WO2002095437A2 - Efficient use of detectors with long dead times - Google Patents

Abstract

The invention provides a system and method for generating a sequence of primary signal pulses (220) in a primary signal pulse source at intervals τs(250); recording detection by a primary signal pulse detector of at least two pulses (240) of said sequence of primary signal pulses in a detector, said detector having an effective detector dead time (290) after a detection during which said primary signal pulse detector is insensitive to primary signal pulses; and wherein τs, is smaller than τd. The system and method of the invention provide for rapid gating of the detector thereby lowering dark counts. The system and method of the invention further comprise correlating times of generation of primary signal pulses in said sequence to times of detections in said detector, thereby defining pulse-detection pairs associating each detection with an element in said sequence.

Description

Efficient use of detectors with long dead times

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 USC 119(e) to United States provisional application serial number 60/292,992, filed May 23, 2001, and incorporates by reference the teachings of that application.

BACKGROUND OF THE INVENTION FIELD OF INVENTION

The present invention relates to signal detectors.

DISCUSSION OF THE BACKGROUND

The term weak signal as used herein means that incident flux at a detector is near the single photon limit. This definition applies to both continuous and pulsed electromagnetic fields.

Dead time τ_d of a detector means the amount of time that the detector, as coupled to its electronics, takes to recover back to essentially full detector efficiency when kept in the dark after a detection, whether the detection event is caused by one or more photon or a dark current. Recovery rate of a detector means the inverse of dead time, l/τ_d. A detector efficiency is the probability of detecting a signal upon receipt of the signal.

An avalanche photo diode (APD) operated in the Geiger mode, means that the

APD is reverse-biased above breakdown. APDs can be operated in Geiger mode to detect weak signals or single photon events with high quantum efficiency and low dark count rates. See R. G. Brown, K. D. Ridley, and J. G. Rarity, Appl Opt 25, 4122

(1986) and Appl Opt 26, 2383 (1987); Dautet et al., Applied Optics 32, 3894 (1993).

APD herein after refers to an APD that operates as a photon counter in the Geiger or a sub-Geiger mode. In a sub Geiger mode, the APD is biased just short of breakdown. The probability of arrival of a photon causing an avalanche current to flow in an APD defines the detection efficiency of the APD. Avalanche current indicates photo detection. A photo count is a determination that a photon was received based upon the existence of an avalanche current. Dark current means an avalanche current in an APD in the absence of a photon. Dark counts incorrectly generate photon counts. Dark counts result in false positive photo detection. G. Ribordy et al., Applied Optics 37, 2272 (1998) discloses that thermal carriers tunneling across the diode's depletion layer and after-pulses from trapped charges cause dark counts in APDs. Silicon APDs detect in the 500-850 nm range. Germanium APDs detect in the near-IR around 1300 nm. InGaAs/InP APDs detect in the wavelength range near 1550 nm used for telecommunications.

Detectors have a dead time related to their structure and composition, based upon their device physics and the detector electronics to which the detector is coupled.

For APDs, the detector electronics produce an output signal for positive detection events from the avalanche current. A. Karlsson et al., Circuits and Devices 11, 34 (1999) discloses that the dead time of an APD is a function of the time to quench the avalanche and restore the detector to a ready state for the next event and after-pulsing. After pulsing means signal noise typically caused by secondary avalanches subsequent to an initial avalanche current.

APD detectors operate as single photon counters in several different electronic configurations. In a continuously biased mode, the bias voltage remains above breakdown except immediately after a positive detection. In an APD, the avalanche forces the voltage below breakdown. Detector control electronics restores the bias voltage by charging the capacitance of the APD after quenching the avalanche.

An APD's dead time is defined to be the sum of the time for quenching the APD's avalanche current and subsequent charging of the capacitance associated with the APD up to the bias voltage. APDs may be quenched by passive or active quenching. For passively quenched APDs, dead times are typically 1 μs. For actively quenched silicon APDs, dead times can be as small as 30-40 ns. After-pulses can extend the time after an avalanche during which the detector noise precludes reliable determination of a subsequent detection. The combined effect of after pulsing and the actual dead time defines and effective dead time, which the total time after a detection during which detector signal is not a reliable indicator of a subsequent detection. APDs that exhibit after-pulsing typical have longer detector dead times than APDs that do not exhibit after-pulsing. For example, A. Karlsson et al., Circuits and Devices 11, 34 (1999), discloses that Ge and InGaAs APDs have relatively long effective dead times whether actively or passively quenched, due to the trapped charge lifetimes of on the order of one microsecond. Detectors can also operate in a gated bias mode, as described below.

Source period τ_s means the time between pulses for a pulsed source with repetition rate l/τ_s. M. Bourennane et al., Journal of Modern Optics 47, 563 (2000) discloses detecting light source pulses at a rate related to l/τ_d where l/τ_d ~ 1 MHz.

Detection rate means the fraction of a signal pulse (whether single or multiple photon signals) that are detected in a detector.

A transmission channel is a medium through which an electromagnetic signal propagates. A transmission channel has a loss coefficient indicating the expected signal loss transmitted through the channel.

The term rapid gating in this application means turning a detector's detection function on and off repeatedly at a rate as large as the inverse of a period during arrival of a signal pulse is anticipated, and at least as rapidly as the inverse of the repetition rate of transmission of signal pulse.

Signal transmission channel losses and detector efficiencies less than unity limit detection rates to less than unity. Detector efficiency for InGaAs APDs near 1550 nm is about 0.1-0.2.

An effective transmission coefficient, η, means the transmission coefficient times the detector efficiency. B. Yurke, Physical Review A 32, 311 (1985) discloses that effective transmission coefficient is less than one even for a perfect detector.

