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Computing with a quantum trick


With a quantum gate Max Planck physicists have developed an essential logic element for quantum computers

Atoms and photons under control: Two spherical mirrors are mounted in a stainless steel holder in the form of truncated cones, one of which can be seen to the right of the middle of the image. Between the mirrors, the Max Planck physicists trap single atoms, which they bring in with a laser beam from the left. Through the glass windows of the vacuum chamber, they can transmit laser pulses. In this way, photons can enter the resonator through one of the mirrors. The physicists take advantage of this in their current experiment to construct a quantum gate, a logical coupling of a photon with an atom in the resonator. The quantum gate changes the state of a photon that is reflected off the resonator depending on the state of the atom. Such a quantum gate could make it possible to interconnect multiple quantum computers.

© Stephan Ritter / MPI of Quantum Optics

Logic with an atom and a photon: The atom (blue) in the resonator, which consists of two mirrors, and an incident photon (red) each encode a single quantum bit. The spin of the atom (indicated by an arrow in the atom) serves as a control bit. It determines whether the target bit, which is stored in the polarisation of the photon, is switched. Polarisation corresponds to the oscillation plane of the light. A logic gate can be characterised by four combinations of initial states, of which only two are shown here (top two illustrations). For a specific setting of the spin (arrow pointing upward, left panel), the linearly polarised light of the input photon splits into two circularly polarised partial waves. The anticlockwise-rotating partial wave penetrates the resonator; the clockwise-rotating wave is directly reflected from the first mirror (left, middle image). The partial wave in the resonator is thereby subject to a change, which physicists refer to as phase shift of 180 degrees. Consequently, the oscillation plane of the input photon rotates by 90 degrees when the modulated partial wave leaves the resonator and reunites with the other partial wave (left, lower image). This happens independently of the linear input polarisation of the photon. For the other orientation of the atomic spin (arrow pointing downwards, right panel), the photon enters fully into the resonator (right, middle image) and leaves it again with its oscillation plane unchanged (right, lower image).

© Graphic: Fritz Höffeler for the Max Planck Society

You better count on quantum information in the future. Physicists from the Max Planck Institute of Quantum Optics in Garching have developed a novel quantum gate, an essential component of quantum computers.

A future quantum computer would be able to handle certain types of tasks far faster than any classical computer. As a central element of their quantum gate, the Max Planck physicists are using an atom trapped between two mirrors of a resonator. By reflecting the photon off the resonator with the atom, they are able to switch the state of the photon. Moreover, the gate operation can entangle the atom with the photon.

When quantum particles are entangled, their properties become interdependent. Entanglement opens up whole new horizons in information processing. The quantum gate recently presented by the Garching-based physicists makes it possible to design quantum networks in which information is transferred between remote quantum processors in the form of photons.

The digital revolution is unlikely to be the final word in the development of information technology. A research team headed by Gerhard Rempe at the Max Planck Institute of Quantum Optics, along with numerous other researchers around the world, are already instigating the next revolution.

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This is now possible because physicists have learned how to manipulate single atoms, photons and other quantum particles more adroitly than Franck Ribéry can handle a football.

The purpose of the experiments is to explore ways to process data in the form of quantum bits, or qubits for short. Whereas classical bits only exist in the states of “0” or “1”, in qubits superpositions of these two states are possible. When several qubits are combined into a single unit – a phenomenon known as entanglement – it is possible to perform parallel calculations that would simply be inconceivable with conventional computers. “A quantum gate such as the one we have developed is an essential component in the construction of a quantum computer,” says Stephan Ritter, who heads the experiment.

A quantum gate consisting of one atom and one photon enables quantum networks

A CNOT gate couples a control bit with a target bit: whether or not the control bit changes the state of the target bit depends on its state. All logic circuits required for quantum calculations can be realized with this logic element and a few other simple operations. Many such logic elements are needed to build a quantum computer.

A quantum computer could, within a reasonable period of time perform intricate searches in databases that would take even the fastest computer today months to complete. In addition, a quantum computer could break the encryption commonly used today. To prevent eavesdroppers from gaining access to transmitted data, quantum information technology has a tried-and-tested trick up its sleeve: quantum cryptography, which stops spies from tapping information from a data line undetected. 

