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computer needs multiple qubits to do calculations, and at least 50 qubits to do anything useful. 14 We might need 4,000 to 8,000 entangled qubits to surpass current encryption technology using very large integers. 3 Programming the devices also requires specialized hardware design skills, not conventional software programing skills. 3

Entangled qubits are difficult to use and scale because of another phenomenon called “decoherence.” The specific correlations between quantum states can dissipate over time, thus destroying the ability of qubits to explore multiple solutions simultaneously. A useful analogy is to think of qubit outputs like smoke rings blown from a cigar. 14 The rings can represent information but disintegrate (lose their “coherence”) quickly. Since entangled qubits have a small probability of taking on different values due to external interactions, the computations require another process to detect and correct errors.

IN 1981, NOBEL Laureate Richard Feynman challenged the com- puting community to build a quantum computer. We have come a long way. In 2015, McKinsey estimated there were 7,000 researchers working on quantum computing, with a combined budget of $1.5 billion.20 In 2018, dozens of universities, approximately 30 major companies, and more than a dozen startups had notable R&D efforts.a Now seems like a good time to review the business.

How do quantum computers work?
Quantum computers are built around
circuits called quantum bits or qubits.
One qubit can represent not just 0 or 1
as in traditional digital computers, but
0 or 1 or both simultaneously—a phe-
nomenon called “superposition.” A
pair of qubits can represent four states,
three qubits eight states, and so on.
N qubits can represent 2n bits of in-
formation, and even 300 qubits can
a https://bit.ly/2OXEA5n
represent information equal to the es-
timated number of particles in the
known universe. 21 To perform calcula-
tions, qubits exploit superposition
and “entanglement.” This refers to
when two quantum systems (such as
an electron or a nucleus), once they in-
teract, become connected and retain a
specific correlation in their spin or en-
ergy states (which represent combina-
tions of 0 and 1), even if physically sep-
arate. Entanglement makes it possible
for quantum bits to work together and
represent multiple combinations of
values simultaneously, rather than
represent one combination at a time.

Once a calculation is finished, you observe the qubits directly as 0 or 1 values to determine the solution, as with a classical computer.

What are the technical hurdles? Qubits resemble hardwired logic gates usually made of atomic particles and superconductor materials chilled to near-absolute zero. A one-qubit system is not so difficult to build, but a quantum

Technology Strategy
and Management
The Business of
Quantum Computing
Considering the similarities of quantum computing development
to the early years of conventional computing.
DOI: 10.1145/3267352