Sunday – Monday, January 27 – 28, 2002
MIT, Building NW14, Room 1112
>>> Sunday’s Schedule <<<
08:30 Breakfast and poster setup
09:45 Seth Lloyd Welcome and introduction to the CMI
QIP project.
10:00 David Cory NMR approaches to quantum inform-
ation processing
10:40 Eddie Farhi The latest news on adiabatic quantum
computation
11:20 Seth Lloyd The origin of square-root of N speed-
ups in quantum information processing
12:00 Pizza & poster session (everyone)
14:30 Terry Orlando Flux-based superconducting qubits
for quantum computation
15:10 Jeff Shapiro Quantum information technology:
Entanglement, teleportation and
quantum memory
15:50 Tim Havel Geometric algebra: A unifying theme
for our CMI QIP project?
16:30 Adjourn but leave posters up
18:00 Irish eyes are smiling (@ Asgard Pub, 350 Mass Ave)
>>> Monday’s Schedule <<<
08:30 Breakfast
10:00 Yu Shi Remarks on universal quantum compu-
tation and multiparticle entanglement
10:40 Yuri Suhov Quantum Gibbs information sources
11:20 Jon Barrett Entanglement and nonlocality
12:00 Pizza & poster session (everyone)
14:30 Gehan Amaratunga Electron wave based quantum gates –
realization using carbon nanotubes
15:10 Sean Barrett An overview of solid-state QIP in the
Cambridge semiconductor physics group
15:50 Daniel Jonathan Collective data bus for “hot” trapped
ions
16:30 Adjourn, take down posters, and / or attend:
Tim Havel Geometric algebra: Imaginary numbers
are real (MIT IAP minicourse lect. #2)
19:00 Workshop banquet (La Groceria, 853 Main, Cambridge)
>>> Concept Abstracts <<<
Electron Wave Based Quantum Logic Gates – Realization Using Carbon Nanotubes
Gehan Amaratunga (Engineering, Cambridge Univ.; ga@eng.cam.ac.uk).
Joint work with Radu Ioniciou, Manish Chhowalla, Aun Shih Teh, Ken Teo (Engineering, Cambridge Univ.); with Sun Beck Lee, David Hasko, Haroon Ahmed
Cavendish Lab., Cambridge Univ.); and with David Williams (Hitachi Cambridge).
Proposals for the implementation of quantum logic gates using electrons has thus far been mainly based on the utilization of spin properties. The other property of electrons which can be used is their wave property. If electrons propagate in well defined energy states, then they can in principle preserve coherent wave like properties. In such a situation it is possible to use single electrons in a manner similar to single photons to establish quantum interactions, and hence quantum logic gates. The main challenge in an electron system, compared to photons, is to isolate them from energy interactions with the environment in order to preserve their coherence. On the other hand it is also this very property which makes it relatively straight forward to establish the electron-electron interactions required for a quantum logic gate.
The elements and structures required for an electron wave based quantum logic gate are presented. It is shown that the single electron devices required for state preparation ( prior to the quantum interactions) and detection after ( after the interactions) have already been demonstrated. The main research task is to couple these device to coherent single electron wave guide structures. The suitability of carbon nanotubes for implementation of the wave guides is reviewed. Finally, work presently underway in Cambridge on carbon nanotube structures and technology is reviewed (Intnl. J. Mod. Phys. B 15:125-33, 2001).
Entanglement and Nonlocality
Jonathan F. Barrett (DAMTP, Univ. of Cambridge; j.s.barrett@damtp.cam.ac.uk).
Joint work with A. Kent (DAMPT).
As is well known, one of the curious features of entangled states is the fact that if measurements are performed on each subsystem at spacelike separation, the results cannot always be simulated by a local hidden variable model. This “nonlocality" finds direct application, for example, in communication complexity scenarios, in which preshared nonlocal states can reduce the number of classical bits which must be sent, and in cryptography, where deep connections have been found between the nonlocality of quantum states and the possibility of using them to extract a secret key. It is obvious that entanglement is necessary for nonlocality. Apart from this, however, the relationship between entanglement and nonlocality turns out to be rather intricate. Here, I will examine what is already known about this relationship and present a new result - a local hidden variable model for positive operator valued measurements on a class of entangled states.
An Overview of Solid-State Quantum Information Processing in the Cambridge Semiconductor Physics Group
Sean Barrett (Cavendish Lab., Univ. of Cambridge; sdb21@cam.ac.uk).
