Quantum states have been shown to endure in a room-temperature solid-state device for a whopping 39 minutes, shattering the previous record of 2 s. The feat was achieved by physicists in Canada, the UK and Germany, who used phosphorus atoms in silicon as their quantum bits – or qubits. The breakthrough offers hope that normally fragile quantum states could be made robust enough to be used in practical quantum computers or even in "quantum money".
Quantum computers are designed to exploit the counterintuitive idea that tiny objects can exist in more than one state at the same time. Rather than processing bits – which are either 0 or 1 – such devices instead manipulate qubits, which can be 0 and 1 simultaneously. Vast numbers of operations could therefore, in principle, be carried out in parallel and rendering these devices far quicker than classical computers.
But anyone trying to build a working quantum computer has to deal with the fact that qubits tend to be incredibly fragile, which means the quantum information they hold is rapidly destroyed by external noise. One way of getting around this problem is to cool the qubit to near absolute zero to minimize its exposure to thermal noise. But working at such low temperatures is not particularly practical, which is why researchers are keen on find qubits that can operate at room temperature.
The new record-breaking system has been created by Mike Thewalt of Simon Fraser University and colleagues, by storing quantum information in the nuclear spins of phosphorous atoms in a silicon crystal. The idea of using these nuclear spins is not new and the system has already been shown to retain quantum information for long times at extremely low temperatures. But even at 10 K, this "coherence time" drops precipitously to just a few milliseconds.
To get around this problem, Thewalt and colleagues took advantage of the fact that phosphorous atoms in silicon at room temperature tend to give up their electrons and become positive ions. Removing the electrons eliminates an important link between the nuclear spins and surrounding electrical noise. Nuclear spins can therefore retain quantum information for much longer than those in neutral phosphorous.
The downside is that removing the electrons makes the nuclear spins so well isolated that they cannot be "read" or "written" to. So to get around this problem, the team first cooled its crystal to 4.2 K and used laser and radio frequency (RF) pulses to put neutral phosphorous atoms into specific quantum states. A laser pulse then ionized the atoms before the crystal was warmed up to room temperature (298 K).
Under these conditions, RF pulses were used to perform a "spin echo" measurement of the coherence time, which was found to be 39 minutes. The crystal was then cooled back down to 4.2 K and another laser pulse was used to neutralize the phosphorus ions before the quantum information was read out using a sequence of laser and RF pulses.