A single photon application means an application involving transmission of photons and requiring detection of photon pulses a substantial number of which contain single photons. Signal states that are essentially empty (a vacuum) can be thrown away. Therefore, the main issue is the ratio of multi-photon signals to single- photon signals. Preferably, this ratio is smaller than fifty percent, and more preferably, smaller than ten percent. Note that with sufficient phase randomization, a coherent state can be regarded as a classical mixture of some vacuum state, single- photon state, two-photon state, etc with a Poisson distribution in the number of photons. However, in single photon applications, η can be 10^"2 or smaller. The signal detection probability per pulse and rate depend on η times , where n is the average photon number per pulse, and ηn is the effective average photon number per pulse impinging on a perfect optical detector. The inequality ηn/τ_s « l/τ_d is satisfied in the prior art quantum cryptography detection systems disclosed in the following references: P. Townsend, PCT/GB95/01940; P. Townsend, US005953421A; P. Townsend, IEEE Photonics Technology Letters 10, 1048 (1998); M. Bourennane et al., Optics Express 4, 383 (1999); M. Bourennane et al., Journal of Modern Optics 47, 563 (2000); W. T. Buttler et al., Physical Review A 84, 5652 (2000); and N. Gisin and coworkers, quant-ph/0101098.

Active period τ_a defines the time duration when a detector is on, or in other words, capable of detecting. Inactive period τ_ina defines the duration when a detector is off, or in other words, not capable of detecting. This paragraph describes Bourennane et al., J. Modern Optics 47, 563 (2000).

Bourennane et al. used an InGaAs APD from EG&G for quantum cryptography at 1550 nm. Their laser source emitted signal pulses at the rate l/τ_s smaller than l/τ_d. Passive quenching of their detector, which was configured in a gated mode, gave τ_d ~ 1 μs, a timing jitter of -300 ps, and detector efficiency ~ 0.18. Their detector had an attenuated Poisson-distributed light containing an average photon number ~ 0.1 per pulse, and an ηw ~ 10^"3-10^'2, ignoring the dark counts. In each detector cycle, their APD electronics alternated the detector bias between an active and inactive period, ^τ _totai ^{= τ} _a ^{+ τ}i_na- ^τ _a ~ 5 ns. During this period the detector electronics raised the bias voltage above breakdown: an incident photon had a probability ~η« to cause a positive detection event (i.e., an avalanche). The detector electronics lowered the bias voltage below breakdown for a time τ_ina set equal to the effective dead time τ_d. To synchronize the source with the detector, their source period τ_s was chosen to be equal to their τ_total. M. Bourennane and coworkers' source emitted pulses such that one signal pulse could arrive at the detector during each active period. Therefore, the source-detector combination gave the signal detection rate or count rate of

Their effective signal detection rate was R _* ηw/τ_d - 1-10 kHz « l/τ_d.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a system and method for increasing the effective signal detection rate of detectors that have either long effective dead times or slow recovery rates.

It is an object of this invention to provide systems and methods for improving the detection efficiency of detecting weak signal pulses using detectors that have either long effective dead times or slow recovery rates. These and other objects of the invention are provided by a system and method for generating a sequence of primary signal pulses in a primary signal pulse source at intervals τ_s; recording detection by a primary signal pulse detector of at least two pulses of said sequence of primary signal pulses in a detector, said detector having a detector effective dead time τ_d after a detection during which said primary signal pulse detector is insensitive to primary signal pulses; and wherein τ_s is smaller than

The invention may also include the following features: a correlator for correlating times of generation of primary signal pulses in said sequence to times of detections in said detector, thereby defining pulse-detection pairs associating each detection with an element in said sequence; a transmitter for transmitting said sequence of primary signal pulses through a fiber optic; a timing signal generator for generating at least one timing signal indicative of time of transmission of each one of said primary signal pulses in said sequence; memory for recording times of detection of said primary signal pulses; primary signal pulse detector electronics for gating said primary signal pulse detector; a controller for gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to signal pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector; a controller for gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector, and not sensitive to pulses during primary signal pulse detector effective dead times; wherein τ_s is less than one half τ_d; a channel from said source to said detector; wherein said detector comprises an APD; wherein said detector comprises an APD and further comprising gating said primary signal pulse detector by raising a reverse bias voltage on said primary signal pulse detector from below breakdown voltage for said detector to above breakdown voltage for said detector and then lowering said reverse bias to below breakdown voltage; a time correlator for correlating time of transmission of said primary signal pulses to times for detection of said primary signal pulses; a secondary timing pulse generator for generating a strong signal secondary timing pulse in advance of a generation of a primary signal pulse; a controller controlling generation of one strong signal secondary timing pulse in advance of generation of each primary signal pulse of said sequence; configured to transmit both said sequence of primary signal pulses and secondary timing pulses over the same channel; active electronics for quenching an avalanche current in said primary signal pulse detector; a memory device for storing data corresponding to each pulse-detection pair; a data processor for processing data corresponding to each pulse detection pair to define a quantum key; the system configured such that detection rate approaches R ~ l/τ_d as the ratio τ_s/τ_d increases; wherein said primary signal pulse source is a semiconductor laser generating radiation at about an 830 nm and said primary signal pulse detector is a silicon detector; and wherein said primary signal pulse source is a semiconductor laser generating radiation at about 1550 nm and said primary signal pulse detector is an InGaAs/InP detector.

In another aspect the invention comprises a system and method for providing a detection count rate R that approaches l/τ_d, such as at least 0.01, preferably at least 0.1, and more preferably at least 0.75 l/τ_d, comprising generating single photon signal pulses in a source; transmitting said single photon signal pulses from source to a single photon detector via transmission channel; gating said detector; recognizing detection events from said detector; time synchronizing the source and detector; correlating successful detector events with the source pulses; and recording and outputting the correlated successful detection-source pulse information.

