2. INTRODUCTION
Quantum cryptography recently made headlines when European Union members announced their
intention to invest $13 million in the research and developmentof a secure communications systembased
on this technology. The system, known as SECOQC
(Secure Communication based on Quantum Cryptography), will serve as a strategic defense against the
Echelon intelligence gathering system used by the United States, Australia, Britain, Canada and New
Zealand.In addition, a handful of quantum information processing companies, including MagiQ
Technologiesand ID Quantique, are implementing quantum cryptography solutions to meet the needs of
businesses, governments, and other institutions where preventing the unauthorized disclosure of
information has become a critical success factor in maintaining a competitive advantage over
.
3. • A Quantum Key Distribution Example
The following is an example of how quantum
cryptography can be used to securely distribute
keys. This example includes a sender, “Alice”, a
receiver, “Bob”, and a malicious eavesdropper,
“Eve” Alice begins by sending a message to Bob
using a photon gun to send a stream of photons
randomly chosen in one of four polarizations that
correspond to vertical, horizontal or diagonal in
opposing directions (0,45,90 or 135 degrees). For
each individual photon, Bob will randomly choose
a filter and use a photon receiver to count and
measure the polarization which is either rectilinear
(0 or 90 degrees) or diagonal (45 or 135
degrees), and keep a log of the results based on
which measurements were taken.
4. WHAT IS QUANTUM ERROR
• Noise” describes all of the things that cause interference in a quantum
computer. Just like a mobile phone call can suffer interference leading it to
break up, a quantum computer is susceptible to interference from all sorts of
sources, like electromagnetic signals coming from WiFi or disturbances in the
Earth’s magnetic field. When qubits in a quantum computer are exposed to
this kind of noise, the information in them gets degraded just the way sound
quality is degraded by interference on a call. This is known as decoherence.
5. HOW TO DO QUANTUM ERROR CORRECTION
• This error-correcting algorithm is able to draw from validated mathematical approaches used
to engineer special “radiation-hardened” classical In QEC quantum information stored in a
single qubit is distributed across other supporting qubits; we say that this information is
“encoded” in a logical quantum bit. This procedure protects the integrity of the original
quantum information even while the quantum processor runs – but at a cost in terms of how
many qubits are required. Overall, the worse your noise is, the more qubits you need
• Depending on the nature of the hardware and the type of algorithm you choose to run, the
ratio between the number of physical qubits you need to support a single logical qubit varies
– but current estimates put it at about 1000 to one. That’s huge. Today’s machines are
nowhere near capable of getting benefits from this kind of Quantum Error Correction.
