One of the fundamental principles of quantum mechanics is that objects can be in two states at the same time. This means that an electron can, for instance, be in two places at once. However, these "superposition" states are never seen in classical, macroscopic objects – one example being Schrödinger's famous cat, who clearly could not be both dead and alive at the same time.

Indeed, the very act of trying to find out whether the cat is alive or dead actually changes its state. This act of measuring disturbs the state of a quantum object so that it collapses and behaves like a classical one. In many experimental systems, measuring this state also actually destroys the qubit itself. Such destructive measurement is akin to needing a new cat and box every time you want to take a measurement.

Applying a microwave tone

In the field of superconducting quantum information, the fragile qubit state can be determined by applying a microwave “tone” to it, explain team members Josh Mutus and Ted White. “The so-called Josephson parametric amplifier (JPA), which is an oscillating resonator containing a SQUID loop, was an important advance here because it allows researchers to use a tone weak enough so that they can extract all the state information without destroying the qubit. Such a level of sensitive, non-destructive readout also allows us to measure the quantum state of the qubit in real time and allows us to observe and, in some cases, control how a single qubit evolves.”

The new device, the Impedance Transformed Parametric Amplifier (IMPA) now means that researchers can measure the states of multiple qubits at the same time by significantly expanding on the bandwidth of existing JPAs.

Strong coupling to a transmission line

The team, led by John Martinis, made its amplifier by coupling a JPA to a transmission line (an external impedance) to create a resonant circuit. “Amplification occurs when we apply a strong pump tone to the device, driving the oscillator into a regime in which energy from the pump is transferred to signals within the resonant bandwidth,” White tells nanotechweb.org. “The bandwidth size is determined by the coupling between the oscillator and the transmission line.”

Previous such amplifiers were only able to weakly couple to the transmission line because of technical constraints – namely the 50 Ohm impedance of the line imposed by the design of most existing commercial microwave hardware, adds Mutus. “To overcome this problem, we designed our IMPA with an on-chip impedance transformer that allows us to change the coupling of the oscillator to the transmission line – and this independently of the oscillator’s properties. Such strong coupling means that we can increase measurement bandwidth (from just 10–20 MHz to up to 700 MHz in this case) and so measure the states of several qubits at once.”

Indeed, the team reveals that it has already succeeded in measuring a record (for the solid state) of five entangled qubits as well as a single qubit with 99% fidelity in a record 140 nanoseconds.

“This fast and independent state readout is a unique feature of superconducting circuit architectures,” says White,” and demonstrates how much progress is actually being made in this field – something that could eventually permit for fault-tolerant computing in the future.”

The research is detailed in Appl. Phys. Lett.. You can read about it for free at arxiv.org/abs/1401.3799.