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- 2. • Introduction to quantum technologies • Quantum bits and quantum gates • The current state of the art: Ion traps, superconducting circuits, linear optic quantum computing
- 3. Quantum technologies • Leave philosophy to philosophers, assume that QM works (which it does) and see if you can do anything useful with it • Quantum 1.0 revolution (20th century): transistors, lasers, integrated circuits, magnetic resonance, … • Quantum 2.0 revolution (21st century): entanglement, coherence, superposition … My brain hurts
- 4. In the Autumn Statement 2013 the UK government announced an investment of £270m over five years into a National Quantum Technologies Programme to accelerate the translation of quantum technologies into the marketplace, to boost British business and make a real difference to our everyday lives. Hubs: • Sensors and metrology • Quantum enhanced imaging • Networked quantum information technologies • Quantum communication technologies Quantum technologies: A £1 billion future industry for the UK It is the mission of UKNQT programme to make the UK a ‘go-to’ place for the development and commercialisation of quantum technologies and a leading player in the global supply chain that will develop to service them.
- 7. Classical computers One bit gates NOT (inverter), (Identity) Two bit gates • AND, OR, NAND, NOR, XOR, XNOR • NAND and NOR are universal: any Boolean function can be implemented using combinations of them INPUT OUTPUT 0 1 1 0 INPUT OUTPUT A B NAND 0 0 1 0 1 1 1 0 1 1 1 0 A B A NAND B INPUT OUTPUT A B=A NAND 0 0 1 1 1 0 NOT A BITS: 1 or 0 Boolean algebra
- 8. Quantum computer Bits → quantum bits (qubits) One bit gates → single qubit rotations Two bit gates → two-qubit gates David Deutsch (1985) Universal quantum computer requires: • Full control of single qubits • ONE two-qubit gate
- 9. Qubits • qubit = 2-state quantum system • Example: | 𝜓ñ = a |ñ + b |¯ñ electron spin in magnetic field |Ψ⟩ = 𝛼|0⟩ + 𝛽|1⟩ 𝛼 + + 𝛽 + = 1 B = 0 B ¹ 0 gµBB S = 1/2 ¯ |0ñ |1ñ x y z q j Bloch sphere Classical bits at north and south poles
- 10. Qubit technologies • ion traps • Single electron or nuclear spin: (P atom in silicon, NV− centre ) • superconducting loops • two-level atoms • photons E2–E1 2 1 (b) 2–level atom B = 0 B ¹ 0 S = 1/2 ¯ (a) spin in B field
- 11. DiVincenzo check-list (2001) 1. The system must possess well–characterized qubits and must be scalable so that it works with large numbers of qubits as well as small ones. 2. It must be possible to prepare the qubits in a simple initial state, such as |000 . . .⟩. 3. The coherence time must be much longer than the gate operation time. 4. Single and two–qubit quantum gates must be demonstrated. 5. There must exist a method to measure the state of each individual qubit.
- 12. Single qubit gates |Ψ⟩ = 𝛼|0⟩ + 𝛽|1⟩ |Ψ⟩ → |Ψ′⟩ 𝛼, 𝛽 → 𝛼′, 𝛽′ 𝛼′ 𝛽′ = 𝑀11 𝑀1+ 𝑀+1 𝑀++ 𝛼 𝛽 𝑴 = 1 |0ñ |1ñ x y z q j 𝛩′ 𝜑′ Single qubit gates implemented as Bloch sphere rotations: • NMR, ESR • Coherent control Gate must be faster than coherence time |Ψ⟩ |Ψ′⟩
- 13. 0p 2p 4p 6p 8p 0 2 4 6 8 Photocurrent(pA) Pulse area |1ñ |0ñ Single qubit rotations in quantum dot •Coherent regime : T(pulse) (10 ps) < T2 (Zrenner, Nature 418, 612 (2002) |0ñ q tuned to neutral exciton θ = Ω(t) dt −∞ +∞ ∫ ; Ω(t) = µ12 E(t) / I ∝sin2 (θ / 2) |1ñ
- 14. Two-qubit gates Controlled NOT gate (CNOT) Input Output q1 q2 q1 q2 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 q2 flipped conditionally on q1 Needs interaction between q1 and q2 CNOT q1 q1 q2 q2 ¢ control target
- 15. Interacting QD excitons: CROT gate CONTROL qubit q1 = |Qñ =½ßñ º ½10ñ TARGET qubit q2 = |Pñ =½Ý¯ñ º ½01ñ (a) UCROT |00ñ = |00ñ (b) UCROT |01ñ = |01ñ (c) UCROT |10ñ = |11ñ (d) UCROT |11ñ = - |10ñ biexciton º two qubit system Quantum = arbitrary single qubit operations computer + ONE two qubit gate |11ñ |X0ñ |00ñ s+ s+ s- s- biexciton |10ñ |01ñ DEB control target ! PL