Joint work with C. H. W. Barnes, A. Moroz, D. Paul, M. Pepper, S. Rahman, T. Stace, V. Talyanskii, and T. Zainiev (Cavendish).
We present a review of research on quantum information processing currently being undertaken in the SP group. We are presently pursuing two technologies for quantum computation in the solid state: electrons bound to sodium ions embedded in silicon MOSFETs, and electrons trapped in surface acoustic wave minima in GaAs (PRB 62:8410-9, 2000). We dicuss experimental progress towards these goals, and review recent theoretical results.
NMR Approaches to Quantum Information Processing
David G. Cory (NED, MIT; dcory@mit.edu)
Joint work with Nicolas Boulant, Joseph Emerson, Evan Fortunato, Tim Havel, Seth Lloyd , Marco Pravia, Chandrasekhar Ramanathan, Suddhasattwa Sinha, Grum Teklemariam and Yaakov Weinstein (MIT).
Nuclear Magnetic Resonance (NMR) spectroscopy is an approach to QIP that works today. It is distinctive in that it operates on macroscopic ensembles of molecules, wherein the spin 1/2 atomic nuclear serve as the qubits. The basic concepts of QIP by NMR will be described, along with its present achievements and future potential (see Fort. Phys. 48:875-907, 2000).
The Latest News on Quantum Adiabatic Computation
Eddie Farhi (Physics & LNS, MIT; farhi@MIT.EDU)
Joint work with Andrew Childs, Jeff Goldstone, Joshua Lapan, Daniel Preda and Michael Sipser (MIT); and with Sam Gutmann and Andrew Lundgren (Northeastern).
The adiabatic theorem of quantum mechanics assures us of being able to continuously deform one Hamiltonian into another while remaining in the ground state – providing only that the change is made slowly enough. By designing a Hamiltonian that vanishes only if the state of a quantum computer corresponds to the solution of a combina-torial problem, e.g. satisfiability, and deforming a Hamiltonian with a known ground state into it, we have obtained a novel approach to solving many hard computational problems. Results of extensive classical simulations on up to 18 qubits suggest that the average-case performance of this algorithm may be polynomial even on NP-hard problems (Science, 292:472-5, April 20, 2001). This talk will focus on the relation- ship between quantum adiabatic evolution and simulated annealing algorithms.
Experimental Implementation of Noiseless Subsystems
Evan Fortunato (NED, MIT; evanmf@MIT.EDU)
Joint work with D. G. Cory, M. A. Pravia (MIT); and with E. Knill, R. Laflamme and L. Viola (LANL).
We demonstrate the protection of one bit of quantum information against all collective noise in three nuclear spins. Because no subspace of states offers this protection, the quantum bit was encoded in a proper noiseless subsystem. We therefore realize a general and efficient method for protecting quantum information. Robustness was verified for a full set of noise operators that do not distinguish the spins. Verification relied on the most complete exploration of engineered decoherence to date. The achieved fidelities show improved information storage for a large, noncommutative set of errors (Science, 293:2059-63, 2001).
Engineering Maximal Entanglement
Vittorio Giovannetti and Lorenzo Maccone (Research Lab of Electronics, MIT, maccone@MIT.EDU).
Joint work with J. H. Shapiro, and F. N. C. Wong (RLE, MIT).
It is shown that parametric downconversion, with a short-duration pump pulse and a long nonlinear crystal that is appropriately phase matched, can produce a frequency-entangled biphoton state whose individual photons are coincident in frequency. Quantum interference experiments which distinguish this state from the familiar time-coincident biphoton state are described.
Geometric Algebra: A Unifying Theme for our CMI QIP Project?
Timothy F. Havel (NED, MIT; tfhavel@MIT.EDU)
Joint work with D. G. Cory (MIT), C. Doran, S. Furuta, R. Parker (Cavendish).
Geometric (aka Clifford) algebra can be regarded as an extension of vector algebra to spaces of all dimensions and signatures, most notably Minkowski space-time. Although GA is now being used in many fields of science and engineering, its intrinsic geometric interpretation makes it especially promising in quantum physics (quant-ph/0004031). The space-time GA, in particular, allows the Bloch vector interpretation of a qubit’s state to be extended to multi-qubit states, including entangled ones. For these reasons, and also because Cambridge University is already something of a Mecca for GA, I believe it could be quite useful in helping the diverse groups of scientists involved in the CMI QIP communicate and find common interests. This talk will only provide a taste of the subject; I will be giving a more extensive introduction in a series of three lectures held in the week following this workshop, as part of MIT’s Independent Activities Period.