Preferably, the detector electronics is configured to gate the detector so that it is sensitive to signal pulses only during time periods when pulses in said sequence of primary signal pulses are anticipated to arrive in said detector. Preferably, the detector is also controlled to not detect during a time interval equal to at least the detector's effective dead time after the detector does detect a signal. Preferably, a system of the invention includes a controller, or control electronics which synchronizes the detector electronics to only detect signals during a time period when primary signal pulses are anticipated to reach said detector, to time correlate primary signal pulses generation times with primary signal pulse detector detection times, to correlate the transmission and detection events, and to associate each detection with a specific element of said sequence of primary signal pulses. Preferably, the system and method of the invention provide a primary signal pulse duration that is substantially less than the duration between primary signal pulses.

Preferably, the rate of primary signal pulses transmitted in sequence is substantially greater that the detector effective dead time τ_d. Preferably, the rate of detection in the detector is on the order of l/τ_d. The invention is most useful when the primary signal source is capable of generating primary signal pulses at a rate much greater than the rate at which the primary detector can detect the signals: l/τ_s » l/τ_d. Preferably, the primary pulse rate l/τ_s is at least ten, at least one hundred, or at least one thousand times l/τ_d. Preferably, the system of the invention comprises a controller for controlling timing of primary signal pulse detector electronics and primary signal pulse source electronics.

Additional elements may include a secondary source, a secondary detector, and a time-interval analyzer. Preferably, the detector is an APD in Geiger mode, in which case the controller controls the detector electronics to sensitize the detector by changing the reverse bias voltage on the APD to above the APDs reverse bias breakdown voltage and to de-sensitize the detector by changing the reverse bias voltage on the APD to below the APDs reverse bias breakdown voltage. The controller may contain electronic memory and input/output ports.

The time correlator includes a timing signal generator and a clock. The timing signal generator and clock may or may not be physically integrated together, and may or may not be located adjacent one another. The time correlator may be located in or integrated with any of the other circuit elements in the system. The timing signal is used as a reference time to enable the time correlator to correlate the time of transmission of a primary signal pulse to a time for detection of said primary signal pulse. The time correlator correlates each detection in the detector with a specified primary signal pulse transmitted from the primary signal source based upon known time delays in a given system between generation of a timing pulse and transmission of a primary source signal pulse and detection of a primary source signal pulse. The clock is used to measure the times to enable the time correlation.

The timing correlator may include secondary source electronics, a secondary timing pulse source, secondary detector electronics, and a secondary detector. Control source signals may control the generation of a secondary timing pulse and a primary signal pulse at times differing by a predetermined interval. Preferably, a secondary timing pulse is transmitted either in advance of each primary signal pulse of a sequence of primary signal pulses, in advance of a first primary signal pulse of a sequence of primary signal pulses, or in advance of every n primary signal pulses, where n is any integer less than one half the number of pulses in a sequence of primary signal pulses. Preferably, secondary timing pulses are strong pulses such that the probability of secondary detector detecting each secondary timing pulse is substantially unity. Preferably, time of detection of each secondary timing pulse controls time of gating of the primary signal pulse detector so that the primary signal pulse detector is sensitive to pulses only when primary signal pulses are anticipated to arrive at the primary signal pulse detector. Secondary timing pulse signals may or may not be transmitted in the same physical channel, such as a fiber optic or free space, as primary signal pulses. Secondary timing pulses may be transmitted substantially concurrently with primary signal pulses over the same physical channel in which case secondary timing pulses must be at a different wavelength than primary signal pulses to enable distinct detection of primary and secondary pulses. Preferably, primary signal pulse detector electronics include a gating circuit designed to gate the detector on and off at the same rate as the inverse of the duration of a time interval or window for anticipated receipt of a primary signal pulse, and to repeat gating at a rate equal to the primary signal pulse generation rate.

When the primary signal pulse detector generates a detection signal, the primary signal pulse detector electronics outputs an electronic signal indicating detection and, preferably, returns the detector to a state of readiness, for example in an APD, by quenching the APD's avalanche current. Additional electronics or electrical connections may exist between electronic components for enabling synchronization, time delays, data storage, and data processing. The method of the invention provides a novel detector bias cycle that increases the efficiency of the detection process. The method assumes that a primary signal pulse train in which each primary signal pulse has an effective average photon number η« impinges the primary signal pulse detector every τ_s time period. Since η« « 1, the probability for any one pulse causing a successful detection event is roughly ηn.

To understand the detector cycle of this method, assume the primary signal pulse detector detects a photon. This detection event creates an avalanche current, which the primary signal detector electronics quenches.

The primary signal detector electronics transform that avalanche current into an electronic output signal indicating detection of a pulse. That signal is preferably used as feed back to the primary signal pulse detector control electronics to turn the primary signal pulse detector off for a duration τ_ιna = τ_d.

An electronic circuit, preferably an AND gate, receives as inputs the pair of detector electronics' output signal and clock time signal, the clock time signal corresponding to the time of detection of a primary signal pulse. However, other electronics can be used to determine the time of detection. The time of detection associated with the primary signal pulse is defined herein as a "pulse detection pair." The pulse detection pair is crucial information about which pulse actually caused each detection, since each different primary signal pulse may convey different information. This information may be encoded for example by relative phase compared to a correlated pulse, absolute polarization, or relative polarization compared to a correlated pulse.

Primary signal pulse detector electronics holds the bias voltage below breakdown for τ_jna, so that the detector does not detect any incident pulses during this time. Once the duration τ_ina elapses, the primary signal pulse detector electronics sets the bias voltage above breakdown.

