Patent number 8766161 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "Single photon counting is useful for many applications, including the measurement of quantum states in tomography or in quantum key distribution systems, as well as in a wide variety of other applications including light-based ranging. Avalanche photodiodes (APDs) are attractive since they are inexpensive, small, and convenient to use. In order to detect single photons thereby acting as a single photon detector (SPD), the bias voltage of an APD is typically brought above the breakdown level, at which point a single photon can set off a macroscopically detectable breakdown event. This is the Geiger mode of operation. Often the bias voltage is time-gated above the breakdown only when optical pulses arrive in order to get acceptable performance (
"Applying a time-gated voltage across the APD causes a charge to feed-through the device that makes detecting small breakdowns difficult. However, the capability to limit the size of the breakdowns is beneficial since large breakdowns correspond to large charge flows through the device, causing more trapped carriers which in turn cause an unwanted afterpulse effect where the device can break-down upon receiving a gate pulse even when no photons are present. This afterpulse effect can be controlled by waiting a suitably long time between gates to allow the carriers to disperse. However that slows down operation.
"Recent work in the field has suggested that the use of either a sine wave gate or the use of differential subtraction can allow small breakdowns to be detected using suitable analog processing (
"Digital sampling using an analog-to-digital converter (ADC) combined with a simple type of digital signal processing (averaging the sampled output over many gate cycles to determine the best threshold) was proposed to digitally process the APD breakdowns (US patent application No. 20090236501 by Takahashi et al.). In principle this is a more flexible method than purely analog methods, however the gate generation circuit is not specified as digital and no substantial control over the gate generation is exploited. Additionally, all the processing is performed in the digital domain after sampling the signal from the APD using an analog-to-digital converter (ADC). The breakdown from the APD is largely repetitive capacitive feed-through with a small breakdown signal. This poses a dynamic range issue for the sampler, since the feed-through signal will saturate the sampler before the breakdown signal does. Ideally, the input to the sampler should be primarily the desired breakdown signal and that breakdown signal should be of a large enough magnitude so that the sampler records it with a high signal-to-noise ratio. This typically means that the (possibly amplified) breakdown signal should consume a significant fraction of the dynamic range of the sampler. The aforementioned analog signal processing methods address this problem by greatly reducing the feed-through via analog processing. However, as previously mentioned analog processing usually carries with it certain limitations and reduced flexibility as well. Analog and digital processing methods have not been optimally combined in prior art. Moreover, prior art does not address estimating various metrics such as the dark count rate (dark count rate is the probability of detecting a photon when none is incident) or the detection efficiency and methods to optimize the operating parameters to optimize these metrics. In a real system these metrics are important and to some degree require a trade-off where improving one will likely degrade another.
"It is also noted that often times multiple SPDs are used in one system, where the output of the multiple SPDs can each be analyzed individually or together such as when measuring coincidence counting statistics, for example when performing a quantum state tomography measurement. In such cases it is desirable to design the system as a whole so as to maximize shared resources and minimize the number of expensive components or limit the number of traces interconnecting the various electronics thereby saving printed circuit board space. APD's can also be operated in linear mode, where their output voltage is linearly related to the optical intensity. In this case the bias voltage to the APD is below the breakdown level. Control over an APD bias such as to optimize its gain in linear mode has been described by Anderson in U.S. Pat. No. 5,929,982. In that work an ADC is used to look at the detector noise with no light incident on the APD and adjust the bias to a desired noise level. This technique is used for gain control when the APD is operated in linear mode so as to address dynamic range issues and issues associated with parameter variations and temperature fluctuations of the uncooled device, as opposed to Geiger mode for detecting single photons where the gain is undefined and the devices are almost always temperature controlled. Additionally, controlling the APD in linear mode is much simpler because effects such as afterpulsing are not present and the APD bias is simply a DC bias level, and therefore issues associated with the shape, magnitude, and phase of the gate pulse, as well gate feedthrough, are absent.
"What is needed is a system of digital control of the SPD such that analog processing can also be implemented and optimized appropriately, preferably over a wide range of operating conditions. Performance metrics should be automatically calculated and optimal performance automatically determined by the system with little or no manipulation by the users. It is desirable to be able to monitor performance including estimating parameters such as dark count rate, detection efficiency, and afterpulse count rate. The output of the SPD can be processed in the analog domain to remove undesired feedthrough, then sampled with a sampler. Ideally the system will require a small number (including just one) of samples per gate pulse. The system should be reprogrammable so that operation over a broad range of conditions, including a wide range of gating frequencies, is possible.
