is 600% of that of the light count. 2.A test of after-pulsing duration can be done by increasing the gating pulse repetition rate (reducing time duration between gating pulse) and observe the increase of DCR. 3.As long as a few micro-seconds after-pulsing duration (equivalently the blind period) can usually be observed and that can put show stoppers to many timing sensitive applications. 4.After –pulsing is a material problem and need long term plan of improvement. However, it can be dealt with short term methods.

Suppression of After-Pulsing 1.The “Current Mode” operation will only allow one avalanche event to take place and prevent any possible after-pulsing for each gating period. This is achieved by limiting the DC supply current and the value of the gating capacitor. 2.During the avalanche process, the APD is switching on and the gating capacitor is discharged. The APD bias drops below the breakdown voltage. Limited by the set current of the DC current source and the capacitance value of the gating capacitor, the charging time to reach breakdown can never happen until the gating period is passed. 3.On the other hand using the voltage mode to operate the APD, the gating capacitor is quickly charged up to above breakdown and the APD is not soon ready to be fired again. At that moment if there still plenty residue carriers in the avalanche region, after-pulses will be generated.

Increasing The DC Source Current Can Reduce The Charging Time.

More on Bias Current Effects

Quantum Imaging - UMBC Objective Study the physics of multi-photon imaging for entangled state, coherent state and chaotic thermal state: distinguish their quantum and classical nature, in particular, the necessary and/or unnecessary role of quantum entangle- ment in quantum imaging and lithography. Study the “magic mirror” for “ghost” imaging. Muti-photon sources and measurement devices. ApproachAccomplishments Using entangled two-photon and three-photon states created via optical nonlinear interaction in spontaneous and stimulated modes for multi- photon spatial correlation study and imaging; Using chaotic light source, coherent light source for two-photon spatial correlation study and ghost imaging; Using photon counting and classical current- current correlation circuit to explore the nature of two-photon correlation. *The experiment: R. Meyers, K.S. Deacon, and Y.H. Shih, “ A new type of ghost imaging experi- ment ”, submitted to Phys. Rev. Lett., (2007). *The physics: Y.H. Shih, “ Quantum Imaging ”, IEEE Journal of Selected Topics in Quantum Electronics, (2007). * The detector: High uniformity, stability, and reliability large-format InGaAs APD arrays. *The photon source: J.M. Wen, P. Xu, M.H. Rubin, Y.H. Shih, “Transverse correlations in triphoton entanglement: geometrical and physical optics, Phys. Rev. A76, (2007). New type of ghost imaging experiment: a successful collaboration with ARL.