Transcript Creating Entanglement
Implementations of Quantum Information I
Barry C. Sanders IQIS, University of Calgary, www.iqis.org
CQCT, Macquarie University, Sydney, Australia, www.qcaustralia.org
Montreal August 2005
Implementations
One 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 Gates
Identity 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 Criteria
1.
2.
3.
Scalability.
Ability to initialize.
Long decoherence times.
4.
Universal set of quantum gates.
5.
Qubit measurement capability.
Additional Criteria
6.
7.
Interconvertibility between physical qubits.
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 x
1
and x
2
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 w , 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 { c
i
}, these are included in the Hamiltonian; extend previous Hamiltonian: The system+environment state according to unitary evolution generated by this bigger Hamiltonian.
evolves
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 from ground to excited state.
Stimulated emission from ground to excited state.
Emitted photon is a copy of the “trigger” photon.
Spontaneous emission from ground to excited state.
Emitted photon is random in direction and phase.
For S-P transitions, rate is once per nanosecond and, for S-D, rate is once per second: S-D is better.
Cirac-Zoller (1995) proposal:
N
ions in a linear trap, each interacting with a separate laser beam. Ions are confined by harmonic potentials in each of
x, y, z
directions with
x
frequency much less than for
y,z
.
Excitation of alkali ion dipole-forbidden transition
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.
e
1
e
0
g
Single-Qubit Rotation
Laser field is tuned to the |g —|e 0 with the laser polarization set to q=0.
transition The Rabi frequency W atom-field interaction; is the strength of the f is the laser phase.
The Hamiltonian for the n th ion is For evolution time t=k p/W
,
Interaction Hamiltonian
Laser acts on n th ion and is detuned by centre of-mass motion angular frequency n
x
.
Change of electronic energy is accompanied by creation or annihilation of one phonon.
For q the angle between laser beam and x-axis, and k = 2p/l for l 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: ^Z
Start |g
m
|g
n
|0 |g
m
|e
0
n
|0
U m 1,0 (0)
|g
m
|g
n
|0 |g
m
|e
0
n
|0
U n 2,1 (0)
|g
m
|g
n
|0 |g
m
|e
0
n
|0
U m 1,0 (0)
|g
m
|g
n
|0 |g
m
|e
0
n
|0 |e
0
m
|g
n
|0 |e
0
m
|e
0
n
|0 -i|g
m
|g
n
|1 -i|g
m
|e
0
n
|1 i|g
m
|g
n
|1 |e
0
m
|g
n
|0 -i|g
m
|e
0
n
|1 -|e
0
m
|e
0
n
|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.
X X
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.)
Electrode Segments Interaction Region Memory region
Decoherence-Free Subspace
Ion transport presents another problem: qubit evolves according to |g +|e |g +e i a |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.
Summary
The Cirac-Zoller proposal for quantum computation in an ion trap is one of the first and influential, especially in the conception of creating two-qubit unitary gates.
The Sorensen-Molmer proposal relaxes constraints on the temperature of the ions by using an off-resonant bichromatic driving field.
The Kielpinski-Monroe-Wineland proposal shows how to surpass the scaling arguments for static ions by using multiple registers, quantum communication, and decoherence-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 g magnetic dipole moment, in a static magnetic field B
0
along z-axis and an alternating time dependent magnetic field B frequency w
1
m its with angular along y axis; the Hamiltonian is Couple to specific qubit by tuning w .
Two-Qubit Coupling
This Hamiltonian generates a unitary evolution that is a combination of single-qubit rotation (by tuning operations.
w ) and two-qubit 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
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.
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.
Effects gate operation:
A-Gates J-Gates B AC (10 -3 ) T B (2 T) 31 P + Barrier e e -
Kane’s Proposal 1998
31 P + Si Substrate
The J-gate controls e-e collisions, hence the coupling of 31 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, (dual-rail qubit).
a superposition of two modes, |0 rail qubit), or exaclty one photon in a a
+
|01 b |1
+
b (single |10
Dual-Rail Photonic Qubit
A beam splitter produces the unitary operation The beam splitter rotates the dual-rail qubit.
Ideal “Kerr nonlinear medium” The Kerr interaction provides the two-qubit operation sufficient for universal circuits.
Unfortunately losses.
c is miniscule or, if enhanced, accompanied by unacceptably high photon
Chuang & 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 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 2001
Summary
Surveyed 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.