"Designs which allow multiple such SPDs to be measured and controlled efficiently with the minimum number of parts and high speed inter-connections are also desired. In particular such multi-SPD systems have applications in quantum state tomography, where the ideal configuration of the SPD may change depending on the properties of the quantum state to be measured."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventor's summary information for this patent: "The invention herein uses a control unit to control one or more SPDs. The optical-to-electrical portion of the SPD can be an APD. The control unit can control and optimize many parameters associated with the SPD system including the generation of gated pulses of controllable frequency, amplitude, shape, and phase to bias the APD. The control system can control the amplitude and phase of an electrical reference signal of one or more spectral frequencies which are subtracted from the APD output signal before being sampled then processed in a discriminator, such as a latched comparator, which will determine if a breakdown event occurred or not and therefore if a photon was detected or not. While various discriminators could be used, it is advantageous to use a sampler such as an analog-to-digital converter (ADC) or the closely related track-and-hold (T/H) circuit. A means to capture the sampled signal into the control unit allows the control unit to use this information to monitor and optimize the system. For instance, the quality of reference signal subtraction can be determined and the phase and magnitude of the reference signal manipulated by the control unit for optimum performance. This method can work at a wide variety of gating frequencies since analog filters of a fixed frequency and self-differencing subtraction with a fixed time delay are not needed.
"It is useful if the control unit can get a good picture of the entire temporal profile of the signal from the SPD, even though the breakdown event to be measured is typically only a small fraction of the time between gates. This will be helpful, for instance, in determining the quality of the reference signal subtraction as well as the ideal sampling point to measure the breakdown signal. One way to do so would be to sample (clock) the sampler at a substantially higher rate than the gate rate but where the sample rate and the gate rate maintain a fixed relationship. For instance the sample rate can be 10 times the gate rate. However, the sampler and associated electronics may not operate at such high rates, especially if multiple SPDs are being operated by one system and where it is undesirable to have many such complex and expensive high speed samplers. In such a case the relative sampling phase of the sampler can be scanned (possibly by gating it with a clock frequency that differs from the gate frequency by a non-integer or by phase-shifting the sampling clock in incremental amounts) to trace out an equivalent time picture of the breakdown. This allows the full temporal profile of the signal generated after subtracting the reference signal from the SPD output to be monitored. The resulting digitized signal can be processed to determine how well the electrical reference signal is being subtracted. The electrical reference signal amplitude and phase can then be adjusted to optimize the quality of the subtraction. The information on the temporal profile of the SPD output signal can also be used to determine the optimal temporal sampling point (phase) of the sampler to discriminate the breakdown events during a measurement.
"In general the invention can be operated in a calibration mode, where information about the system is estimated so as to optimize and characterize the performance level, and a measurement mode where the actual measurements take place. The modes can be separated in time, for instance initially or periodically entering a calibration period followed by a measurement period. In some cases the two modes can be interleaved. The use of a low rate sampling clock with a varying phase with respect to the breakdown signal arrival time (or alternatively the gate pulse arrival time at the photon detector) to the T/H circuit can occur during the calibration period, so as to trace out the temporal profile of the SPD without requiring a very high sample rate ADC.
"Especially when multiple SPDs are being monitored, as may happen for instance when performing a quantum state tomographic measurement, it is helpful to use T/H circuits to sample the output of the SPDs with the T/H output being sent both to a comparator in order to threshold the breakdown signal as well as to a multiplexer. The multiplexer can select one of the multiple input signals to be sent to its output and which of the multiplexer input signals is selected is controlled by the control unit. The multiplexer input singles each represent a sampled output of one of the SPDs. The desired signal to be monitored is selected by the control unit and the corresponding output of the multiplexer is sent out to an ADC to digitize the desired signal. The ADC sends the digitized signal into the monitor unit (MU). Note that the MU may be implemented in the same electrical platform as the control unit, such as in a field programmable gate array (FPGA) or integrated circuit. The T/H circuits may be operated at a sampling frequency significantly below the gate frequency during the monitoring period so that a low rate inexpensive ADC can be used as the front end digitizer for the monitor unit. By employing a multiplexer the number of ADCs required in the system can be reduced, simplifying the system.