Preferably, the primary signal pulse detector electronics controls the detector to implement rapid gating cycles at the same rate as the primary signal pulse rate until another detection occurs. Preferably, the primary signal pulse detector is gated so that it is on only during that portion τ_on of the τ_s primary signal pulse periodicity during which a primary signal pulse is anticipated to arrive. The anticipated arrival time is defined by the signal transmission time from the primary signal pulse source to the primary signal pulse detector and the properties of the primary signal pulse source and associated source control electronics defining the duration during over which the source may generate a pulse.

To achieve rapid gating of the primary source detector, a controller sends a signal to the primary signal pulse detector electronics to raise the bias voltage above breakdown and lower the bias voltage below breakdown once during each rapid gating cycle. Preferably, the primary signal source detector bias voltage is only above its breakdown for τ_on during the expected arrival time of each pulse, and the time duration of the expected arrival time of each pulse is only a small fraction of τ_s. Since the average photo detection probability is only η«, many τ_s cycles will occur on average before a detection. Preferably, rapid gating cycles of duration τ_on < τ_s continue until either (1) a photo detection event or dark count causes the next avalanche or (2) a set number of rapid on-off cycles occurs. After a detection, detector control electronics places the detector in a non-detect mode for τ_ina after which the detector is placed into a detect mode and preferably rapid gating cycles commence.

Electronic memory connected to the controller may store the pulse-detection pairs. The controller outputs the pulse-detection pair information either continuously or when queried by an external device. This allows external devices, such as programmed digital or quantum computers to process any information represented by the detected primary signal pulses. Such information may be the polarization or phase of a photon. This information is useful for example in quantum cryptography applications. The invention provides a detection count rate R > l/( X + T_J), where X is the average time duration between a detection event. The count rate approaches R ~ l/τ_d as X becomes much smaller than the effective dead time. The rapid gating of the invention also reduces the dark count rate compared to a detector that is continuously biased above breakdown, thereby enabling larger R. The system of the invention may have other means to time synchronize transmission from the primary pulse signal source with the time of detection when direct electronic synchronization through the controller becomes difficult. These means may include a secondary detector to detect a strong secondary timing pulse signal sent by from the location of the primary signal source either from the primary pulse signal source or from a secondary timing signal source. The secondary detector, would output a detection signal to a discriminator circuit that in turn sends a clock signal to the controller. The secondary timing signal detection time thereby provides both a system clock and synchronization with the source.

The system may also include a time-interval analyzer. The controller may trigger a time-interval analyzer at the beginning of the rapid gating cycles to start measuring the elapsed time until a detection event. After a successful detection event, the detector electronics may output an electronic signal to the time-interval analyzer to stop the elapsed time measurement. Therefore, the time-interval analyzer may measure the elapsed time between the trigger from the controller and a successful photo detection in order to record the one-to-one correspondence between the detected event and the particular primary signal pulse that caused the detection. This is an alternative method to specify each pulse detection pair.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in connection with the following drawings.

Fig. 1 is a schematic of a system of the invention.

Fig. 2 is a schematic showing timing sequences helpful in describing a method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a transmission system 1 including primary signal pulse source electronics 20, primary signal pulse source 30, fiber optic transmission channel 40, primary signal pulse detector 60, primary signal pulse detector electronics 70, controller 80, data lines 90 and 100, controller input data line 110, controller output data line 120, secondary timing pulse electronics 130, secondary timing pulse sourcel40, secondary timing pulse detector electronics 150, and secondary timing pulse detector 160. In addition, Fig. 1 schematically shows propagation of pulses 50 in fiber optic 40. Controller 80 controls when primary signal pulse electronics 20 and secondary timing pulse electronics 130 energize primary signal pulse source 30 and secondary timing signal pulse source 140. Controller 80 sends control signals to the pulse electronics via control signals transmitted over data line 100. Controller 80 also controls primary signal pulse detector electronics 70 and secondary timing pulse electronics 150 via control signals transmitted from controller 80 over data line 90.

Controller 80 controls primary signal pulse detector electronics 70 to gate voltage across detector 60 to a level sufficient for detector 60 to detect primary signal pulses when primary signal pulses should arrive at the detector. When primary signal pulses should arrive is based upon time of generation of primary signal pulses and primary signal pulse transit time from primary signal pulse source 30 to primary signal pulse detector 60. Primary signal controller 80 may determine when primary signal pulses should arrive based upon time of generation and known signal transit times from controller 80 to source 30, from controller 80 to detector 60, and from source 30 to detector 60. Data processor 170 may receive pulse detection pair information from controller 80 via data line 110, and send instructions to controller 80 via data line 120. Data processor 170 may process information received from controller 80. Preferably, data processor 170 processes pulse detection pair information in a quantum cryptography application. Part of that application may include defining a quantum key based upon the information received from controller 80. Data lines shown may each be single or multiple physical lines. Controller 80 may physically separate from or adjacent to source or detector electronics. Alternatively, primary signal pulse detector electronics 70 gates voltage across detector 60 to a level sufficient for detector 60 to detect primary signal pulses at a specified time after detector 160 detects a secondary timing signal pulse. In this embodiment, secondary timing pulse electronics 150 may control primary signal pulse electronics 70 to bias voltage across primary signal pulse electronics 60 to a detection level a first specified time delay after detection of each secondary timing pulse and for a second specified duration. In this alternative, controller 80 controls pulse generation such that each secondary timing pulse is generated prior to each primary signal pulse preferably by said first specified time delay.

In embodiments wherein primary and secondary pulses travel over different channels, said first specified time delay is adjusted based upon the predetermined delay between receipt of timing and signal pulses such that detector 60 is sensitive to corresponding primary signal pulses when they are anticipated to arrive at detector 60.

