seth_lloyd's picture
Professor of Quantum Mechanical Engineering, MIT; Author, Programming the Universe

My job is to design and build quantum computers, computers that store and process information at the level of individual atoms. Even at the current rapid rates of progress of current computer technology, with the computer components halving in size every two years or less, and computers doubling in power over the same time, quantum computers should not be available for forty years. Yet we are building simple quantum computers today. I could tell you that quantum computers will drastically change the way the world works during our lifetime. But I'm not going to do that, for the simple reason that I have no idea whether it's true or not.

Whether or not they change the world, quantum computers have something to offer to all of us. When they flip those atomic bits to perform their computations, quantum computers possess a several useful features. It's well known that quantum computers, properly programmed, afford their users privacy and anonymity guaranteed by the laws of physics. A less well-known virtue of quantum computers is that everything that they do, they can undo as well. This ability is built into quantum computers at the level of fundamental physical law. At their most microscopic level, the laws of physics are reversible: what goes forward can go backward. (By contrast, at the more macroscopic scales at which classical computers operate, the second law of thermodynamics kicks in, and what is done cannot be undone.) Because they operate at the level of individual atoms, quantum computers inherit those atoms' ability to undo the present, and recall the past.

While quantum computers afford their users protection and anonymity that classical computers cannot, even classical computers can be programmed to share this ability to erase regret, although they currently are not. Although classical computers dissipate heat and operate in and a physically irreversible way, they can still function in a logically reversible fashion: properly programmed, they can un-perform any computation that they can perform. We already see a hint of this digital nostalgia in hard-disk 'time machines,' which restore a disk to its state in an earlier, pre-crash era.

Suppose that we were to put this ability of computers to run the clock backward to the service of undoing not merely our accidental erasures and unfortunate viral infections, but to undoing financial transactions that were conducted under fraudulent conditions? Credit card companies already supply us with protection against theft conducted in our name. Why should not more important financial transactions be similarly guaranteed? Contracts for home sales, stock deals, and credit default swaps are already recorded and executed digitally. What would happen if combined digital finance with reversible computation?

For example, if a logically reversible computer—quantum or classical—were used to record a financial contract and to execute its terms, then at some later point, if the parties were not satisfied with the way those terms were executed, then those terms could be 'un-executed,' any money disbursed, reimbursed, and the contract deleted, as if it had never been. Since finance is already digital, why not introduce a digital time machine: let's agree now that when the crash comes, as it inevitably will, we'll restore everything to a better, earlier time, before we clicked those inauspicious buttons and brought on the blue screen of financial death.

Can it be done? The laws of physics and computation say yes. But what about the laws of human nature? The financial time machine erases profits as well as losses. Will hedge fund managers and Ponzi schemers sign on to turn back the clock if schemes go awry, even if it means that their gains, well-gotten or ill-, will be restored to their clients/victims? If they refuse to agree, then you don't have to give them your money.

I make no predictions, but the laws of physics have been around for a long time. Meanwhile, the only true 'law' of human nature is its intrinsic adaptability. Microscopic reversibility is the way that Nature does business. Maybe we can learn a thing or two from Her to change the way that we do business.