Barry C. SandersIQIS, University of Calgary, www.iqis.org

CQCT, Macquarie University, Sydney, Australia, www.qcaustralia.org

Implementations of Quantum Information

I

Montreal August 2005

ImplementationsOne task in physics is to implement quantum

information processing, i.e. realize quantum communication and quantum information processing.

There are many challenges because of imperfections in systems and decoherence, but there are promising techniques such as quantum error correction to overcome these problems.

Many candidates for physical quantum information processing, and we study some of these here.

I. Introduction General qubit state:

General multiqubit state:

Density matrix:

One Universal Set of GatesIdentity and Not gates:

Hadamard gate:

Phase gates:

Controlled phase gate (equivalent to ^X=CNOT under local unitaries:

Goals Encode qubits in physical system. Process these qubits. Universal set of gates for quantum

computation. Qubit-specific readout. Store qubits. Minimize decoherence. Correct errors.

Problems Qubit may in Hilbert space larger than

two dimensions: truncate! Coupling to environment ==>

decoherence. Imperfect gates so they don’t effect

precisely the desired transformation Preparation and readout

DiVincenzo Criteria1. Scalability.2. Ability to initialize.3. Long decoherence times.4. Universal set of quantum gates.5. Qubit measurement capability.

Additional Criteria6. Interconvertibility between physical qubits.7. Faithfully transmit flying qubits.

Investigate proposals Trapped ions Nuclear spins Spin-based quantum dot qubit Photons

Our General Approach Identify the physical qubits in a given

system. Determine the Hamiltonian(s) governing

dynamics of the qubits:

The Hamiltonian generates the unitary evolution operator, which performs processing:

The Hamiltonian The Hamiltonian operator is a function of

operators concerning degrees of freedom of the system.

If quantum information is encoded in positions x1 and x2 of two particles, then

with … representing other relevant operators.

Real systems are highly complicated, and creating an effective model is high art!

Harmonic Oscillator A simple harmonic oscillator in one

dimension is described by

The particle has mass m and angular frequency , which is independent of amplitude.

Number operator has spectrum 0,1,2,…. Eigenstates corresponding to number of

quanta are {|n}, e.g. photons (for light) or phonons (for vibrations).

Phonons Number of quanta are increased or decreased by creation or annihilation

operators:

The position operator can be represented by

The conjugate momentum operator can be represented by

The Environment The Hamiltonian generates unitary

evolution, which corresponds to dynamics in a closed system, but the system must be open for preparation and readout.

The openness is the coupling of the system to the environment; e.g. a puck sliding on ice is slowed by frictional coupling to ice and air resistance so ice and air are part of the puck’s environment.

Growing the Hamiltonian The Hamiltonian for the entire model

must include system and environment. If the environment has dynamical

degrees of freedom {i}, these are included in the Hamiltonian; extend previous Hamiltonian:

The system+environment state evolves according to unitary evolution generated by this bigger Hamiltonian.

The Reduced State The state of the system+environment is not

useful to us; we just want to know the state of the system.

We discard all information about the environment by tracing the density matrix for system+environment over environment degrees of freedom:

The state of the system is, in general mixed. Decoherence-free subspaces and quantum

error correction are designed to protect purity.

Summary Goals are to encode quantum information in a

physical system and realize quantum gates and single qubit measurements, perhaps with subsequent dynamics dependent on these measurement results.

Dynamics determined by Hamiltonian, which generates the evolution operator describing the gates and circuits.

Systems are necessarily coupled to the environment, and decoherence-free subspaces and quantum error correction are designed to protect against environment-induced degradation.

II. Trapped ions

Trapped Ions The trapped ion system is an early and

promising medium for realizing quantum information processing.

Ions are charged atoms, and electric fields are used to confine or move these ions in a lattice.

Quantum information is encoded in the electron energy level.

Coupling is obtained via collective motion, which is quantized (with the quanta called photons).