While in Figure 1, the primary and secondary pulses are shown to be transmitted in the same communication link, generally they may also be transmitted in different communication links, for instance, different spatial paths of a point-to-point quantum key distribution (QKD) set-up. Preferably, the two communication links have related time delay properties that would allow easy time synchronization. Optional elements for alignment and/or security and/or convenience, etc such as polarization randomizers, attenuators, beam-splitters, pulse modulations, fiber-couplers, isolators, etc may be combined with the invention.

Fig. 2 shows a time sequence schematic 200 including time flow arrow 210, signal pulse power sequence 220 illustrating signal pulse power arriving at detector 60, detector gate voltage sequence 230 of gate voltage across detector 60, and detection event sequence 240 of detections by detector 60. Time is correlated between sequences 220, 230, and 240 such that events shown as at the same p*osition along the time flow direction 210 in Fig. 2 occur at the same time. In some applications such as quantum key distribution (QKD), the sender and the receiver may apply some random modulation to each source signal before it reaches the detector. In fact, Bob may have two detectors or more in QKD. Part of the signal goes to detector 1 whereas the remaining part of the signal may go to detector 2. For instance, in the standard BB84 QKD scheme, Alice and Bob each randomly chooses between two bases. When Bob is using a different basis from Alice, the strength of the signal reaching say detector one of Bob is halved. When Bob is using the same basis from Alice, then the signal strength is either full or nothing, depending on the specific modulation applied by Alice. Effectively, in BB84, for each signal, it has to go through a random filter which allows full strength to go through with probability %, half with probability ^lA and zero with probability ^lA. Signal pulse power sequence 220 shows primary signal pulses 250 having a duration of τ_p and a periodicity τ_s as indicated by time segment 250. Sequence 220 shows a continuous sequence of primary signal pulses wherein one pulse arrives at detector 60 every τ_s.

Detector gate voltage sequence 230 shows voltage pulse signals 300 of duration τ_on separated by detector low gate voltage during period τ_off, as illustrated by time segment 290.

Detection event sequence 240 shows a first detection event 400 concurrent with second pulse of signal pulse power sequence 220 and second gate voltage pulse 300. Detection event sequence 240 shows a second detection event 500 concurrent with gate voltage pulse 320 of gate voltage sequence 230.

In operation, detector 60 receives the sequence of weak pulse signals power sources, and occasionally, perhaps frequently but not always, detects one of those pulses, such as pulse 300 shown in the time sequence. Upon detection of a photon pulse (or any detection including a dark current induced false detection), primary signal pulse electronics 70 reduces the voltage of the detector to below its detection threshold (reverse bias avalanche breakdown voltage for an APD) for a detector inactive period approximately as long as the detector effective dead time τ_d. At the expiration of the inactive period, detector 60 is controlled to repeat the rapid gating cycle, as shown starting with pulse 310.

Since the primary signal pulse rate may be substantially greater than the detector effective dead time, at least one primary signal pulse is likely to be detected in a time less than the detector effective dead time, resulting in a data transmission rate approaching one pulse every detector effective dead time, or l/τ_d. Rapid gating prevent dark current detection outside of times when primary signal pulses could arrive at detector 60.

To understand the complete detector biasing cycle of this method, assume a successful photo detection occurs in the system's detector. This detection event creates an avalanche current that the detector electronics transforms into an electronic output signal. The output signal is sent to controller 80. Controller 80 feeds back that signal to the AC voltage supply of detector electronics 70 as a trigger to lower the bias voltage below breakdown, initiating detector 60's inactive period of duration τ_ιna = τ_d. During the inactive period detector electronics 70 holds the bias voltage below breakdown, so that detector 60 cannot detect any incident pulses. The inactive period allows detector 60 to recover fully from the detection, e.g., by allowing trapped charges to relax without causing after-pulsing. Once the duration τ_ιna elapses, the rapid gating cycles of detector 60 begins again. To achieve rapid gating, the controller sends an electronic signal to detector electronics 70 to raise and lower the bias voltage 230 above and below breakdown during each rapid gating cycle of time duration τ_s. The bias voltage is only above breakdown for a short time duration τ_on > τ_p (or approximately equal to τ_p) surrounding the expected arrival time of each pulse, as synchronized by controller 88. For example, for τ_p = 1 ns and τ_s = 5 ns, rapid cycle on- and off-times of τ_on = 2 ns and τ_ofT= 3 ns, respectively, work within the 300 ps timing jitter of the detector. (By definition, τ_s = τ_on + τ_off to synchronize detector 60 and source 30.) The detector cycle repeats after the successful detection event causes an avalanche.

In some applications, such as quantum key distribution, the controller 80 may physically be divided into two separate controllers between the sender and the receiver. Information is generally compartmentized in such a set-up. For instance, the sender, traditionally called Alice, selects a random basis and a random polarization/pulse for each signal she transmits to Bob. Her choice is kept private and is, thus, generally not available to Bob's controller. Similarly, Bob's choice of measurement basis is, a priori, kept private and is, thus, not generally available to Alice's controller. In summary, Alice and Bob's controllers may choose to keep some information private from each other and subsequently disclose only relevant partial information in subsequent steps specified by a protocol. Such a compartmentization may be crucial for the security of quantum key distribution. Note that our invention applies directly also to the "plug and play" set-up, D. Stucki et al, "Quantum Key Distribution over 67km with a plug & play system" available at http://xxx.lanl . gov/abs/quant-ph/0203118. Fig. 2 also shows a modulated signal pulse power sequence 225, which results from the aforementioned random modulation.

