Photon number generation with the visible light photon counter

The multi-photon counting capability of the Visible Light Photon Counter (VLPC), along with the process of parametric downconversion, is used to experimentally generate photon number states containing 1,2,3 and 4 photons with high fidelity.

A liquid helium cryostat has been designed and operated for the purpose of testing visible light photon counter (VLPC) chips at 6.5 K. To achieve the desired operational characteristics for the VLPC devices their operating temperature is restricted to 6.5 K +/− 0.1 K. They will be used in a scintillating fiber tracker being proposed as part of an upgrade of the DO detector at Fermilab. The final version of the scintillating fiber tracker will contain roughly 100000 VLPC channels. Two cryostats with identical thermal design (a 128 channel and a 3072 channel design) have been built to perform the initial VLPC testing. The heat transfer needed to maintain the VLPC at its operating temperature occurs by conduction across an annular helium gas gap to a liquid helium reservoir. Helium boiloff is used to intercept conduction heat leak to the liquid reservoir. ANSYS finite element heat transfer analysis was utilized in the thermal design of the cryostat. The cryostat design and thermal performance (predicted and measured) are presented.

The upgrade of the D/spl phi/ detector at Fermilab is scheduled to run in 1999. One component of the upgrade is a cylindrically shaped preshower detector, placed on the outside of a solenoidal magnet that will surround the tracking volume. The technology for this detector will be plastic scintillating strips, with axial wavelength shifting (WLS) fiber and Visible Light Photon Counter (VLPC) readout. The VLPCs, with their quantum efficiency of /spl sim/70%, perform much better than traditional photomultipliers. We have performed extensive tests on different detector variations; investigating different scintillators, WLS fibers, and construction techniques. Prototype modules were placed in a cosmic ray test stand and over 120000 clean events per detector variant were collected. The large event sample and superior performance afforded by the VLPCs allow precise characterisation of the detector response. In this paper, we present results of our test programs, including uniformity of response, yields and interchannel crosstalk. We have achieved effective attenuation lengths exceeding 11 meters and yields of 4.5 photoelectrons per millimeter of scintillator traversed. An additional factor of 2 increase in yield seems achievable.

This paper reviews the research work on the Visible Light Photon Counters (VLPC) that have been developed for the scintillating fiber tracking at high luminosity colliders and high rate fixed target experiments. The devices originated from the joint work between UCLA and Rockwell International Science Center. The VLPCs are capable of counting photons very efficiently down to a single photon level with high avalanche gain, producing pulses at very high rates with very short rise times. Due to small gain dispersions they can be used in counting photons with high quantum efficiencies, therefore they are excellent devices for charged particle tracking using small diameter scintillating plastic fibers. In this paper, fiber tracking for the CDF and D0 upgrades and a possible usage of the VLPC readout for the experiment E803 at Fermilab will be discussed.

Visible light photon counters (VLPCs) offer many attractive features as photon detectors, such as high quantum efficiency and photon number resolution. We report measurements of the single-photon timing jitter in a VLPC, a critical performance factor in a time-correlated single-photon counting measurement, in a fiber-coupled closed-cycle cryocooler. The measured timing jitter is 240 ps full-width-at-half-maximum at a wavelength of 550 nm, with a dark count rate of 25 000 counts per second. The timing jitter increases modestly at longer wavelengths to 300 ps at 1000 nm, and increases substantially at lower bias voltages as the quantum efficiency is reduced.

The low signal to noise ratio produced by the VLPC (Visible Light Photon Counter) chip in the detection of a single photon necessitates the use of a filtering capacitor. Maximum performance of the filtering capabilities dictate the placement of the capacitor be as close to the VLPC chip as possible. However, the chip operates at extremely low temperatures (7K). In addition, available space within the VLPC cassette is limited. Therefore it is desired to find a capacitor which provides good temperature stability at cryogenic temperatures within a minimal space (e.g. surface mounted). This engineering note presents the results from a test of the effect of low temperature on surface mounted capacitors. Preliminary testing suggested that capacitors of the tantalum type would provide the best te mperature stability at cryogenic temperatures, therefore two different tantalum capacitors were tested. A ceramic capacitor was also included in the results as a comparison, even though preliminary results suggested the ceramic type would not provide sufficient temperature stability.

This engineering note documents the design of the control dewar and vacuum pump platform that is to be installed on the D-Zero detector. It's purpose is twofold. Firstly it is a summary and repository of the final design calculations of the structure. Secondly, it documents that design follows the American Institute of Steel Construction (AISC) manual and applicable OSHA requirements with respect to walking working surfaces. The information contained in the main body of this note is supported by raw calculations included as the appendix. The platform is a truss type frame strucrure constructed primarily of rectangular steel tubing. The upper platform is for support of the control dewar (cryogenic/electrical interface for the solenoid), visible light photon counter (VLPC) cryogenic bayonet can, and infrequently, personnel during the connection and disconnection of the detector to building services. Figure 1 shows a layout of the structure as mounted on the detector and with the installed equipment. The connection of the platform to the detector is not conventional. Two main booms cantilever the structure to a location outside of the detector. The mounting location and support booms allow for the uninhibited motion of the detector components.

