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Single photon detectors based on semiconductor devices with InAs quantum dots Beata E Kardynał 1 / Quantum Information Group, Toshiba Research Europe Ltd Patrick See Neil Beattie Ian Farrer Andrew J Shields James Blakesley Dave Ritchie Cambridge Research Laboratory Toshiba Research Europe Ltd Cambridge, UK 2 / Semiconductor Physics Cavendish Laboratory University of Cambridge Cambridge, UK Quantum Information Group, Toshiba Research Europe Ltd Motivation Development of single photon detector for quantum cryptography and more generally for quantum information systems. Requirements: high detection efficiency very low dark count rate fast >10 MHz and faster better 3 / Quantum Information Group, Toshiba Research Europe Ltd Overview Devices for single photon detection: resonant tunnelling diodes containing InAs quantum dots (QDRTD) field effect transistors containing InAs quantum dots (QDFET) Principle of Operation: sensing electrostatic potential due to captured photo-hole. Single photon counting. Conclusions. 4 / Quantum Information Group, Toshiba Research Europe Ltd Quantum Dots Field Effect Transistor photon (semi)-transparent gate 20nm GaAs electron channel h Photon absorption takes place in quantum well (20nm) and the e optional absorber under the well quantum dot layer n+ GaAs back gate (up to 200nm). photon absorber AlGaAs barrier Si doping Photo-holes are captured on negatively charged dots. V(x,y) At 4K the effect is persistent. (x,y) I EF EC 5 / Quantum Information Group, Toshiba Research Europe Ltd Effect of Light on QDFET 100 dark 101 100 -0.5 0.0 105 10-1 10-2 10-3 0.5 103 101 10-1 illuminated -0.5 0.0 0.5 Conductance (µS) Conductance (µS) 102 0 .0 V - 0 .0 2 V - 0 .0 4 V - 0 .0 8 V 10 - 0 .1 2 V - 0 .1 4 V - 0 .1 8 V - 0 .2 V 0 Gate Voltage (V) 200 400 600 800 Time (s) Negative charge on the dots shifts the I-V characteristics of the FET. Reduced by illumination charge on the dot results in increased conductance of the FET. Electrical detection of single photon possible if the FET is small (about 1 – 2 µm mesa, 1 – 2 µm gate). Steps in current are up to 10% of the total current. 6 / Quantum Information Group, Toshiba Research Europe Ltd 0.2 Detection efficiency of up to 1.0% at 650 nm. Limited by: 0.1 NiCr gate (25% transmission). 0.0 0 5 10 Incident Photon Flux 15 (s-1) Both GaAs and AlGaAs absorption detected. No losses due to the tunnelling of holes from GaAs. 20-25 nm of AlGaAs around the dots “active” assuming uniform collection efficiency. Abs20nm Abs25nm 1 1 0.1 400 600 Wavelength (nm) 7 / 10 Quantum Information Group, Toshiba Research Europe Ltd 800 Absorption near Dots (%) Low absorption of 20nm thick GaAs well at 650 nm (6%). Quantum Efficiency (%) Detection rate (s-1) Spectral Response of the QDFET Quantum Dots Resonant Tunnelling Diode 50nm n+ GaAs collector quantum dot layer Photon absorption in the absorber on the collector side of the device (100 - 600 nm) photon absorber AlGaAs /GaAs RTD n+ GaAs emitter Photo-holes are captured on negatively charged dots. Photo-holes recombine on the dots producing photons of lower energy that GaAs bandgap – no re-absorption. Tunnelling current repopulates depleted dot with τ ∝ Itunnel. Independent contacts are obtained using cross-wire geometry of the device. 8 / Quantum Information Group, Toshiba Research Europe Ltd Effect of Light on QDRTD Charged dots detune the device (in I e emitter emitter- collector voltage bias) from the resonance compared with RTDs with no collector dots. h Reduction of the charge on a dot with a 0.02 0.5 0.00 Current Density -2 (Acm ) 0.04 photo-hole brings the RTD locally closer to 0.0 the resonance: tunnelling current rises sharply. 0.5 1.0 Bias (V) -0.2 Electrical detection of single photon possible if the RTD is small (1 – 2 µm diameter). -0.4 9 / Quantum Information Group, Toshiba Research Europe Ltd 4 0.04 RTD has 300 nm absorber region under 50nm n+ collector. λ=550nm 3 The best fit between the measurement and theory is obtained when it is assumed that only bottom 150 nm of the absorber is “active”. 