More concretely, given the original signal pulse power sequence 220, the sender and receiver may apply their modulation, which leads to a modulated pulse power sequence 225. In the particular example shown in Fig. 2, the modulated pulse power sequence 225 corresponds to the strength of a time sequence of signals at a specific detector in the standard BB84 quantum key distribution scheme. In other words, the detector actually sees the strength shown in the modulated signal pulse power sequence 225, rather than the strength shown in the original signal pulse power sequence 220. Note that corresponding to each peak in the original signal pulse power sequence 220, the modulation ideally gives three possible strength (height) in the modulated signal pulse power sequence 225, namely, full strength with probability

1/4, half strength with probability ^lA and zero strength with probability 1/4. Except for imperfections such as dark counts and misalignment, nothing happens to a detector when the modulated signal strength is zero. When the modulated signal strength is full, a detector will detect an event with certain probability, p, depending on the signal strength and the detector's intrinsic detection efficiency. When the modulated signal strength is half, a detector will detect an event with essentially a probability, p/2 , assuming that the signal strength is weak.

In quantum cryptography, each signal pulse typically contains n < 1 photon. The detection fraction r\n is typically much less than one. As a result, the probability for any one pulse to cause a detection P = (1- e^" ") « η«. The mi . pulse in succession has a probability P(w) = exp[-(m-7)η«]-exp[-mη«] «

to cause a successful detection. Analysis gives the average time duration of rapid gating cycles as X * For

> 10², X > 500 ns. For simplicity, in the above discussion, we have ignored the filtering effect in quantum key distribution discussed earlier.

When a detection occurs, controller 80 uses an AND gate between detector electronics 70 output signal and a clock signal synchronized with source 30 to pair the clock time corresponding to the arrival of the primary signal pulse with the successful detection of that pulse. This process creates a pulse detection pair that provides the information about which pulse actually caused each photo detection.

Electronic memory, connected to controller 80 may store the pulse-detection pairs from a sequence of detected pulses. Using the controller input/output ports, controller 80 outputs the pulse-detection pair information continuously and/or when queried by computer 170. As a result, computer 170 can process any information represented by the detected signal pulses, e.g., the polarization or phase of a photon used for quantum cryptography. The invention also provides timing information that connects the successful detection of an event with the system clock, which for example may be of used in quantum cryptography or fluorescence studies.

The dark count rate of the preferred method of the invention is small compared to a dark count rate of a detector that is continuously biased above breakdown. The average time that detector 60 spends biased above breakdown is Xτ_on /t_s. For the example given above, the dark count probability is -0.008 for the average detector cycle duration. When other system components and parameters remain the same, faster gating electronics and a shorter laser pulse - as fast as the timing jitter - allow τ_on > 300 ps and a dark count probability of -0.001. Thus, dark count probability can be reduced to below 0.01, and preferably to below 0.05 per photon detection. Using the preferred embodiment of the system of the invention, the method gives a detection count rate R ~ 1/(X + τ_d). The count rate approaches R ~ l/τ_d as X becomes much smaller than the effective dead time. For X ~ 500 ns and τ_d = 1 μs as above, R ~ 670 kHz. Achieving the limit of R ~ l/τ_d only requires a higher repetition rate source and faster electronics. For an APD detector, the source must operate in the detector's wavelength range of sensitivity. For example, a coherent or partially coherent source may be a solid-state, semiconductor, dye, free electron or other laser. An 830 nm (1550 nm) semiconductor laser works well with silicon (InGaAs/rnP) APDs. The source may also originate from scattered light or spontaneous emission from a spectroscopic sample. Single photon sources, those that emit exactly one photon per pulse, may be available at visible and near-IR wavelengths in the future for use with APDs. Spontaneous parametric down-conversion creates approximately a single photon source. For example, amplified -800 nm Ti:Al₂O₃ laser pulses, frequency-doubled to -400 nm, impinge on a nonlinear crystal to create a pair of photons, the signal and idler, with some probability. In this case a second APD detector may detect the idler photon to confirm the presence of the signal photon used for certain single photon applications, like quantum cryptography.

Longer wavelength sources, such as infrared lasers, microwave and millimeter wave antennas, or novel GHz and THz emitters, may operate with detectors such as bolometers or cooled photo conductors or cooled CCDs. Shorter wavelength sources, whether ultraviolet lasers, harmonically generated ultraviolet and x-ray pulses, synchrotrons, charged particle impacts in metal targets, laser-excited plasmas, UV and x-ray free electron lasers, or other processes that generate bursts of short- wavelength radiation, can be used with the methods of the invention when used with efficient detectors like the Bicron detectors, Si APDs, or diamond detectors. An alternative to embodiment to coherent or partially coherent sources are incoherent sources including an ultraviolet, visible, or near-infrared LED pulsed by an external current source. An LED that emits light centered at 850 nm works with silicon APDs.

In one embodiment, the primary signal pulse source and primary signal electronics comprise an Optocom OPT3200 series 1550 nm InGaAsP/InP laser system that pulses at the rate l/τ_s = 200 MHz (τ_s = 5 ns) with pulse duration τ_p ~ 1 ns and transmission channel 40 is optical fiber which has low loss of ~0.2db/km at this wavelength. Alternatively, the transmission channel medium may be free space. Most coherent and incoherent light sources should give Poisson detection statistics in an APD but the photon-number detection statistics are not critical to the method of the invention. In the preferred embodiment, detector 60 comprises an

InGaAs/InP APD from Fujitsu, NEC, or EG&G, and it detects light at single-photon light levels in Geiger mode. A thermo-electric cooler stage can help to provide temperature and operational stability, to the APD. APD model C306444EJT-07 from EG&G/Perkin-Elmer provides detection efficiencies of about -0.18 when passively quenched. In operation, a voltage source DC biases this detector at - 40 V, roughly