Ion Charged atom - number of electrons is

greater than or less than number of protons. Concerned with outermost electron orbiting

“shielded” nucleus. Angular momentum J is a vector sum of spin s and orbital angular momentum L: J=s+L.

Spectroscopic notation: For L, use S for L=0, P for L=1, D for L=2, …

Atom-Photon interactions

Photon absorption fromground to excited state.

Stimulated emission fromground to excited state.

Spontaneous emission fromground to excited state.

Emitted photon is random in direction and phase.For S-P transitions, rate is once per nanosecondand, for S-D, rate is once per second: S-D is better.

Emitted photon is a copy of the “trigger” photon.

Cirac-Zoller (1995) proposal: N ions in a linear trap, each interactingwith a separate laser beam. Ions are confined by harmonic potentialsin each of x, y, z directions with x frequency much less than for y,z.

Excitation of alkali ion dipole-forbiddentransition

Driving the Ion Each laser beam acts on one ion located at the

node of the laser field standing wave. There are two excited states, with transition to

q=0 or q=1 determined by laser polarization. Ions share a collective centre-of-mass motion

with energy restricted to zero or one phonon.

0e

g

1e

Single-Qubit Rotation Laser field is tuned to the |g—|e0

transition with the laser polarization set to q=0.

The Rabi frequency is the strength of the atom-field interaction; is the laser phase.

The Hamiltonian for the nth ion is

For evolution time t=k,

Interaction Hamiltonian Laser acts on nth ion and is detuned by

centre-of-mass motion angular frequency x. Change of electronic energy is accompanied

by creation or annihilation of one phonon. For the angle between laser beam and x-

axis, and k = for the laser field wavelength.

Unitary Evolution Apply the laser beam for time Evolution is given by

This transformation doesn’t change Effects transformation

Two-Qubit Gate: ^ZStart Um

1,0(0) Un2,1(0) Um

1,0(0)

|gm|gn|0 |gm|gn|0 |gm|gn|0 |gm|gn|0

|gm|e0n|0 |gm|e0n|0 |gm|e0n|0 |gm|e0n|0

|e0m|gn|0 -i|gm|gn|1 i|gm|gn|1 |e0m|gn|0

|e0m|e0n|0 -i|gm|e0n|1 -i|gm|e0n|1 -|e0m|e0n|0

Universal Quantum Computation The Cirac-Zoller scheme enables

universal quantum computation for ions by combining single-qubit rotations with the two-qubit ^Z.

Requires atoms to be cold. Sorensen and Molmer introduced a

bichromatic off-resonant driving schemes that allows ions to be ‘warm’: two-photon processes interfere to minimize sensitivity of ions to vibrational state.

XX

Independent of Phonon Number Two-photon transition via intermediate

states

Two-photon transition rate for |ggn —|een:

Same transition rate for |egn —|gen. Two-photon transitions interfere to remove

n from the effective two-photon Rabi frequency.

Architecture for a Large-Scale Computer

Kielpinski-Monroe-Wineland, Nature 2002. Static ion traps may be limited to a few

dozen ions so KMW suggested multiple quantum registers with quantum communication between these registers.

Trapped ions are stored in quantum memory registers, with ions shuttled between registers.

Ions are transported quickly and then recooled by sympathetic cooling (via cooling process on a different species of ion in the neighbourhood.)

Memory regionInteraction

Region

Electrode Segments

Decoherence-Free Subspace Ion transport presents another problem:

qubit evolves according to |g+|e|g+ei|e.

The parameter a is random and is due to varying magnetic field strength along the ion’s path, resulting in random fluctuations in the energy separation of|g and |e.

Reduce this decoherence by encoding the logical qubit as |0=|ge and |1=|eg.

The logical qubits are invariant (up to unimportant phase) under stochastic magnetic fields.

SummaryThe Cirac-Zoller proposal for quantum computation inan ion trap is one of the first and influential, especiallyin the conception of creating two-qubit unitary gates.

