The goal of our group is to implement a quantum computer with superconducting
qubits. There exist many quantum systems that could be used as part
of a quantum computer, but the class of superconducting qubits based
on Josephson junction circuits is one of the most promising. One
advantage of these qubits is the ability to precisely engineer the
Hamiltonian of the system, which includes single qubit design, multiple
qubit design, and measurement design. The superconducting circuit
we study is the PersistentCurrent Qubit (PC Qubit).
A fully functional single qubit is the first step towards a quantum
computer. This is why single qubit characterization and the demonstration
of single qubit control is our top priority. One of the hallmarks
of a quantum system is its discrete energy levels, and so characterization
always includes a study of the qubit energy spectra via spectroscopy.
Single qubit control is studied by applying radiation to the qubit
and using nanosecondscale resolution measurements to watch population
transfer between eigenstates.
Just as important as a fully functional qubit is a good way to
measure the qubit. The conventional way of measuring flux qubits
is by ramping the bias current of a DC SQUID magnetometer until
a voltage is seen across the DC SQUID. The qubit state is then inferred
from the current at which the switch to finite voltage takes place.
However, strong qubit decoherence can result due to this readout
process. To address this we have experimentally implemented a resonant
readout technique that only requires the readout SQUID to be biased
at low currents along the supercurrent branch. The low current bias
tends to maintain the firstorder noise isolation, helping to minimize
the level of decoherence of the qubit. Since the SQUID does not
switch to the voltage state, the number of quasiparticles is also
drastically reduced. In addition, the resonant readout approach
utilizes a narrowband filter that shields the qubit from broadband
noise.
The greatest applications of a quantum computer require thousands
of qubits coherently coupled. To avoid having enormous amounts of
room temperature electronics, as well as the noise they contribute,
it would be extremely advantageous to integrate qubit and control
electronics monolithically. Modern superconducting foundries, such
as the one at MIT Lincoln Laboratory, have the capability of manufacturing
classical digital and analog electronics alongside quantum bits
in the same integrated circuit process. We have recently studied
a single Josephson junction coupled inductively to the PC Qubit.
When sufficient dc current is provided, the device acts as a current
controlled oscillator. Utilizing radiation sources directly integrated
with qubits permits individual control radiation for many different
qubits with a modest overhead in roomtemperature electronic complexity.
The eventual need for very largescale control circuitry to control
timing and order of operations on different qubits also has us investigating
Rapid Single Flux Quantum (RSFQ). RSFQ is capable of shuttling fundamental
quanta of magnetic flux throughout a microprocessor at clock speeds
of 10’s of GHz. These magnetic quanta are used as digital
data bits, and can also be used to stimulate the state of the qubit.
A thorough understanding
of the nature of the decoherence present in our system is another
extremely important step towards a quantum computer. Probing decoherence
with Electromagnetically Induced Transparency is a technique we
have mapped from the atomic world to the PC Qubit. The basic idea
is that you utilize three states of the qubit, two metastable states
1 and 2, and a third, shorterlived state 3, that may spontaneously
decay (to other unmentioned states) at a relatively fast rate R3.
A strong “control” microwave source couples the 23
transition, and a weak “probe” microwave source couples
the 13 transition. Individually, the microwaves from the probe
and control sources are readily absorbed by the qubit and thus the
transmittance of the radiation through the PC Qubit is quite low.
However, when the control and probe sources are applied simultaneously,
destructive quantum interference between the qubit states involved
in the two driven transitions causes the qubit to become "transparent"
to both the probe and control radiation. Thus, the fields pass through
with virtually no absorption. This can serve as a sensitive probe
of decoherence since even the smallest deviations from perfect destructive
interference between the qubit states results in a loss of transparency.
With the daunting tasks involved with building a large quantum
computer, the ability to do nearterm quantum computation is quite
attractive. This is why we have designed schemes to implement the
Factorized Quantum LatticeGas Algorithm (FQLGA) for fluid dynamics
simulation, as well as a scheme to run our quantum computer as an
Adiabatic Quantum Computer. The FQLGA is the quantum version of
classical latticegases. By replacing bits with qubits you gain
an exponential decrease in required memory and the ability to simulate
arbitrarily small viscosities. These advantages are seen with only
modest numbers of qubits, making the implementation foreseeable
in the not so far future. We have demonstrated multiple schemes
to implement this algorithm with the PC Qubit. Adiabatic quantum
computation, while mathematically equivalent to conventional quantum
computation, runs by exploiting the ability of coherent quantum
systems to adiabatically follow the ground state of a slowly changing
Hamiltonian. This has great practical interest because encoding
a quantum computation in a single eigenstate, the ground state,
offers intrinsic protection against dephasing and dissipation. We
have demonstrated a scalable superconducting architecture for adiabatic
quantum computation that can handle any class NP problem without
requiring interqubit couplings that vary during the course of the
computation. The proposal we have given requires neither interqubit
couplings to extend beyond nearest neighbors nor qubit measurements
to be highly efficient.