1.5 V below breakdown. This is the low detector gate voltage shown in Fig. 2. At this bias the detector is off. In accordance with the method of the invention, primary signal pulse detector electronics 70 includes a fast rise time, input-controlled AC voltage supply, which may provide a transient voltage of roughly 5 V in series with the 40 V DC when triggered. This additional voltage biases the APD above breakdown and thus turns on the detector. When biased 3.5 V above breakdown, a successful detection event or dark count causes an avalanche current to flow through the APD. When biased above breakdown, the dark count rate in this detector is roughly 40,000/sec. The remaining primary signal pulse detector electronics 70 preferably includes an avalanche-current limiting resistor of -300 kΩ that connects the APD in series to a grounded -50 Ω load resistor in parallel with an amplifier stage. The detector circuit quenches the avalanche and provides a timing resolution of -300 ps. The effective dead time τ_d is about 1 μs owing to the electronics and after-pulsing. An amplifier stage sends the amplified output current to a discriminator, which clearly identifies the avalanche events. The discriminator then outputs an output pulse or transient voltage level to controller 80 to indicate detection events. Commercial amplifier and discriminator systems are available, for example, from Ortec/Perkin-Elmer.

Controller 80 provides several control functions for the system, as mentioned above. In the preferred embodiment, controller 80 comprises a Xilinx FPGA unit, time synchronized with an electronic clock integrated into primary signal pulse source electronics 20. An alternative controller includes a combination of standard analog and digital chips, including logic gates, counters, and digital input/output ports, preferably an application-specific printed circuit board layout, designed to perform all control functions.

System 1 may include other means to synchronize source 30 with detector 60 when direct electronic synchronization through controller 80 is difficult. For example, these means may include secondary timing pulse detector and its electronics 150, 160 to detect a strong secondary optical synchronization signal originating near source 30. The secondary timing pulse may be triggered by the primary source's electronics before a primary source pulse is sent. The secondary timing pulse may have a wavelength of 650 nm originating from a pulsed diode laser. Secondary timing pulse detector may be a standard PIN photo diode, may output to a discriminator circuit that sends a clock signal to controller 80, providing both a system clock and synchronization with the source. Not every primary source pulse would require a secondary pulse for synchronization. Controller 80 may maintain its own clock, which controller 80 may synchronize with the primary source only periodically or aperiodically by using a secondary source pulse.

The system of the invention may also include a time-interval analyzer. Ortec/Perkin-Elmer makes time-interval counting devices based on time-to-amplitude conversion and a multi-channel analyzer. Controller 80 may trigger a time-interval analyzer at the beginning of the rapid gating cycles to start measuring the elapsed time until a photo detection event. After a successful detection event, the detector electronics may output an electronic signal to the time-interval analyzer to stop the elapsed time measurement. Therefore, the time-interval analyzer may measure the elapsed time between the trigger from controller 80 and a successful photo detection in order to record the one-to-one correspondence between the detection and the particular source pulse that caused the detection. This is an alternative method to specify each pulse-detection pair, compared to the direct controller-detection method,.

The method of using the preferred embodiment of the system provides the rapid gating cycles that increase the efficiency of the detection process for weak signals. The system and method of the invention provide for increased efficiency of detecting useful information from weak signals, particularly for detectors having effective dead times that are long compared to signal pulse rates generated by a signal source.

The correlation of the time of receipt of certain primary signal pulses with the time at which they are received enables subsequent correlation of the detected train of signal pulses with the transmitted train of signal pulses. So that substantial amounts of information encoded in the transmitted train of pulses can be recovered from the received/detected train of pulses.

While only certain features of the present invention have been outlined and described herein, many modifications and variations will be apparent to those skilled in the art. Therefore, the claims appended hereto are intended to cover all such modifications and equivalents that fall within the broad scope of the invention.

Claims

I CLAIM

1. A system comprising: a primary signal pulse source for generating a sequence of primary signal pulses at intervals τ_s; a primary signal pulse detector for recording detection of at least two pulses of said sequence of primary signal pulses, said primary signal pulse detector having a detector effective dead time τ_d after a detection during which said primary signal pulse detector is insensitive to primary signal pulses; and wherein τ_s is smaller than τ_d.

2. The system of claim 1 further comprising a coπelator for correlating times of generation of primary signal pulses in said sequence to times of detections in said detector, thereby defining pulse-detection pairs associating each detection with an element in said sequence.

3. The system of claim 1 further comprising a transmitter for transmitting said sequence of primary signal pulses through a fiber optic.

4. The system of claim 1 further comprising a timing signal generator for generating at least one timing signal indicative of time of transmission of each one of said primary signal pulses in said sequence.

5. The system of claim 1 further comprising memory for recording times of detection of said primary signal pulses.

6. The system of claim 1 further comprising primary signal pulse detector electronics for gating said primary signal pulse detector.

7. The system of claim 1 further comprising a controller for gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to signal pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector.

8. The system of claim 1 further comprising a controller for gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector, and not sensitive to pulses during primary signal pulse detector effective dead times.

9. The system of claim 1 wherein τ_s is less than one half τ_d.

10. The system of claim 1 further comprising a channel from said source to said detector.

11. The system of claim 1 wherein said detector comprises an APD.

12. The system of claim 1 wherein said detector comprises an APD and further comprising gating said primary signal pulse detector by raising a reverse bias voltage on said primary signal pulse detector from below breakdown voltage for said detector to above breakdown voltage for said detector and then lowering said reverse bias to below breakdown voltage.

13. The system of claim 1 further comprising a time correlator for correlating time of transmission of said primary signal pulses to times for detection of said primary signal pulses.

14. The system of claim 1 further comprising a secondary timing pulse generator for generating a strong signal secondary timing pulse in advance of a generation of a primary signal pulse.

15. The system of claim 1 further comprising a controller controlling generation of one strong signal secondary timing pulse in advance of generation of each primary signal pulse of said sequence.

16. The system of claim 1 configured to transmit both said sequence of primary signal pulses and secondary timing pulses over the same physical medium.

