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January 7, 2014

The Promise of Quantum Money

Tiffany Trader
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In their continuing efforts to stop counterfeiting the United States Federal Reserve released a newly-designed $100 note with an eye-popping color palette. The newest iteration sports a decade’s worth of security advances, anti-counterfeiting measures such as holograms, raised printing and artifacts that appear to move and change color depending on the viewing angle.

Despite these sophisticated security features, currency forgers have a big financial incentive to succeed in manufacturing convincing fakes, as an article at Physics Central points out. In the computing world, experts have long looked to quantum mechanics to make unbreakable encryption. Encryption and decryption are seen as the number one killer apps for quantum computers. What if it were possible to extend quantum technology to make un-copyable currency?

In quantum computing, units of information are known as “qubits.” Unlike a classical bit which can only exist in one of two states, quantum mechanics allows a qubit to exist in a superposition of both states at the same time. According to theory from quantum physics, this same principle can also be used to create ultra-secure currency, referred to as quantum money.

“All dollar bills have serial numbers on them,” the articles notes. “Having a unique identifier on each bill helps law enforcement track and identify counterfeit bills. Quantum dollars would have a second serial number, one made of qubits that can only be read with a quantum computer.”

“Whenever someone buys something with quantum money, the clerk runs the bill’s serial number through a central database, checking to make sure that the bill’s qubit code matches the one on file. It’s really a bit more like checking than an ordinary cash in this way.”

The qubit codes are safe from counterfeiting because the act of measuring qubits would destroy them. This principle is referred to as the “no cloning theorem.” If a counterfeiter attempts to measure the polarizations of the qubits without knowing their value in advance, the code will be changed, making the bill worthless.

The author details the process: “To check a bill’s authenticity, the polarized qubits pass through a filter. If the filters match what is on the bill, then the qubits pass through unchanged, but if they’re different, the qubits change to match the filters. The reason that the clerk can measure the quibits without changing them, is because by running the serial number first, the computer can pull up the correct filters and scan the bill to make sure it’s legit.”

The idea for “quantum money” can be traced to physicist Stephen Wiesner, a graduate student at Columbia University, who worked on quantum information theory in the 1960s and 70s. Although the principles are sound, the idea has been called impractical owing to the fact that the necessary technology does not yet exist.

Quantum computing is considered the holy grail for the field of cryptography. Whether or not “true quantum computing” exists is a rather contentious topic, but it’s safe to say that much progress has been made in the last few years by the likes of D-Wave, Google, NASA, and others.