A future quantum computer would be able to handle certain types of tasks far faster than a conventional computer. As a central element of their quantum gate, the Max Planck physicists are using an atom trapped between two mirrors of a resonator. Moreover, by reflecting the atom off the resonator with a photon, they are able to switch the state of the photon. Moreover, the computational 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 parallel quantum processors in the form of photons.
Atoms and photons under control: Two spherical mirrors are mounted in a stainless steel holder, one of which can be seen to the right of the middle of the photo. Between the mirrors, the Max Planck physicists trap single atoms, which enter from below. Laser pulses can be transmitted into the apparatus through the glass window of the mount. In this way, photons can enter the resonator through the backs of the mirrors. The physicists use a similar setup to construct a quantum gate, involving the logical coupling of a photon with an atom in the resonator. The quantum gate alters the state of a photon that is reflected off the resonator depending on the state of the atom in it. Such a quantum gate could make it possible to interconnect multiple quantum computers. (Image: Stephan Ritter / MPI of Quantum Optics)
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. 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 exists in the states of “0” or “1”, in qubits the opposite states are superimposed. 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. “Our quantum gate 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 opens up the possibility of 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 time, perform intricate searches in databases that would take even the fastest computer today months to complete. In addition, a quantum computer could crack most types of encryption used today. To prevent eavesdroppers from gaining unrestricted 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 mobile medium.
“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 enabled us to link multiple quantum processors together.” 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.
Logic with an atom and photon: The atom in the resonator, which consists of two mirrors, and an incident photon (red) code for one quantum bit. The nuclear 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. Four combinations of initial states are possible, of which only two are shown here (top two illustrations). If the nuclear spin points upward (left panel), part of the incident photon enters the resonator (left, middle image). Then the linearly polarised light of the input photon splits into two circularly polarised partial waves. The partial wave in the resonator rotates clockwise, while the partial wave outside the resonator rotates anticlockwise (indicated by the arrows). The partial wave in the resonator is modulated in the process. Consequently, the oscillation plane of the input photon rotates 90 degrees when the modulated partial wave leaves the resonator and reunites with the other partial wave (left, lower image). Conversely, if the nuclear spin of the atom points downward (right panel), the photon penetrates 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)
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 with an atom in a suitable state off the resonator, 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 resonator-atom system. The force of the electromagnetic field of the laser beams holds the particles stationery between the mirrors. By delivering 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 atom-coupled photon in the resonator changes, depending 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 determines 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 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. Not content with this, however, 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 laid the required foundation 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.