17. The system of claim 1 further comprising active electronics for quenching an avalanche current in said primary signal pulse detector.

18. The system of claim 1 further comprising a memory device for storing data corresponding to each pulse-detection pair.

19. The system of claim 1 further comprising a data processor for processing data coπesponding to each pulse detection pair to define a secure key in quantum key distribution.

20. The system of claim 1 configured such that detection rate approaches R ~ l/τ_d as the ratio τ_s/τ_d increases.

21. The system of claim 1 wherein said primary signal pulse source is a semiconductor laser generating radiation at about an 830 nm and said primary signal pulse detector is a silicon detector.

22. The system of claim 1 wherein said primary signal pulse source is a semiconductor laser generating radiation at about 1550 nm and said primary signal pulse detector is an InGaAs/InP detector.

23. The system of claim 1 further comprising a controller for gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to signal pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector, thereby providing a ratio of dark current detections to photon detections of less than 0.01.

24. The system of claim 1 wherein said detector further comprises a photon pair creator.

25. The system of claim 1 wherein said effective dead time is longer than a dead time of said primary signal pulse detector.

26. The system of claim 1 wherein after pulsing occurs for a period of time longer than a dead time of said primary signal pulse detector.

27. A method comprising: generating a sequence of primary signal pulses in a primary signal pulse source at intervals τ_s; recording detection by a primary signal pulse detector of at least two pulses of said sequence of primary signal pulses in a detector, said detector having a detector effective dead time τ_d after a detection during which said primary signal pulse detector is insensitive to primary signal pulses; and wherein τ_s is smaller than τ_d.

28. The method of claim 27 further comprising correlating times of generation of primary signal pulses in said sequence to times of detections in said detector, thereby defining pulse-detection pairs associating each detection with an element in said sequence.

29. The method of claim 27 further comprising transmitting said sequence of primary signal pulses through a fiber optic.

30. The method of claim 27 further comprising generating at least one timing signal indicative of time of transmission of each one of said primary signal pulses in said sequence.

31. The method of claim 27 further comprising recording times of detection of said primary signal pulses.

32. The method of claim 27 further comprising gating said primary signal pulse detector.

33. The method of claim 27 further comprising gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to signal pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector.

34. The method of claim 27 further comprising gating said detector so that said detector is not sensitive to pulses between times when primary signal pulses are anticipated to arrive at said detector, and not sensitive to pulses during detector effective dead times.

35. The method of claim 27 wherein τ_s is less than one half τ_d.

36. The method of claim 27 further comprising transmitting timing signals from said source to said detector.

37. The method of claim 27 wherein said detector comprises an APD.

38. The method of claim 27 wherein said detector comprises an APD and further comprising gating said primary signal pulse detector by raising a reverse bias voltage on said detector from below breakdown voltage for said detector to above breakdown voltage for said detector and then lowering said reverse bias to below breakdown voltage.

39. The method of claim 27 further comprising coπelating time of transmission of said primary signal pulses to times for detection of said primary signal pulses.

40. The method of claim 27 further comprising generating a strong signal secondary timing pulse in advance of time of generation of a primary signal pulse.

41. The method of claim 27 further comprising generating one strong signal secondary timing pulse in advance of generating each primary signal pulse of said sequence.

42. The method of claim 27 further comprising transmitting both said primary signal pulses and secondary timing pulses over the same physical medium.

43. The method of claim 27 further comprising quenching an avalanche cuπent in said detector.

44. The method of claim 27 further comprising storing data corresponding to each pulse-detection pair in a memory device.

45. The method of claim 27 further comprising processing data corresponding to each pulse detection pair to thereby define a secure key in quantum key distribution.

46. The method of claim 27 wherein detection rate approaches R ~ l/τ_d as the ratio τ_s/τ_d increases.

47. The method of claim 27 wherein said source is semiconductor laser generating radiation at about an 830 nm and said detector is a silicon detector.

48. The method of claim 27 wherein said source is a semiconductor laser generating radiation at about 1550 nm and said detector is an InGaAs/InP detector.

49. The method of claim 27 further comprising gating said primary signal pulse detector so that said primary signal pulse detector is not sensitive to signal pulses between times when primary signal pulses are anticipated to arrive at said primary signal pulse detector, thereby providing a ratio of dark current detections to photon detections of less than 0.01.

50. The method of claim 27 further comprising generating a photon pair from said primary pulse signal.

51. The method of claim 27 wherein said effective dead time is longer than a dead time of said primary signal pulse detector.

52. The method of claim 27 wherein after pulsing occurs for a period of time longer than a dead time of said primary signal pulse detector.

53. A system comprising: means for generating a sequence of primary signal pulses in a primary signal pulse source at intervals τ_s; means for recording detection by a primary signal pulse detector of at least two pulses of said sequence of primary signal pulses in a detector, said detector having a detector effective dead time τ_d after a detection during which said primary signal pulse detector is insensitive to primary signal pulses; and wherein τ_s is smaller than τ_d.

54. A system for providing a detection count rate R that approaches l/τ_d, comprising: a single photon detector; a transmission channel to convey the photons from the source to the detector; means for gating a detector; means for recognizing successful detection events from a detector; means for synchronizing the source, detector, and other components of the source-detection system; means for correlating successful detector events with the detected source pulses; and means for recording and outputting the coπelated successful detection-source pulse information.

55. A method for providing a detection count rate R that approaches l/τ_d, comprising: generating single photon signal pulses in a source; transmitting said single photon signal pulses from source to a single photon detector via transmission channel; gating said detector; recognizing detection events from said detector; time synchronizing the source and detector; correlating successful detector events with the source pulses; and recording and outputting the correlated successful detection-source pulse information.