Collective Data Bus for “Hot” Trapped Ions
Daniel Jonathan (DAMTP, Cambridge Univ.; D.Jonathan@damtp.cam.ac.uk).
Joint work with M. Plenio (Imperial College).
The quest for practical ion-trap quantum information processing has motivated in-creasingly sophisticated schemes for realising two-qubit gates in this system. Here I present a new such scheme, where quantum information is conveyed from one ion to another using all vibrational modes simultaneously as a 'collective' data bus. The modes are only ever 'virtually' excited, so the method functions even when the ions are `hot' (have nonzero motional temperature and suffer external heating effects). It may also be experimentally simpler to realise than existing `hot gate' proposals.
The Origin of Square Root of N Speed-Ups in Quantum Information
Seth Lloyd (Mech. Eng., MIT; slloyd@MIT.EDU)
Joint work with everyone who is anyone
A variety of quantum effects – quantum search, quantum clocks, interferometry, quantum positioning, and quantum channel capacity – exhibit square root speedups or enhancements over their classical counterparts. This talk explores the origins of such effects and presents a unified picture of how they are obtained (see e.g. Nature 412:417-9, 2001).
Flux-based Superconducting Qubits for Quantum Computation
Terry P. Orlando (EECS, MIT; orlando@MIT.EDU)
Joint work with K. Segall, Lin Tian, D. Crankshaw, D. Nakada, Janice Lee, Bhuwan Singh, D. Berns, W. Kaminsky, S. Lloyd, & L.S. Levitov (MIT); M.J. Feldman and M.F. Bocko (Rochester); M. Tinkham, N. Markovic and S.0. Valenzuela (Harvard); K. Berggren, (MIT Lincoln Lab); J.E. Mooij, C.J.P. Harmans & C. H. van der Wal (Delft)
Superconducting quantum circuits have been used to show the superposition of macroscopic quantum states, the so-called ``Schroedinger Cat'' states. One type of quantum circuit is a superconducting loop of Al interrupted by three Josephson junctions and the other is an RF SQUID fabricated in Nb. Microwave spectroscopy experiments indicate symmetric and anti-symmetric quantum superpositions of macroscopic states. The two classical states have persistent currents of about a microamp and correspond to the center-of-mass motion of millions of Cooper pairs.
The goal of the present research is to use superconducting quantum circuits to model the measurement process, understand the sources of decoherence, and to develop scalable algorithms. A particularly promising feature of using super-conducting technology is the potential of developing high-speed, on-chip control circuitry with SFQ electronics. The picosecond time scales of SFQ electronics means that the superconducting qubits can be controlled rapidly on the time scale that the qubits remain phase-coherent. Recent progress and the major challenges will be discussed (Science, 290:773-7, 2000).
Applying Geometric Algebra to Multiparticle Entanglement
Rachel Parker (Cavendish Lab, Cambridge Univ.; rfp23@cam.ac.uk).
Joint work with C. Doran (Canvendish); and with T. F. Havel (MIT).
When two or more subsystems of a quantum system interact with each other they can become “entangled”. For systems with only two subsystems this entanglement can be completely described using the well-known Schmidt decomposition. This selects a preferred othernormal basis for expressing the wavefunction, and gives a measure of the degree of entanglement present in the system. The extension of this to the more general case of more than two subsystems is not yet known. I shall preservent a review of this process using the standard representation, and also apply this method in the setting of geometric algebra, which has the advantage of suggesting a generalization to more than two subsystems (quant-ph/0106055 & 0106063).
Coherent Control of NMR Qubits Using Strongly Modulated RF Pulses
Marco Pravia (NED, MIT; praviam@MIT.EDU)
Joint work with N. Boulant, D. G. Cory, E. M. Fortunato, T. F. Havel, G. Teklemariam (MIT).
We describe a method for improving coherent control through the use of detailed knowledge of the system's Hamiltonian. Precise unitary transformations were obtained by strongly modulating the system's dynamics to average out unwanted evolution. With the aid of numerical search methods, pulsed irradiation schemes are obtained that perform accurate, arbitrary, selective gates on multi-qubit systems. Compared to low power selective pulses, which cannot average out all unwanted evolution, these pulses are substantially shorter in time, thereby reducing the effects of relaxation. Liquid-state NMR techniques on homonuclear spin systems are used to demonstrate the accuracy of these gates both in simulation and experiment. Simulations of the coherent evolution show that the control sequences faithfully implement the unitary operations, typically yielding gate fidelities on the order of 0.999 and, for some sequences, up to 0.9997. The experimentally determined density matrices resulting from the application of different control sequences on a 3-spin system have overlaps of up to 0.99 with the expected states, confirming the quality of the experimental implementation.
