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Opening a Remote Quantum Gate

Physicists have created and demonstrated a remote “quantum gate” – a key component for long-range quantum information transfer and an essential element of one plan for a quantum computer – by carefully manipulating the atomic states of two separately trapped ions.

A quantum computer would be able to handle certain currently intractable problems, such as factoring huge numbers and searching enormous unstructured databases, by exploiting a phenomenon called “superposition”: the ability of a quantum-mechanical object to be in multiple states at the same time. 

One indispensable part of any computing system is a logic gate, a device that can perform operations on fundamental units of data. In a conventional electronic computer, voltages and currents in transistors carry out that function on fixed-value binary digits. In a quantum computer, gates must operate on superpositions of many potential values. There are several possible designs, and scientists worldwide have been pursuing various approaches.

Now a team of researchers at the Joint Quantum Institute (JQI) have devised a remote gate which can “entangle” two widely separated ytterbium ions. Entanglement is a uniquely quantum-mechanical condition in which the states of two or more objects become inextricably linked even if they are physically far apart. Prior to taking a measurement, each object exists in a superposition of many states at the same time. But when a measurement forces one entangled object to take on specific characteristics, the characteristics of the other object are automatically determined. 

Overview of the Experimental SetupThe JQI gate design thus combines the function of an ordinary electronic logic gate – performing operations on the values of binary data bits – with the power of superposition, which allows multiple values in quantum bits, or “qubits,” simultaneously.

“Our experimental gate produces exactly the desired output for any chosen input about 90 percent of the time,” says JQI scientist Peter Maunz, “and we expect to improve the fidelity through modifications. Our gate could be used to create ‘quantum repeaters’ that facilitate the transfer of quantum information over long distances. In addition, it could be used to generate clusters of entangled arrays required for a so-called ‘one-way’ quantum computer.”

The research team used much the same laboratory configuration that produced the first teleportation between distant matter qubits. (See First Teleportation Between Distant Atoms)

Two individual ytterbium ions are confined in identical but completely independent vacuum traps a meter apart. An initial light pulse places each ion in the ground state. Then a microwave burst with carefully controlled phase and duration “writes” the desired superposition information onto the ions. This process prepares the ions in either of two hyperfine ground states --- an excellent quantum memory --- or various superpositions of both. Finally, a very brief laser beam boosts the ions into an elevated energy state. Each ion sheds that energy by emitting a single photon that can be one or the other of two frequencies, which the researchers call red and blue. At this point, each photon is entangled with the state of its ion.

Those photons are sent via optical fiber to opposite sides of a beamsplitter in which each photon has a 50-50 chance of passing through or being reflected. Thanks to rules of quantum interference, there is only one particular combination of red and blue photons that can cause both detectors to register a photon simultaneously. In that case, it is impossible to determine which photon came from which ion, and the ions are projected into an entangled state that forms the two-ion quantum gate.

Thereafter, the researchers performed numerous electromagnetic operations and measurements on the ions to see if a particular input state would dependably produce the anticipated output state. That occurred 89 percent of the time.

Because of experimental constraints on the existing laboratory configuration, the probability of successful two-photon coincidence detection is currently low, occurring about once in every 10 million attempts.  However, the scientists in JQI Fellow Christopher Monroe's group repeated the experiment about  70,000 times per second, resulting in a successful event every 11 minutes -- less than the 12-minute coherence time of the chosen qubit states. "On the other hand," Maunz notes, "we know exactly when we succeed." The researchers believe that the success rate can be improved considerably by capturing a higher percentage of emitted photons.  But the present success rate does not affect one of the principal strengths of the design: "A major strength," Maunz says, "is the operation of the gate between two quantum memories. Atoms and ions are able to store quantum states for a long time."

The research was supported by the Intelligence Advanced Research Project Activity under Army Research Office contract, by the National Science Foundation (NSF) Physics at the Information Frontier program, and by the NSF Physics Frontier Center at JQI.

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