The logic gate devised by the physicists in Garching could be interesting both for the construction of quantum computers and for transmitting quantum information, as it uses tools from both technologies. Earlier quantum-computer concepts relied on the use of infinitesimally small but solid particles, such as atoms or ions. Physicists have since constructed quantum gates using various methods. They have been particularly successful with ion-based designs, which Austrian researchers have used to perform 100 sequential logical operations. By contrast, quantum communication, the basis for quantum cryptography, uses photons as a carrier for quantum information.

“With our quantum gate we have created a hybrid system consisting of a photon and an atom in a resonator”, says Andreas Reiserer, who conducted the recent experiment as part of his doctoral dissertation. “The gate could enable us to link multiple quantum processors." In this way, the researchers were able to overcome a daunting problem: that it might not be possible to link up enough quantum gates to form a processor and thus exploit the full potential of quantum computing. In a quantum network using hybrid quantum gates as interfaces, especially tricky tasks would not be performed by one large quantum computer but by multiple small processors linked together by photons.

A new mechanism to logically couple qubits

Stephan Ritter emphasises another property that distinguishes the team’s quantum gate. “We are presenting a new mechanism of interaction for coupling qubits,” says the researcher. “Not many are known, and new ones are very difficult to identify.” In physical terms, an interaction is any process in which particles or fields mutually influence each other. They play a role in what happens in the world around us. However, most interactions between particles or between light and particles cannot be adequately controlled for use in specific computing operations.

Recently the Max Planck researchers have achieved such fine control of an interaction that they can use it to operate a logic gate. They are able to alter the polarisation of a photon by letting it interact with a rubidium atom in a resonator. Polarisation corresponds to the oscillation plane of the light wave inherent in the photon. When the photon is reflected off the resonator with an atom in a suitable state, the interaction rotates the oscillation plane.

Several years ago, the Garching-based researchers succeeded in trapping individual atoms between the mirrors of a resonator for many seconds – under ideal conditions for longer than a minute, using laser beams that are finely tuned to the atom-resonator system. The force of the electromagnetic field of the laser beams holds the particles stationery between the mirrors. Using more laser pulses, the physicists are able to manipulate the spin of the rubidium atom. Spin is a quantum mechanical property that causes the atom to act like a tiny magnet. “The polarisation of the photon changes upon reflection from the resonator, conditioned on the direction of the atom’s spin,” Andreas Reiserer explains. So the qubit of the photonic input signal switches from “0” to “1” or vice versa, depending on the state of the atom – precisely what is required of a CNOT gate.

A quantum computer can perform parallel calculations with entangled particles

If the initial states are suitable, the switching operation also causes entanglement of the atom and photon. The properties of entangled particles subtly depend on each other: In the curious world of quantum physics, the atom’s spin is inextricably linked with the photon’s polarisation. The concrete state of the two properties – the direction of spin and the polarisation – remain ambiguous until the property of the particle is actually measured. Measuring one of the particles then simultaneously defines the state of both particles – irrespective of the distance separating them. It is this effect – Albert Einstein called it “spooky action at a distance” – that allows parallel processing, which could make quantum computers incomparably fast for some tasks.

The quantum gate can entangle multiple photons with an atom

Not only can the physicists in Garching entangle an atom with a single photon by skilfully selecting the spin of the atom and the polarisation of the photon, they can even make several photons “spookily” dependent on the atom. The inevitable consequence is that all the photons and the atom are entangled. So far, the Garching team have achieved this feat with two photons. In addition, they later managed to remove the atom from the entangled ménage-à-trois, leaving only a pair of entangled photons. The atom in the resonator is then available for new tasks.

“In our current work we have reached a pinnacle of our research that stretches back several years,” says Gerhard Rempe, Director of the Max Planck Institute of Quantum Optics. “We first stored information in individual atoms and read that information. We then transferred qubits from one atom to the next. Now we have also processed quantum information for the first time with our system.” Although it’s still a long way from here to a network of multiple quantum computers, the physicists in Garching have paved the way by steadily extending their influence in the quantum world. “In the meantime, we can control many effects that could eventually be used in quantum information technology,” says Rempe.


Dr. Stephan Ritter

Max Planck Institute of Quantum Optics, Garching

Phone: +49 89 3290-5728


Prof. Dr. Dr. habil. Gerhard Rempe

Max Planck Institute of Quantum Optics, Garching

Phone: +49 89 32905-701
Fax: +49 89 32905-311


Original publication

Andreas Reiserer, Norbert Kalb, Gerhard Rempe und Stephan Ritter
A quantum gate between a flying optical photon and a single trapped atom
Nature, 10. April 2014

Dr. Stephan Ritter | Max-Planck-Institute
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