"In addition to the detection signal processing, the control unit can also control the gating signal. For instance a digital-to-analog converter (DAC) could generate a fully programmable bias gate pulse for an APD which can be varied in shape, time duration, amplitude, phase, and repetition rate. For instance, for sinusoidal gating a programmable oscillator followed by a variable attenuator and variable phase shifter can be used to control the sinusoid's frequency, amplitude, and phase. The control unit can adjust the APD bias voltage as well as the DAC generated gate pulse profile and the detection signal processing while also estimating performance parameters in order to find an ideal operating condition. A clock signal that is synchronous to the clock of the optical input pulses is distributed to the system components as needed.
"There are several competing performance parameters in single photon detection systems including dark count rate, detection efficiency, and after-pulse probability. These competing performance parameters are typically balanced to the users' best judgment by varying device and system parameters like the APD bias voltage, breakdown threshold (threshold level which separates a measured photon counting event from a dark event), APD device temperature, etc. The invention here can help automatically set such parameters or provide performance data to help the user to better set them directly. For instance, periodically some gate pulses can be intentionally offset from the known optical pulse arrival locations so that the gate pulses arrive at the APD when no light is incident. The statistics of these dark events can be built up over time to estimate the dark count rate. Various metrics can be estimated so as to choose the digital filtering parameters (if the SPD signal is digitized it could be digitally filtered), breakdown threshold voltage, APD bias level, reference signal amplitude and phase, and gate pulse profile to optimize performance. This is especially the case if a reference light source is used or the approximate incident photon rate is known so that the detection efficiency can be estimated. After-pulse rates can be estimated by comparing the average photon counting rate with the photon counting rate for those pulses immediately following a breakdown or after a designated dead-time. The gates can be placed so as to detect the optical input pulses, or intentionally temporally offset so as to not detect the optical pulses and therefore estimate the dark count rate, or offset in a manner that detects a reference light source to calibrate the detection efficiency. Alternatively, an optical switch can configure the input as the desired input to be measured, or a reference optical input for detection efficiency calibration, or no input for dark count calibration, however such a configuration will add some insertion loss due to the optical switch. If the gates are offset from the arrival of the optical pulses periodically then the dark count rate can be estimated without added optical insertion loss. If a reference optical pulse is inserted into the system and the gate is shifted so as to detect the reference pulse then the detection efficiency can be calibrated. Such a dynamic change in the gating output can be generated because of the high level of control the control unit has over the gate and/or sampling instant. For instance the gate location can be changed by using a DAC, or by changing the phase of a sine wave gate, or by other such gate temporal control means, and the sampling phase of the sampler can also be changed by the control unit
"It is possible for the DAC to lengthen the pulse width, including making it a continuous wave (CW) signal. Thus the gate duration can be lengthened for detecting temporally longer pulses. The system could even choose to pull the APD out of Geiger mode into linear mode if desired, for instance to increase the dynamic range of a particular measurement. The presence of the reference optical signal allows the resulting detection efficiency to be characterized easily, even if the APD performance changes over time.
"In some cases one wants to collect the statistics from multiple photon counters, including perhaps joint statistics such as correlation information. The photon counting system can be modified to control two or more SPD detectors using largely the same electronics. Since one may want to vary the optical delay between the detectors, an optional delay controlled by the controller unit is used to allow for the gate pulses to each APD to be independently adjusted. Reference pulses can be sent simultaneously to both detectors, which will allow for both detection efficiency calibration of both detectors as well as a check to see that the co-incidence count rate between the two detectors behaves as expected (the reference optical inputs are uncorrelated so the probability of correlated detection should be the multiplication of the probability of detection at each detector).
"One application for the use of multiple detectors is in quantum state tomography. It is desirable to minimize the measurement time in such a system, but optimal measurement time for a given accuracy will depend on the SPD characteristics and the characteristics of the input state being measured. An initial measurement of the optical input state can be made which the control unit can use to estimate the properties of the input optical signal. The control unit can use this information to update the SPD parameters so as to more optimally measure the input state, allowing for faster and/or more accurate tomographies."
URL and more information on this patent, see: Kanter, Gregory S.. System for Controling and Calibrating Single Photon Detection Devices. U.S. Patent Number 8766161, filed
Keywords for this news article include: Electronics,
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