With the demise of the Superconducting Supercollider (SSC) Project, there was great concern that the technological developments for that accelerator and its associated detectors might well be lost in the aftermath. In the case of scintillating fiber tracking, such as not been the case. During the period 1990--1993, several tracking technologies were under development for SDC, including Scintillating Fiber Tracking, Straw-tubes, and Microstrip Gas Chambers. In summer 1990, several members of the Fiber Tracking Group (FTG) proposed the use of Scintillating Fiber Tracking to the D0 experiment at Fermilab. This proposal was accepted, and D0 now is building a 75,000 fiber channel tracking detector with readout via Visible Light Photon Counters (VLPC) which were devices pioneered by the SDC Fiber Tracking Group. In addition, all the preshower detectors for D0 also make use of fiber readout (in this case waveshifting fibers) and VLPC for photosensing. In February 1993, a full 7 months prior to cancellation of the SSC project by Congress, the SDC experiment rejected scintillating fiber tracking for further development. Fortunately for all concerned, the D0 experiment had already embraced this technology, so this important detector concept could be further developed, refined, and utilized for physics experimentation. In early 2000, data will be taken with the D0 fiber tracker to study Top Quarks, Beauty Particles, Electroweak Physics, QCD phenomena, and to search for new phenomena. The University of Notre Dame has played a fundamental and seminal role in the development and implementation of this detector technology. R. Ruchti has served as cospokesman of the Fiber Tracking Group since its inception in 1989, and has been a pioneer of fiber tracking technology since 1980. In addition, at least one other experiment at Fermilab, E835, has utilized scintillating fibers with VLPC readout to study Charmonium in proton-antiproton collisions using a gas-jet target in the Tevatron antiproton accumulator ring. This fiber detector has been operated successfully for over 1 year of actual beam conditions.

<sec>High-order photon correlations of light fields are important for characterizing the quantum nature. Since Hanbury Brown and Twiss conducted the pioneering experiments in the 1950s, the HBT effect has inspired extensive research on high-order photon correlation in quantum optics, quantum information, and quantum imaging. The Single-photon counting module is one of the most widely used single-photon detectors. Due to its high detection efficiency and low dark counts in the visible and near-infrared region, it is reasonably chosen for basic research on quantum mechanics. Many researches have demonstrated that the maximum value of second-order photon correlation <i>g</i><sup>(2)</sup>(<i>τ</i>) at zero delay (<i>τ</i> = 0) can be used to distinguish different light fields. Therefore, the HBT scheme containing two single photon detectors have been widely used in many advanced studies, such as space interference, ghost imaging, single photon detection with high efficiency, etc. However, higher-order photon correlations <i>g</i><sup>(<i>n</i>)</sup> (<i>n</i> > 2) can reveal more measurable characteristics of light fields, such as information about the non-Gaussian scattering process, the skewness and kurtosis of photon number distribution, etc. When the extended HBT scheme is used to measure higher-order photon correlations, the experimental conditions including quantum efficiency and background noise greatly affect the photon correlation measurement. The influences of the counting rate and resolution time of the detection system on the measurements are also very important and cannot be ignored. Therefore, the comprehensive considering of various influence factors is necessary for accurately measuring the high-order photon correlations and also a challenge.</sec><sec>In this paper, we present a method based on double Hanbury Brown-Twiss scheme for the accurate measuring of high-order photon correlations <i>g</i><sup>(<i>n</i>)</sup> (<i>n</i> > 2). The system consists of four single photon counting modules and is used to detect and analyze the joint distribution probability of temporal photon correlation. Considering the effects of the background noise and overall efficiency, theoretically, we analyze the correlations of the third- and fourth-order photon with the incident light intensity, squeezing parameter and photon number respectively for thermal state, coherent state, squeezed vacuum state, and Fock state. Meanwhile, experimentally we study the influences of resolution time and counting rate on correlations of the coherent state and thermal state with third- and fourth-order photon. On condition that the resolution time is 2<sup>10</sup> ns and the counting rate is 80 kc/s, the correlations of third and fourth-order photon with the thermal state at zero time delay are accurately measured, and the relative statistical deviations of the measured vales from the theoretical values are 0.3% and 0.8%, respectively. In addition, the third- and fourth-order photon correlations of the thermal state at different delay times are also observed. It is demonstrated that the high-order photon correlations of light fields are measured accurately by comprehensively analyzing various influencing factors. This technique provides a promising and useful tool to investigate quantum correlated imaging and quantum coherence of light fields.</sec>