The Sorensen-Molmer proposal relaxes constraints onthe temperature of the ions by using an off-resonantbichromatic driving field.

The Kielpinski-Monroe-Wineland proposal shows howto surpass the scaling arguments for static ions byusing multiple registers, quantum communication, anddecoherence-free subspaces.

III. Various Proposals

Various proposals In ion-trap quantum computation, quantum

information is stored in electronic levels, and laser fields drive transitions with vibrations acting as the bus.

Another early and promising proposal concerns encoding quantum information onto nuclei and using magnetic fields for readout.

Here we discuss this nuclear magnetic resonance technique plus coupled quantum dots and also photons.

Nuclear Spin Quantum Computation Nuclear spins - DiVincenzo Science 1995,

Cory et al and Gershenfeld & Chuang both 1995.

Qubit is realized as a nucleus with gits magnetic dipole moment, in a static magnetic field B0 along z-axis and an alternating time-dependent magnetic field B1 with angular frequency along y axis; the Hamiltonian is

Couple to specific qubit by tuning .

Two-Qubit Coupling

This Hamiltonian generates a unitary evolution that is a combination of single-qubit rotation (by tuning ) and two-qubit operations.

Quantum computing has been performed in liquid state; unfortunately initialization is difficult as the density matrix is always close to identity.

Scalability may be resolved using solid state.

Loss and DiVincenzo QDot QComputer

The T-gate is a magnetic gate that rotates the electron spin: single-qubit rotations.

The J-gate is the coupling term effected by inter-dot electron quantum tunneling.

A quantum dot confines an electron in all 3 dimensions: artificial atom.

Qubit corresponds to spin state of quantum dot’s excess electron.

Higher energies ignored.

Effects gate operation:

31P+

31P+

Barrier

Si Substrate

BAC

BJ-Gates

e-

e-

A-Gates

(2 T)

(10-3) T

Kane’s Proposal 1998Kane’s Proposal 1998

The J-gate controls e-e collisions,The J-gate controls e-e collisions,hence the coupling of hence the coupling of 3131P’s.P’s.

Photons The electromagnetic field consists of modes

identified, for example, by frequency, spatial characteristics, and polarization.

Each mode is a harmonic oscillator, and the number state |n corresponds to n photons.

A qubit can be a superposition of no photon and one photon in a mode, |0+|1 (single-rail qubit), or exaclty one photon in a superposition of two modes, |01+|10 (dual-rail qubit).

Dual-Rail Photonic Qubit

The Kerr interaction provides the two-qubit operation sufficient for universal circuits.

Unfortunately is miniscule or, if enhanced, accompanied by unacceptably high photon losses.

A beam splitter produces the unitary operation

The beam splitter rotates the dual-rail qubit. Ideal “Kerr nonlinear medium”

Chuang & Yamamoto 1995Chuang & Yamamoto 1995

Knill-Laflamme-Milburn 2001 A solution to weak

nonlinearity provided by GC teleportation gate, which teleports input to “processed” output by feeding entangled ancillas produced offline. Gottesman & Chuang 1999Gottesman & Chuang 1999

KLM developed a circuit to produce Kerr nonlinearity rarely, but successes are signaled by photodetection events.

Successful outputs are provided as ancillas for a GC type of teleporter; by only supplying proper states, the quantum computation is not hindered by the local success rate of the nonlinear gate.

One-way computation More recently Raussendorf and Briegel introduced one-

way quantum computation with cluster states, based on processing highly entangled states via successive single qubit measurements.

This approach seems promising for realizing quantum computation with photons.

Raussendorf and Briegel 2001Raussendorf and Briegel 2001

SummarySurveyed a few proposals for quantum

computation using ions, nuclei, quantum dots, and photons.

Ignored schemes such as optical cavities, superconducting qubits, and fullerene.

The theory can be understood once the model Hamiltonian is determined, and a lot of work is underway to realize such Hamiltonian dynamics and understand the environment.