- 16. 0p 1p 2p 3p 4p 5p 0.0 0.4 0.8 1.2 1.6 no pre-pulse cross-polarized pulses co-polarized pulses DPhotocurrent(pA) Pulse Area time delay = 28ps |11ñ |01ñ -|10ñ |00ñ |10ñ Conditional X0 - 2X0 Rabi oscillation (CROT) |10ñ |11ñ |00ñ x x y |01ñpre CROT Boyle et al PRB 78, 075301 (2008) control |Qñ º |10ñ target |Pñ º |01ñ control target
- 17. Oxford Q20:20 The Hub’s flagship goal is to realise the Q20:20 engine: a hybrid light-matter quantum computer involving twenty nodes, optically interlinked, where each node is a small quantum processor of twenty qubits. Each processing node will be an ion trap, a device within which a small number of charged atoms – ions – are held suspended in a vacuum and manipulated by laser and microwave systems. Interlinking between the traps will be realised by single photon emissions, which are combined and measured by fibres, splitters, switches and detectors. Prof. David Lucas Prof. Andrew Steane
- 18. in Fig. 1b. This interface is described by the interaction hamiltonian Hint(t), where for typical states 〈Hint(t)〉≈ᐜχ(t), with ᐜ being h/2π (where h is Planck’s constant) and χ(t) being the time-dependent coupling strength between the internal material system and the electromagnetic field. Desirable properties for a quantum interface include that χ(t) should be ‘user controlled’ for the clocking of states to and from the in Fig. 1c, d. In the first example (Fig. 1c), single atoms are trapped optical cavities at nodes A and B, which are linked by an optical fib External fields control the transfer of the quantum state ᎂΨ〉 stored in t atom at node A to the atom at node B by way of photons that propaga from node A to node B6,18 . In the second example (Fig. 1d), a sing photon pulse that is generated at node A is coherently split into tw a b c d Quantum node Quantum channel out (t) Node A Node B k in (t) (t) k Node BNode A c ≈ 〈Hint 〉 ᐜ k c ≈ 〈Hint 〉 ᐜ g Y AW Y BW BW g g The quantum internet Kimble, Nature 453, 1023 (2008) laser beams detectors trapped ions vibrational displacementquantum data–bus
- 21. D-Wave 2X • A lattice of 1000 tiny superconducting circuits, known as qubits, is chilled close to absolute zero to get quantum effects • Operates in an extreme environment: 0.015 K, shielded to 10−9 T, ultrahigh vacuum 10−10 atmospheres • Enables quantum algorithms to solve very hard problems Ø searches for the “lowest point in a vast landscape” Ø The processor considers all possibilities simultaneously to determine the lowest energy required to form those relationships Ø Multiple solutions are returned to the user, scaled to show optimal answers
- 22. IBM 50-qubit quantum computer IBM Raises the Bar with a 50-Qubit Quantum Computer IBM established a landmark in computing Friday, announcing a quantum computer that handles 50 quantum bits, or qubits. The company is also making a 20-qubit system available through its cloud computing platform. The announcement does not mean quantum computing is ready for common use. The system IBM has developed is still extremely finicky and challenging to use, as are those being built by others. In both the 50- and the 20-qubit systems, the quantum state is preserved for 90 microseconds—a record for the industry, but still an extremely short period of time. Nonetheless, 50 qubits is a significant landmark in progress toward practical quantum computers. MIT Technology Review 10 November 2017
- 27. The state of the art in LOQC Demultiplexers Detectors Single-photon device Ultra-low-lossphotonic circuit PC PC PC PC TDC l set-up for multiphoton boson-sampling. The set-up includes four key parts: the single-photon device, demultiplex tectors. The single-photon device is a single InAs/GaAs quantum dot coupled to a 2-µm-diameter micropillar cavit S NATURE PHOTONICS DOI: 10.1038/ Wang et al., "High-efficiency multiphoton boson sampling," Nat Photon,, 6, pp. 361-365, (2017) QD source
- 29. Conclusions • Heavy investment in quantum technologies by national funding agencies and EU • Several big companies also investing • Superconducting systems advancing very fast