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Billions of molecules are used, each one a separate quantum computer. Most advanced experimental demonstrations to date, but poor scalability as molecule design gets difficult and SNR falls. Qubits are stored in nuclear spin of flourine atoms and controlled by different frequencies of magnetic pulses.
Used to factor 15 experimentally.
Vandersypen, 2000
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Qubits are stored in the spin of the nucleus of phosphorus atoms embedded in a zero-spin silicon substrate. Standard VLSI gates on top control electric field, allowing electrons to read nuclear state and transfer that state to another P atom.
Kane, Nature, 393(133), 1998
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High electron mobility transistor (HEMT)
http://www.fqd.fujitsu.com/
Quantum dot structure
N < 30
# of electrons
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N) orbital degree of freedom
&
spin degree of freedom
double quantum dot
bonding & anti-bonding orbitals (charge qubit)
(spin qubit)
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S. Tarucha et al., Phys. Rev. Lett. 77, 3613 (1996).
1s 2p 3d
|ψ(r)| 2 500 nm
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dissipation (T 1 ), decoherence (T 2 ) & manipulation
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A double quantum dot fabricated in AlGaAs/GaAs
2DEG
(schematically shown by circles)
Approximate number of electrons: 10 ~ 30 Charging energy of each dot: ~ 1 meV Typical energy spacing in each dot: ~100 ueV Electrostatic coupling energy:~200 ueV Lattice temp. ~ 20 mK (2 ueV)
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Two-qubit device
Pashkin et al., Nature, 421(823), 2003
One-qubit device can control the number of Cooper pairs of electrons in the box, create superposition of states.
Superconducting device, operates at low temperatures (30 mK).
Nakamura et al., Nature, 398(786), 1999
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J. Martinis, NIST
Qubit representation is phase of current oscillation.
Device is physically large enough to see!
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The qubit representation is
a quantum of current (flux)
moving either clockwise or
counter-clockwise around
the loop.
NIST, Oxford, Australia, MIT, others Ions (charged atoms) are
suspended in space in an oscillating electric field.
Each atom is controlled by a laser.
NIST, Oxford, Australia, MIT, others Number of atoms that can
be controlled is limited, and each requires its own laser. Probably <100 ions max.
T
Deutsch, UNM
Neutral atoms are held in place by standing waves from several lasers.
Atoms can be brought together to execute gates by changing the waves slightly.
Also used to make high-precision atomic clocks.
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All-optical CNOT gate composed from
beam splitters and wave plates. O'Brien et al., Nature 426(264), 2003
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Various “threshold theorems” have suggested that
we need 10^4 to 10^6 gates in less than the
decoherence time in order to apply quantum error
correction (QEC). QEC is a big enough topic to
warrant several lectures on its own.
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