The prototype of a quantum random number generator is a single photon which impinges onto a beam splitter and is then detected by single photon detectors at one of the two output paths. Prior to detection, the photon is in a quantum mechanical superposition state of the two possible outcomes with –ideally– equal amplitudes until its position is determined by measurement. When the two output modes are observed by a single photon detector, the generated clicks can be interpreted as ones and zeros – and a raw random bit stream is obtained. Here we implement such a random bit generator based on single photons from a defect center in diamond. We investigate the single photon emission of the defect center by an anti-bunching measurement. This certifies the “quantumness” of the supplied photonic input state, while the random “decision” is still based on the vacuum fluctuations at the open port of the beam-splitter. Technical limitations, such as intensity fluctuations, mechanical drift, and bias are discussed. A number of ways to suppress such unwanted effects, and an a priori entropy estimation are presented. The single photon nature allows for a characterization of the non-classicality of the source, and allows to determine a background fraction. Due to the NV-center’s superior stability and optical properties, we can operate the generator under ambient conditions around the clock. We present a true 24/7 operation of the implemented random bit generator.

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Interferometric spectroscopy with undetected photons (ISUP) utilizing quantum mechanically path-entangled photon pairs has received considerable attention as an optical-measurement platform. Recently, we carried out a proof-of-concept experiment to show that ISUP can be used to measure the transmission spectrum of a Fabry-Perot resonator. Here, we demonstrate that ISUP with dual stimulated parametric down-conversion (StPDC) processes, which allows us to perform infrared rovibrational spectroscopy with visible photons. In our ISUP method, quantum coherence between two independent signal photons from each StPDC crystal is induced by the indistinguishability of conjugate idler fields and results in high visibility of the signal single-photon interferometry at the nondegenerate wavelength. If the seed-beam intensity is imbalanced due to the sample absorption, the corresponding envelope modulation of the interference fringe as a function of the seed-beam frequency reveals the absorption spectrum of the optical sample. As a proof-of-principle experiment, the full rovibrational transmission spectrum at 1550 nm of the hydrogen cyanide (${\mathrm{H}}^{13}{\mathrm{C}}^{14}\mathrm{N}$) molecules in a gas cell is measured from the single-photon interference fringe of the signal fields at 807 nm. We thus anticipate that the single-photon ISUP technique with dual StPDC crystals will find broad use for the development of high-resolution atomic and molecular spectroscopy with undetected photons.

Conventional Glauber coherent states (CS) can be defined in several equivalent ways, e.g., by displacing the vacuum or, explicitly, by their infinite Poissonian expansion in Fock states. It is well known that these definitions become inequivalent if applied to finite $d$-level systems (qudits). We present a comparative Wigner-function description of the qudit CS defined (i) by the action of the truncated displacement operator on the vacuum and (ii) by the Poissonian expansion in Fock states of the Glauber CS truncated at $(d\ensuremath{-}1)$-photon Fock state. These states can be generated from a classical light by its optical truncation using nonlinear and linear quantum scissors devices, respectively. We show a surprising effect that a macroscopically distinguishable superposition of two qudit CS (according to both definitions) can be generated with high fidelity by displacing the vacuum in the qudit Hilbert space. If the qudit dimension $d$ is even (odd), then the superposition state contains Fock states with only odd (even) photon numbers, which can be referred to as the odd (even) qudit CS or Schr\"odinger's cat state. This phenomenon can be interpreted as an interference of a single CS with its reflection from the highest-energy Fock state of the Hilbert space, as clearly seen via phase-space interference of the Wigner function. We also analyze nonclassical properties of the qudit CS including their photon-number statistics and nonclassical volume of the Wigner function, which is a quantitative parameter of nonclassicality (quantumness) of states. Finally, we study optical tomograms, which can be directly measured in the homodyne detection of the analyzed qudit cat states and enable the complete reconstructions of their Wigner functions.

A fundamental constraint on the generation rate in quantum random number generators with a continuous variable is obtained by minimal mathematical means. This constraint is valid for any signal intensity—any number of photons in a quantum state. For large occupation numbers, the random number generation rate increases logarithmically with the number of photons, but always remains finite. For a small number of photons, the generation rate is proportional to the number of photons. For any number of photons, the limiting random number generation rate is proportional to the frequency band of the signal—a quantum state.

We have studied the two-qubit quantum Rabi model in the asymmetric case and its generalizations with dipole and Heisenberg-type qubit–qubit interactions. The solutions are obtained analytically with eigenstates given in terms of the extended coherent states or photon number states. For identical qubit–photon couplings, a novel type of quasi-exact solution which exists for all coupling values with constant eigenenergy is found, leading to level crossings within the same parity subspace even for non-identical qubits. In contrast to the quasi-exact eigenstates of the single-qubit model, these exceptional eigenstates are formed by just a few Fock states (photon number bounded from above at one or three), and the condition for them depends only on a fine-tuning of the qubit level splittings with respect to the photon energy, but not on the coupling to the photon field if the qubit–qubit interaction is not taken into consideration. This makes them excellent candidates for direct experimental observation within circuit quantum electrodynamics (QED) and application in single photon experiments. Besides, exceptional solutions with finite photon numbers N are also found.