Single-photon sources... and their applications Andrea Fiore Photonics and Semiconductor Nanophysics Department t of Applied Physics
Photonics Transmission and processing of information A. Fiore
Shrinking sizes (Moore s law) The first commercially manufactured silicon transistors (1954): Ruby laser (invented 1960): Bell labs A 22-nm node transistor (2011): Microcavities: AMOS/ MOTIF 60 nm Intel 500 nm EPFL-CNR, 2005 A. Fiore
Shrinking energies Reduced size Reduced energy Electronics: ~1 fj to switch a CMOS gate (decreased by /1000 in 30 years) Photonics: ~0.3 fj to detect a "1" bit (decreased by /1000 in 30 years) What is the limit? A. Fiore
Energy quantisation Planck, 1900: Energy in EM field is 1 multiple of a "quantum": 2 E n h 2 h 1 0 E h Energy quantum ("photon"): h 3 x 10 19 J for visible light Ex.: 40 W ~ 3x10 18 photons in 1 sec
Single photons On Earth: ~ 1 photon/sec in a 40 cm aperture
Single-photon applications (1) Example: LIDAR for altimetry and atmospheric sensing LIDAR altimetry of Mars surface (Mars Orbiter Laser Altimeter) www.nasa.gov A. Fiore
Single-photon applications (2) Biomedical imaging IC failure analysis Fluorescence-lifetime in-vivo detection of pre-cancer (Duke Univ.) Fluorescence imaging g of hot spots in ICs (Scanalitics) A. Fiore
Outline Single photons as quantum bits Quantum key distribution Single-photon sources and detectors Quantum photonic integrated circuits A. Fiore
Single photons: Particles or waves? They can be emitted and detected one by one, as particles They interfere, as waves S M 1 BS 0 BS M D counts Number of Quantum mechanics: A single photon can be in a coherent superposition of states 0 1 (0 and 1: Coded in position, time, phase, polarisation...) A. Fiore
Quantum bits and cats Another example: Polarisation 0 0 1 quantum bit (qubit) 1 NB: Measurement makes the cat dead (or alive) 1 Cat alive A. Fiore Schrödinger's cat 0 Cat dead
Quantum key distribution Alice and Bob want to exchange a secret message They need to exchange a cryptographic key With classical bits: Insecure Eve Eve 1 0 0 1 0 1 0 0 1 Bob Alice A. Fiore
Quantum key distribution Alice and Bob want to exchange a secret message They need to exchange a cryptographic key With single-photon quantum bits: Secure Eve Bob Measurement of a qubit changes its state Can be detected Alice A. Fiore Quantum Key Distribution is physically secure
Example of QKD protocol "BB84" (Bennet&Brassard 1984) protocol: Two polarization coding sets: = 1 = 1 = 0 = 0 Alice produces a set of random bits: 1 1 0 0 0 1 0 1 1 Alice chooses a polarization set randomly: Alice codes the bits accordingly: Bob chooses an analyzer set randomly: Only the bits where the same polarization sets were used are kept 1 1 0 0 1 1 A. Fiore
Eavesdropping on BB84 QKD: Intercept and resend attack Photon can be in any of 4 non-orthogonal states Impossible for Eve to measure its state Intercept & resend attack: Measure all bits in random basis, retransmit to Bob the measured state Eve 1 Bob Sent a 0, received a 1 0 0 Error Alice Introduces 25% error rate By monitoring the error rate Alice and Bob can check the channel security
Security of QKD: Cloning of quantum states What about amplifying photons? Eve A fundamental theorem in quantum mechanics: "No cloning theorem" Wootters & Zurek, Nature, 299, 802 (1982): "A single quantum cannot be cloned" (This amplifier would violate linearity of QM)
QKD applications QKD commercial today Limited distance ( 100 km) Limited key rate (<1 Mb/s) Need for better devices idquantique Vectis Single-photon sources Single-photon detectors A. Fiore
Single-photon sources A single-photon source is not just a low-power source! Laser Attenuator 2 1 security threat Statistical distribution of photon numbers time Single- 2 emitter 1 Well-defined photon number time A. Fiore
Single-photon sources Single atom: 2 1 Atoms are very difficult to control! time "photon dropper" "Semiconductor atoms" (quantum dots): GaAs CB Nanoscale confinement 10 nm InGaAs VB Energy quantisation A. Fiore
Now you have a photon Are you sure?? Need a single-photon detector...