0.03 2 0.02 0.01 20 0 0.00 0.0 0.1 0.2 0.3 0.4 Incident Photon Flux (s-1) Maximum efficiency of 15 % is measured at 620 nm wavelength. Efficiency is limited by: photon reflection from the surface. losses in the collector contact layer. Quantum Efficiency (%) Detection rate (s-1) Spectral response of RTD Absorption in 150nm GaAs above InAs dots 15 10 5 0 400 600 Wavelength (nm) 10 / Quantum Information Group, Toshiba Research Europe Ltd 800 Single photon counting – amplifier Source-Drain Bias Gate Voltage + Reset Pulse Vout Vac SPD ISPD Band-Pass Amp VDC LHe cryostat Low-Pass Amp Gain (mV/nA) Optic Fibre Cascode Bias 100 10 1k 10k 100k 1M 10M Frequency (Hz) Transimpedance amplifier is used with the detector for single photon counting. Amplifier is configured to operate with high resistance of the device. Amplifier input referred noise is about 0.14 pA/Hz0.5 and is dominated by the input transistor. 11 / Quantum Information Group, Toshiba Research Europe Ltd 100 0.0 50 0.0 0.5 Typical traces of the output signals from the low-pass and ac amplifiers captured on the oscilloscope. AC output 10.0 1.0 Time (s) Current (nA) Low-pass amp output (nA) 20.0 Band-pass amp output (mV) Single photon counting – amplifier II 185 180 175 0 12 / Quantum Information Group, Toshiba Research Europe Ltd 2 4 6 Time (ms) 8 10 Counting Setup Pulsed Laser Power Supplier Laser Sync Signal Minimum time gate 5 ns. Time Delay Laser wavelength 684 nm. Laser Diode Discriminator Level Pulse filling factors: 10-4 - 103 photons. SPD T = 4.2K. Amp Period of the Measurement 13 / Quantum Information Group, Toshiba Research Europe Ltd Photon Counting with QDRTD a) Device with 3 nm Al0.33Ga0.67As barriers => Counts (a.u.) 1.0 Light induced step size about 15nA. 0.5 Jitter (FWHM) measured to be 150 ns => solely the circuit property, not the detection process speed. 0.0 0 1 2 Time (µs) Dark counts: amplifier noise and RTS noise 1 Count rate (pulse) 10 of the device . 0 10 -1 10 -2 10 light dark -3 10 -4 10 20 40 60 Discriminator level (mV) 14 / Quantum Information Group, Toshiba Research Europe Ltd Photon Counting with QDRTD a) dark counts (per gate) -2 10 -3 10 Maximum counting efficiency at 684 nm -4 10 -5 laser wavelength 9% and 1% at a very low -6 dark count of 10-6 per gate (10-8 ns-1). 10 10 -7 10 -8 10 0 2 4 6 8 10 approximate quantum efficiency (%) gain : (step height) / (reset rate)/e =2 ·105 6 10 5 10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 From the current step and reset rate we get -2 peak current (Acm ) -2 modelled current (Acm ) step height (nA) -1 reset rate (s ) for 3 nm barriers device. We measure current: more important is the step height itself. 3 4 5 6 7 8 Barrier thickness (nm) 15 / 9 10 Exponential increase of photon-induced signal with RTD barrier thickness. Quantum Information Group, Toshiba Research Europe Ltd Jitter at strong signal Current (nA) Count rate (arb. units) illumination 400 300 200 100 0 0 200 400 600 data Gaussian fit 1.0 0.5 0.0 100 Time (s) 120 140 160 180 200 Time after laser pulse (ns) The jitter was measured (for FET) to be 6.5 ns (FWHM) and it is still limited by the capacitance of the circuit. Frequency of operation limited by width and jitter of the signal: 100% of count in 25 ns gate. 16 / Quantum Information Group, Toshiba Research Europe Ltd Counting with QDFET 10 10 10 0 light dark -1 -2 -3 10 -4 10 -5 20 40 60 Dark count rate (per gate) Count rate (per gate) 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 0.0 Discrimintor level (mV) 17 / Quantum Information Group, Toshiba Research Europe Ltd 0.5 Quantum efficiency (%) 1.0 Summary Maximum quantum efficiency is obtained in the QDRTD and is about 15% at 620nm. It should increase to 65% at 475 nm in optimised devices. Maximum counting efficiency at 684nm laser was measured to be about 9%. Dark count rate is very low for both devices: at 1% counting efficiency dark count rate is 10-8 ns. Jitter of the counting is limited by the signal to noise ratio and the minimum jitter was measured to be 150 ns FWHM when the current step was about 15 nA. We expect reduction of the jitter to 100 ps – 1 ns in optimised circuit. Wavelength of detection can be tuned with material system used. InGaAs/InAlAs on InP can be used for telecommunication wavelengths. Since the absorption region is different from the detection region it may be possible to increase the absorption area. 18 / Quantum Information Group, Toshiba Research Europe Ltd