Defense Intelligence Reference Document Quantum Computing And Utilizing Organic Molecules In Automation Technology

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THE CLOSED BOX AND FAULT TOLERANCE
In traditional silicon transistor circuits, once a bit is set to 1 or O by a gate or other device
locking the output voltage, that value is expected to remain through subsequent clock cycles
until deterministically changed.h Additionally, traditional circuits behave by the same rules for
each clock cycle; that is, as more information is processed, repeated gate operation does not
deteriorate the bit latching mechanism. This behavior is achieved by constantly providing
energy to the circuits. Any interruption in this constant need for power from the outside and
the integrity of the information in the computing circuit is lost.;
A fundamental principle of quantum mechanics is that any external influence on a system
necessarily disturbs the state of that system. This influence could be external disruption or
internal leakage - either interaction will change the internal quantum state. This destructive
process is known as decoherence. External influences will disturb the system and thus the
circuits need to be isolated from the rest of the universe, also known as the 'closed box'
requirement.
Trial to trial variations in a quantum circuit produce an increasing deviation from an initial
phase as the wave function evolves. This trial-to-trial deviation causes decoherence on a
timescale termed T2*. Although a single trial in a quantum circuit could retain coherence
longer than T2*, absent external influence the isolated internal circuit components must
eventually come to thermal equilibrium through random processes: this occurs on timescale
Tl. Moreover, the closed box cannot be perfect since a useful device requires some kind of
input and output, thus some small interaction with the external environment is necessary.
Random interactions with the environment from this isolation 'leakage' will dephase internal
signals on timescale T2. These three time constants that describe the internal signal decays
are very similar to the same named quantities in nuclear magnetic resonance (NMR). NMR is
indeed a technology path under development for quantum circuits. Each technology and
hardware design will be characterized in its isolation from external and internal influences by
T2* (stable repeatability), Tl (resistance to entropy), and T2 (isolation from the rest of the
universe).
No design can be completely free of decoherence and the next consideration is how much
decoherence is acceptable, or more precisely, what is the fault tolerance threshold for
successful operation? Fault tolerance isn't much of a consideration in traditional processing
architecture,i although it is a major consideration in storage and communication of
information. To illustrate fault tolerance, consider a scheme of hard disk storage
configuration where each 8-bit byte is written across 9 disks in a stripe-set configuration
(one bit per disk, read in parallel). The extra disk holds parity information about the byte.
Consecutive bytes shift the location of all bits one disk so the parity information isn't all
stored on the same disk.k In the event one disk fails, each byte can be reconstructed from
the other 8 disks using a software algorithm that automatically starts when the disk failure is
detected. The broken hardware can be replaced and the data reconstructed while the system
is operating in this slower 'limp' mode. This storage system is said to be fault tolerant. The
° For those new to quantum phenomena, it may seem redundant to use the phrase "deterministically changed." The
phrase emphasizes the point that absent any error, classical circuits are always changed by intent, while quantum circuits
include the element of random occurrence.
'Here we refer to the processing circuits themselves. Some types of memory and long term storage can of course hold
information in isolation indefinitely.
1 A non-deterministic result in processing hardware causes a fatal error in current designs.
" The bitwise stripe-set is instructive on the principle at hand, but for engineering considerations, more complex
configurations are used in practice. See Wikipedia "RAID" for actual data d,stribution schemes.
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