已有 2825 次阅读 2019-7-22 09:35 |个人分类:杂说|系统分类:科普集锦| 量子物理 |文章来源:转载

这是science alert上的一篇科普文章:



Physicists Have Reversed Time on The Smallest Scale by Using a Quantum Computer

MIKE MCRAE    20 JUL 2019


It's easy to take time's arrow for granted - but the gears of physics actually work just as smoothly in reverse. Maybe that time machine is possible after all?

An experiment earlier this year shows just how much wiggle room we can expect when it comes to distinguishing the past from the future, at least on a quantum scale. It might not allow us to relive the 1960s, but it could help us better understand why not.
近来的实验向我们表明我们在区分未来和现在的时间上有多大的空间,至少是在量子级(尺寸上)。 它可能不能够让我们回到1960年代,但是却能够让我们理解为什么不能回去。

Researchers from Russia and the US teamed up to find a way to break, or at least bend, one of physics' most fundamental laws on energy.

The second law of thermodynamics is less a hard rule and more of a guiding principle for the Universe. It says hot things get colder over time as energy transforms and spreads out from areas where it's most intense.
热力学第二定律是一个不那么塑性的定律,而更像是一个引导宇宙规律的定律。它说热的东西随时间流逝会渐渐冷却,因为能量由高到低的传导和散失 。

It's a principle that explains why your coffee won't stay hot in a cold room, why it's easier to scramble an egg than unscramble it, and why nobody will ever let you patent a perpetual motion machine.

It's also the closest we can get to a rule that tells us why we can remember what we had for dinner last night, but have no memory of next Christmas.

"That law is closely related to the notion of the arrow of time that posits the one-way direction of time from the past to the future," says quantum physicist Gordey Lesovik from the Moscow Institute of Physics and Technology.
“热力学第二定律与时间由过去向未来单向流动密切相关”,莫斯科物理技术学院(Moscow Institute of Physics and Technology)的量子物理学家Gordey Lesovik说。

Virtually every other rule in physics can be flipped and still make sense. For example, you could zoom in on a game of pool, and a single collision between any two balls won't look weird if you happened to see it in reverse.

On the other hand, if you watched balls roll out of pockets and reform the starting pyramid, it would be a sobering experience. That's the second law at work for you.

On the macro scale of omelettes and games of pool, we shouldn't expect a lot of give in the laws of thermodynamics. But as we focus in on the tiny gears of reality - in this case, solitary electrons - loopholes appear.

Electrons aren't like tiny billiard balls, they're more akin to information that occupies a space. Their details are defined by something called the Schrödinger equation, which represents the possibilities of an electron's characteristics as a wave of chance.

If this is a bit confusing, let's go back to imagining a game of pool, but this time the lights are off. You start with the information – a cue ball – in your hand, and then send it rolling across the table.

The Schrödinger equation tells you that ball is somewhere on the pool table moving around at a certain speed. In quantum terms, the ball is everywhere at a bunch of speeds … some just more likely than others.

You can stick your hand out and grab it to pinpoint its location, but now you're not sure of how fast it was going. You could also gently brush your finger against it and confidently know its velocity, but where it went... who knows?

There's one other trick you could use, though. A split second after you send that ball rolling, you can be fairly sure it's still near your hand moving at a high rate.

In one sense, the Schrödinger equation predicts the same thing for quantum particles. Over time, the possibilities of a particle's positions and velocities expands.

"However, Schrödinger's equation is reversible," says materials scientist Valerii Vinokur from the Argonne National Laboratory in the US.
 “然而,薛定谔方程是可逆的”,美国Argonne国家实验室物理学家Valerii Vinokur说。

"Mathematically, it means that under a certain transformation called complex conjugation, the equation will describe a 'smeared' electron localising back into a small region of space over the same time period."

It's as if your cue ball was no longer spreading out in a wave of infinite possible positions across the dark table, but rewinding back into your hand.

In theory, there's nothing stopping it from occurring spontaneously. You'd need to stare at 10 billion electron-sized pool tables every second and the lifetime of our Universe to see it happen once, though.

Rather than patiently wait around and watch funding trickle away, the team used the undetermined states of particles in a quantum computer as their pool ball, and some clever manipulation of the computer as their 'time machine'.

Each of these states, or qubits, was arranged into a simple state which corresponded to a hand holding the ball. Once the quantum computer was set into action, these states rolled out into a range of possibilities.

By tweaking certain conditions in the computer's setup, those possibilities were confined in a way that effectively rewound the Schrödinger equation deliberately.
通过仔细的调节计算机的初始状态,从而对其中的量子的状态的可能性进行限制 ,从而有效地使薛定谔方程逆向过程发生。

To test this, the team launched the set-up again, as if kicking a pool table and watching the scattered balls rearrange into the initial pyramid shape. In about 85 percent of trials based on just two qubits, this is exactly what happened.

On a practical level, the algorithms they used to manipulate the Schrödinger equation into rewinding in this way could help improve the accuracy of quantum computers.

It's not the first time this team has given the second law of thermodynamics a good shake. A couple of years ago they entangled some particles and managed to heat and cool them in such a way they effectively behaved like a perpetual motion machine.

Finding ways to push the limits of such physical laws on the quantum scale just might help us better understand why the Universe 'flows' like it does.

This research was published in Scientific Reports.
这篇研究在科学报告Scientific Reports上发表。

A version of this article was first published in March 2019.

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