Detecting single (or few) photons Technical solution n. 1: Our eye! Mehmis and Mohseni, SPIE 2008 Limitations: Sensitive only in visible range Slow (~300 ms) A. Fiore
Artificial detectors Linear detectors A Need internal gain urrent Cu time Do not reach single-photon sensitivity due to amplifier noise Eg photomultipliers But low sensitivity at wavelengths of interest A. Fiore Molecular expressions
A recent technology: Superconducting singlephoton detectors (SSPDs) Superconducting nanowires as photon-sensing elements NbN + V - R hs T>T C ~ 4nm I~I C Substrate Golts'man et al., Appl. Phys. Lett. (2001) Invented in 2001 Commercialised in 2004 Now 4 companies selling SSPD systems Technology of choice for all single-photon research in the near-ir A. Fiore
SSPD fabrication challenge 60 nm A. Fiore Film growth and meas. @ EPFL, nanofab. @CNR-IFN
SSPD performance x1000 more sensitive, 10x faster than photodiodes and photomultipliers Record-distance QKD over an installed optical fiber: 150 km between Geneva and Neuchatel (QKD group at Univ. Geneva) Vericold closed-cycle cryostat Courtesy Univ. Geneva A. Fiore
The power of quantum mechanics... But can we do more than sending a photon in a fiber? Bob Alice
Single photons: What's next? Quantum computing: Maybe, one day... (but most likely not with photons) www.sciencedaily.com http://northtexasdrifter.blogspot.nl NB: Quantum computers, if made, will have to communicate with single photons
Quantum mechanics with photons Photons: a model system to investigate quantum mechanics Example: Entanglement and nonlocality 1 2 (ex. Aspect et al., PRL 1982) 1 2 H V V H 1 2 1 2 Example: Quantum evolution of interacting many-body systems (quantum walk of photons) A. Fiore (Peruzzo et al., Science 2010)
From scientific fun to applications Use photons to implement the quantum physics you (think you) know (more qubits more nonclassical) Use photons to simulate the quantum physics you don't know Example: Calculating the energy structure of complex molecules On a classical computer, the resources required to solve Schrödinger eq. for N particles scales exponentially with N 30% of computation time in supercomputers used for quantum chemistry and band structure calculations A. Fiore martin-protean.com
Feynman once again... Quantum physics is too hard to simulate on a classical computer... we should better use a quantum system
Quantum simulators Quantum simulators as modern orreries: A. Fiore Gilkerson orrery arm.ac.uk
Quantum simulators Complex problem Optical circuit ĤH massey.dur.ac.uk Get result from a measurement Quantum simulators with 50-100 qubits would beat classical supercomputers in molecular structure calculations A. Fiore
Why integration phas.ubc.ca Scaling beyond few qubits impossible
The solution: Quantum photonic integrated circuits Aspuru-Guzik et al., Nature Phys. 2012 Essential components: Single-photon sources Single-photon detectors Passive circuit Nice extra's: Fast modulators Quantum memory (spin?) Deterministic quantum gate A. Fiore
What has been done: Passive circuits Increasingly complex passive circuits being realised: (on SiON, LiNbO 3, Si, GaAs) Active groups: Univ. Bristol, Oxford, Rome, Macquarie,... CNOT quantum gate: Reconfigurable quantum circuit: Problems: Scalability (n. external sources/detectors) t In- and out-coupling efficiency Shadbolt et al., Nature Photon. 2011 A. Fiore Politi et al., Science 2008 Thermo-optic phase shifter
Our approach: Active quantum photonic integrated circuits Generate, process and detect single photons on a chip GaAs A. Fiore Why GaAs: Low-loss waveguides and passive circuits Quantum dots as single-photon emitters Superconducting detectors on GaAs Fast modulators using E-optic effect
Where we are: Sources Easy part: Generate single photons into waveguides Source QD Intensi ity (a.u.) 600 side collection ~100 ev 300 0 1285 1290 1295 1300 1305 1310 Wavelength (nm) Related work at Stanford, TU Denmark, Toshiba, TU Munich Coupling efficiency to WG mode up to 80% Hoang et al., Appl. Phys. Lett. 2012, Opt. Express 2012
Where we are: Sources More difficult: Efficiently couple single photons to low-loss waveguides Source low-loss ridge WG Taper 70% coupling effic. between PhC and ridge WG <1 db/mm loss >3 MHz single-photon emission rate into ridge WG ( 10 times higher than in fibers) Fattahpoor et al., Appl. Phys. Lett. 2013
Where we are: Sources A lot more difficult: Scale to 2 identical photons Requirements: Quantum interference: a 1 Coherence Need cavity Indistinguishability Same wavelength for QD and cavity b 2 2 0 0 2 a b a b Produces entangled states Can be used for CNOT A. Fiore Control, control, control unit) PL Inte ensity (arb. 1200 800 400 Cavity Mode QD2 QD1 0 1280 1290 1300 1310 Wavelength (nm)
Control of QD energy Combine Stark tuning with PhC cavities: QD under applied field: E c E v Hoang et al., Opt. Express 2012 Tuning of single exciton lines over >9 nm A. Fiore
Controlling the cavity: PhC NOEMS Double-membrane PhC cavity: Cavity tuning over >10 nm Control coupling to single QDs A. Fiore Midolo et al., Appl. Phys. Lett. 2011, 2012
Combining cavity and QD control in WGs Combine PhC tuning with Stark tuning: V 1 V 2 p-cavity n-gnd p n p n-gnd p-cavity p-qd n-gnd work under way Couple tuneable PhC cavity to ridge WG:... towards fully controlled, coherent single- source in WG A. Fiore
What's next? A benchmark experiment: Photon bunching on chip Quantum interference: a 1 b 2 2 0 0 2 a b a b This would "qualify" our sources for quantum information processing on chip... Next steps: Fully integrated CNOT gate Scale to higher photon numbers: Ex. quantum interference of 4 photons (for that we need: site-controlled QDs, increase effic. by x10 and have detectors on chip) A. Fiore
Where we are: Detectors NbN nanowires GaAs Al 0.75 Ga 0.25 As Sprengers et al., Appl. Phys. Lett. 2011 20% % efficiency demonstrated A. Fiore
Where we are: Detector integration Two detectors on same waveguide: Corr. card Hanbury-Brown and Twiss autocorrelator Sahin et al., Optics Express 2013 Photon-number-resolving detectors in WGs demonstrated Detectors integrated with MMIs demonstrated Detectors integrated with sources on their way (Sahin et al., APL 2013) (Gaggero et al., unpublished) A. Fiore
Quantum Photonic Integration: Perspectives Potential show-stoppers: Efficiency Yield Dark counts Need to improve our technology & set-ups What can we achieve? >2 photons: New science (quantum information processing) >50 photons: New applications (quantum simulators) A. Fiore