Quantum Information Technology: Entanglement, Teleportation and Quantum Memory
Jeffrey H. Shapiro (EECS & RLE, MIT; jhs@MIT.EDU)
Joint work with S. M. Haas, S. Lloyd, M.S. Shahriar and N.C. Wong (MIT); and with P. R. Hemmer (Hanscom AFB).
The preeminent obstacle to the development of quantum information technology is the difficulty of transmitting information over noisy and lossy quantum communication channels, recovering and refreshing the quantum information that is received, and then storing it in a reliable quantum memory. A team of researchers from the Massachusetts Institute of Technology and Northwestern University have undertaken a Multidisciplinary University Research Program to overcome this obstacle. In this presentation, we shall review our progress in the areas of: singlet-state quantum communication architecture; entanglement sources based on parametric downconversion; trapped-atom quantum memories; and applications of entanglement (see PRL 87:167903, 2001).
Measurement on A Superconducting Flux Quantum Bit
Lin Tian (Physics, MIT; tianl@MIT.EDU).
Joint work with D.S. Crankshaw, D. Nakada, K. Segall, T.P. Orlando, S. Lloyd and L. Levitov (MIT)
During measurement, information is transferred from the measured quantum system to the detector via their coupling. The same coupling that extracts information from the quantum system transmits noise from the detector's environment to the system as well. We developed a method to calculate the effective noise transferred to the superconducting persistent-current qubit from the environment of the detector – an underdamped dc SQUID – that inductively interacts with the qubit. The method, with a simple linear circuitry approach, can be applied to other interacting quantum systems such as a qubit and a control radiation source. The effect of this noise on qubit relaxation and decoherence is also studied.
Remarks on Universal Quantum Computer and on Multiparticle Entanglement
Yu Shi (Cavendish Labs, Cambridge Univ.; ys219@cam.ac.uk).
We remark that it is still open whether there exists a universal quantum Turing machine, proposed by Deutsch in 1985. We also discuss the halting problem first noticed by Myers, as well as the programmable quantum gate array discussed by Nielson and Chuang. In the rest of the talk, we discuss all possible entanglements possessed by a multiparticle state. It is noted that for a pure state of more than two parties, all kinds of GHZ-like states do not furnish a reversible entanglement generating set under local operations and classical communication (quant-ph/ 0201079).
Effect of Stochastic Noise on Laser Pulses for Quantum State Transfer
Tom Stace (Cavendish Lab, Univ. of Cambridge; tms29@cam.ac.uk).
We consider the effect of classical stochastic noise on control laser pulses in a scheme for transferring quantum information between atoms, or quantum dots, in separate optical cavities via an optical connection between cavities. We develop a master equation for the dynamics of the system subject to stochastic errors in the laser pulses and use this to evaluate the sensitivity of the transfer process to stochastic pulse shape errors for a number of different pulse shapes. We show that under certain conditions, the sensitivity of the transfer to the noise depends on the pulse shape, and so we develop a method for determining a pulse shape that is minimally sensitive to specific errors.
Quantum Gibbs Information Sources
Yuri M. Suhov (University of Cambridge; y.m.suhov@statslab.cam.ac.uk).
We discuss generalisations and refinements of Schumacher's coding theorem to the case where a quantum source is represented by a density matrix of a Gibbs ensemble. The talk will be accessible to a wide audience: no preliminary knowledge of information theory or statistical mechanics (classical or quantum) is assumed.
NMR Analogs of Quantum Erasers
Grum Teklemariam (Physics, MIT; gtek@MIT.EDU).
Joint work with: D. G. Cory, E. M. Fortunato, T. F. Havel and M. A. Pravia (MIT).
We report the implementation of two- and three-spin quantum erasers using liquid-state NMR spectroscopy. Traditionally, quantum erasers have been of interest as a means of demonstrating causality and locality paradoxes in isolated quantum systems. Although our NMR analogs cannot do this, they do show how the essential kinematics of entanglement can be reproduced even in a very weakly polarized ensemble. The trick is to use magnetic field gradients to render unobservable the same phase information that would be lost in a strong measurement on every system in the ensemble. In addition to a simple two-spin eraser illustrating the loss of “which path” information, we also describe implementations of a three-spin GHZ state “disentanglement eraser” due to Garisto and Hardy, as well as a more recent three-spin eraser based on Werner’s so-called “W-state” (PRL 86, 5845-9, 2001).